early ordovician (arenig) brachiopods from volcaniclastic ... - BioOne

J. Paleont., 77(2), 2003, pp. 212–242 Copyright q 2003, The Paleontological Society 0022-3360/03/0077-212$03.00

EARLY ORDOVICIAN (ARENIG) BRACHIOPODS FROM VOLCANICLASTIC ROCKS OF THE FAMATINA RANGE, NORTHWEST ARGENTINA JUAN L. BENEDETTO Ca´tedra de Estratigrafıá y Geologıá Histo´rica, Departamento de Geologıá, Facultad de Ciencias Exactas, Fı´sicas y Naturales, Universidad Nacional de Co´rdoba, CONICET, Av. Velez Sarsfield 299, 5000 Cordoba, Argentina

ABSTRACT—This paper constitutes the first monographical study of the rich brachiopod faunas from the Early Ordovician Suri and Molles Formations of the central Famatina Range, which form a nearly continuous, more than 2,000 m thick succession of fossiliferous clastic and volcaniclastic rocks. Conodonts from the brachiopod-rich levels of the upper third of the Suri Formation and Los Molles Formation indicate the upper part of the Oepikodus evae Biozone (mid-Arenig). The systematic study of brachiopod faunas reveals the presence of 22 species belonging to 19 genera, three of which are new. The new genera recognized are the orthid Suriorthis, the hesperonomiid Mollesella, and the rectostrophiid Trigonostrophia. The following 12 new species and subspecies are described and illustrated: the clitambonitoidean Tritoechia mollesensis; the skenidioideans Crossiskenidium? stelzneri and Skenidioides kayseri; the orthoideans Paralenorthis suriensis, Paralenorthis riojanus brevis, Panderina? ambigua, Productorthis angulensis, Hesperonomiella arcuata, and Monorthis transversa; the plectorthoideans Ffynnonia famatinensis and Desmorthis? bifurcata; and the porambonitoidean Rugostrophia protoandina. Associated forms are Tritoechia sp., Pinatotoechia acantha Benedetto, 2001b; Protoskenidioides cf. revelata Williams, 1974; Hesperonomia orientalis Benedetto, 1998a; Paralenorthis riojanus (Levy and Nullo, 1973), Famatinorthis turneri (Levy and Nullo, 1973); and Camerella sp. Brachiopods from the Famatina Range display strong affinities with Welsh and Central Newfoundland, Maine and New Brunswick volcanic assemblages forming with them a statistically well defined Celtic cluster. Faunal evidence suggests that the Famatina volcanic belt continues northward into the western Puna belt.

INTRODUCTION

been known from the Famatina Range for over a century, since the work published in 1876 by the German paleontologist Emanuel Kayser who first noticed the Ordovician fossils of Argentina. Among other fossils, he described and illustrated with drawings Orthis caligramma Dalman, Orthis vespertilio Sowerby, Orthis disparilis Conrad, and Orthisina adscendens Pander. No further descriptions were made until the revisions by Levy and Nullo (1973, 1980) based on specimens collected by J. M. Turner during the realization of the Famatina geological map (Turner, 1964). Though geographically localized, taxa described in the Levy and Nullo papers lack precise stratigraphic indications. Brachiopods and other macrofossils from the less complete but more accessible outcrops of the northern part of the Famatina System (Chaschuil area), however, have been subject of several recent studies (brachiopods: Acenõlaza and Toselli, 1977; Benedetto, 1994; trilobites: Acenõlaza and Ra´bano, 1990; Vaccari et al., 1993; Vaccari and Waisfeld, 1994; bivalves: Sańchez and Babin, 1993; Sańchez, 1994, 1997). This paper constitutes the first monographical study of the rich brachiopod faunas from the Early Ordovician succession of the central Famatina Range (Fig. 1). It was based on material collected by the author during the past five years from the continuous and slightly deformed volcano-sedimentary rocks exposed along the Cachiyuyo, Saladillo Grande, Saladillo Chico and Los Molles Rivers. Over 40 fossiliferous levels were sampled from the Suri and Molles Formations, including approximately 3,000 brachiopod specimens, most of them finely preserved as external and internal molds. This study shows that brachiopod faunas from these units are far more diverse than had been acknowledged and constitute the richest known Arenig assemblage from west Gondwana. Although it is generally agreed that the Famatina System developed along an active margin during the Early Paleozoic (Acenõlaza and Toselli, 1984), the spatial relationship between the magmatic arc and the Gondwana continental margin remains controversial. For instance, based on geochemical evidence, Mannheim (1993) proposed a volcanic island-arc model with an adjacent back-arc basin (see Astini and Benedetto, 1996, for other possible settings for the island-arc hypothesis). Recent evidence, however,

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appears to favor an ensialic marginal basin coupled with a continental arc (Pankhurst et al., 1998). A causal relationship between the eastward subduction beneath the proto-Andean margin responsible for the Famatinian volcanic activity and the approach and subsequent accretion of the exotic, Laurentian-derived Precordillera Terrane was proposed by Benedetto (1993) and Benedetto and Astini (1993), and later analyzed in detail by Astini et al. (1995). Pankhurst and Rapela (1998) and Ramos and Keppie (1999) recently edited complete collections of papers dealing with plate tectonics of the south Iapetus and possible interactions between Gondwana, Laurentia and intervening terranes during the Early Paleozoic. In light of this complex geodynamic scenario, taxonomic knowledge of the Famatina brachiopods and resulting faunal affinities can give valuable clues to the paleogeographic reconstruction of the proto-Andean margin of Gondwana. GEOLOGICAL SETTING, STRATIGRAPHY, AND AGE OF THE FAUNAS

The Sierra de Famatina (‘‘Famatina System’’) is an over 6,000 m altitude mountain chain located to the west of the Pampean Ranges and to the northeast of the Precordillera (Fig. 1). Geologically it is characterized by large volumes of Early Paleozoic calcalkaline granites and volcanics interbedded with marine volcanosedimentary rocks. The basement of the Famatina System consists of low-grade metasedimentary rocks formed during the Neoproterozoic-mid Cambrian Pampean Cycle. The oldest sedimentary rocks, the Volcancito Formation, consists of black shales, sandstones and some interbedded carbonate horizons bearing Late Cambrian-earliest Tremadoc trilobites and graptolites (Harrington and Leanza, 1957; Tortello and Esteban, 1997). The overlying volcano-sedimentary rocks are superbly exposed in the core of the Famatina Range, where form a nearly continuous, more than 2,000 m thick succession from which the brachiopods described here came. Less continuous and more deformed sequences crop out in the Chaschuil area (Fig. 1), in the northernmost part of the Famatina System, Catamarca Province (Mańgano and Buatois, 1996, 1997). This succession forms the Famatina Group (Turner, 1964) that includes the Suri and Molles Formations previously recognized by Harrington and Leanza (1957). Astini and Benedetto (1996) and Astini (1998) reviewed the stratigraphy and depositional setting of these units in the central region of the Famatina Range. The Suri Formation is well

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FIGURE 1—Geologic map of the Cachiyuyo River area in the eastern flank of the Famatina Range showing outcrops of the Suri and Molles Formations and location of fossiliferous localities (asterisks). Modified from Astini (1998, fig. 3).

exposed along the Saladillo Grande, Saladillo Chico, and Cachiyuyo Rivers (Fig. 1). It can be subdivided into the following four informal members, from bottom to top (Fig. 2): 1) silicified black shales with thin tuff partings; 2) gray-bluish laminated shales; 3) bioturbated gray sandy-siltstones with calcareous nodules; and 4) greenish sandstones and bioturbated siltstones. The overlying Molles Formation, best exposed along the rio Los Molles, the type section, consists of three members: 1) lower reddish-purple sandstones and siltstones; 2) green pyroclastic breccias and volcanic sandstones interbedded with tuffaceous fossiliferous mudstones and siltstones; and 3) upper red and purple siltstones and sandstones. The overall succession shows a clear shallowing upward trend, from oxygen-deficient, starved-basin graptolitic black shales (lower part of the Suri Formation) to tidally influenced sandstones (top of the Molles Formation). The upper part of the Suri Formation is characterized by richly fossiliferous mudstones interbedded with lenticular, hummocky cross-bedded sandstones interpreted as deposited in a narrow, high-gradient, storm-influenced platform subjected to cyclic drops in sea level and prograding volcanic aprons (Astini and Benedetto, 1996; Mańgano and Buatois, 1997; Astini, 1999). Mud-dominated platform facies are recurrent and reappear in the upper part of the Molles Formation. These muddy and silty facies yielded most of the brachiopods included in this study (Fig. 2). The remaining brachiopods occur as shell concentrations within sandstone storm-beds

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FIGURE 2—Generalized stratigraphic column of Famatina Group beds exposed in the Cachiyuyo River area (left), and detail of the sampled interval of the Molles Formation (right) showing position of brachiopod-bearing levels. CA: Cachiyuyo River section; QS: Saladillo Chico River section; S: Saladillo Grande section; Mo: Los Molles River section. Geographic location of sections on Figure 1.

near the top of the Suri Formation and more frequently in the lower part of the Molles Formation. The age of the Famatina Group, formerly considered as Early Llanvirn on the basis of trilobites (Harrington and Leanza, 1957; Acenõlaza and Ra´bano, 1990), is now well constrained on conodont and graptolite evidence. Graptolites from the lower member of the Suri Formation, described by Toro and Brussa (1997) indicate an Early- to mid-Arenig age (Baltograptus deflexus and Didymograptellus bifidus Zones). Conodonts recovered from coquinites of the Suri Formation (Loma del Kilometro Member, Mańgano and Butois, 1996) in the Chaschuil region were referred to the mid-Arenig Baltoniodus navis Biozone by Albanesi and Vaccari (1994). This age assignment is consistent with trilobite data from the immediately underlying beds, which have yielded a typical mid-Arenig (Whitlandian) fauna including Oopsites sp., Carolinites genacinaca Ross, and a species of Merlinia closely related to the British M. ryakos Fortey and Owens (Vaccari and Waisfeld, 1994). Conodonts from the central Famatina (Cachiyuyo and Los Molles Rivers), suggest an Early Arenig age (Prioniodus elegans Zone) for the upper part of the Suri Formation and the Baltic Paroistodus originalis Zone for the lower part of the Molles Formation (Lehnert et al., 1997). New samples from the

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carbonate-rich, brachiopod-bearing coquinites of the upper member of the Suri Formation in the Saladillo Grande section as well as from the base of the Molles Formation at its type section have both produced a conodont association referable to the upper part of the Oepikodus evae Zone (Albanesi and Astini, 2000). In conclusion, the paleontological evidence now available supports a mid-Arenig age (Upper Ibexian) for the brachiopod assemblages described in this paper. These levels can be confidently correlated with other Early Ordovician brachiopod-rich stratigraphic units of Argentina such as the upper part of Acoite Formation of the Central Andean basin (Benedetto, 1998a) and the middle part of the San Juan Formation of the Precordillera basin (Huacoella and Niquivilia Brachiopod Zones) (Benedetto, 1998b and references therein). PALEOBIOGEOGRAPHIC SIGNIFICANCE OF THE FAMATINIAN BRACHIOPODS

The Celtic affinity of the Famatinian brachiopods was initially noted by the author in studying the low-diversity assemblage from the Chaschuil area (Benedetto, 1994). New taxonomic data from the rich central Famatina faunas described here confirm this paleobiogeographic signature and provide new evidence to evaluate the relationships of the Famatina arc-related basin with the Gondwana margin and other Iapetus terranes. Nineteen genera have been positively identified from the Famatina basin, and two (Panderina, Desmorthis) remain questionably assigned. Four of them are widespread forms without biogeographic significance (Tritoechia, Paralenorthis, Hesperonomiella, Camerella) even though the former two are systematically associated with Celtic faunas. The genus Incorthis, recorded at the Chaschuil locality, is the sole representative of autochthonous western Gondwana faunas (northwest Argentina, Bolivia and Morocco; Havlıćˇek and Branisa, 1980; Mergl, 1991; Benedetto, 1994). Protoskenidiodes has been recorded in Ireland (Laurentia) (Williams and Curry, 1985) and Shelve District (Williams, 1974), although the species to which the Famatinian form has been compared occurs in the latter locality. Crossiskenidium, present with doubt in the Famatina basin, has only been reported from the Tourmakeady Limestone of Ireland that contains a Laurentian brachiopod association (Williams and Curry, 1985). Most of the remaining genera, including Ffynnonia, Productorthis, Monorthis, Famatinorthis, Rugostrophia, Skenidioides and Ahtiella (the latter not yet described) are distinctive of Celtic assemblages; two of them, Ahtiella and Productorthis, are shared with the Baltic province. For statistical analysis a database of 60 Arenig genera from 16 localities belonging to the four brachiopod-based paleobiogeographic entities (Toquima-Table Head, Celtic, Baltic, and Mediterranean) was assembled. The Mediterranean region includes the Klabava Formation of the Praga basin and the northwest Africa (Morocco) assemblages (Havlıćˇek, 1971; Havlıćˇek et al., 1991). In the data matrix, the Mytton Flags fauna of Shropshire (Shelve) (Williams, 1974) was treated separately, and Celtic faunas were subdivided into ‘Welsh’ assemblages (Anglesey, Tagoat and Dyfed localities) and Central Newfoundland, Maine and New Brunswick assemblages (‘CENFL’). In this analysis, the present-day neighboring Precordillera and Central Andean basins (Andean) were discriminated as independent sites to show their faunal relationships with the Famatina assemblage (Fig. 1). The Central Andean basin includes brachiopods from the Cordillera Oriental of Argentina (Benedetto, 1998a; unpublished data) and some Bolivian localities (Havlıćˇek and Branisa, 1980). Endemic genera restricted to a single province or site were not included in the data matrix. Figure 3 illustrates the dendrogram obtained by cluster analysis based on the Jaccard similarity measure by unweighted pair group algorithm. The Welsh and CENFL assemblages cluster together

FIGURE 3—Dendrogram of Arenig brachiopod faunas from selected localities generated by Cluster analysis of a matrix using Jaccard coefficient by unweighted pair group linkage.

with a coefficient linkage over 0.45 and both form with the Famatina faunas a statistically well-defined Celtic cluster. The Arenig Precordilleran assemblages are strongly linked (coefficient of similarity of 0.65) with the Toquima-Table Head faunas. The mixed ‘Celtic’-Toquima-Table Head signature of the Precordillera brachiopods noted by Herrera and Benedetto (1991) and Neuman and Harper (1992) does not becomes evident until the Early Llanvirn when a number of Celtic taxa immigrate to this region. The Shelve and Central Andean faunas form an independent cluster without evident links with the Celtic and Baltic-Mediterranean clusters. The distinctiveness of the Celtic province was demonstrated by Neuman and Harper (1992) in using the technique of correspondence analysis and later by Harper et al. (1996) by means of cladistic and other phenetic methods. According to these authors, the Celtic province developed mainly around northwest Gondwana at mid- to high-latitudes, in both autochthonous margins and allochthonous volcanic islands. The Famatina brachiopod assemblage supports the geographic continuity of the Celtic province in regions far removed from the Appalachian-Caledonian sites (Neuman and Harper, 1992). Discovery of a species comparable to Famatinorthis turneri in the Arenig Shin Brook Formation of Maine (Neuman, 1997) and the reassignment to Monorthis of specimens previously identified as aff. Eostrophomena (McKerrow and Cocks, 1986) from the Gander Group of Central Newfoundland (Benedetto, 1998b; Neuman, 1999) strengthens the link between the Famatina and the eastern North America accreted terranes. On the basis of available paleontological and paleomagnetic data from the Famatina and Puna volcanics (Conti et al., 1996) the author proposed a reconstruction of the south Iapetus (Fig. 4) in which a long, probably nearly continuous volcanic island-arc system extended along the western Gondwanan margin, named

BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA

FIGURE 4—Arenig paleogeographic reconstruction of the Iapetus Ocean and intervening terranes. A: Avalonia; An: Anglesey; F: Famatina; FPA: Famatina-Puna-Avalonia volcanic island-arc system; MIR: MidIapetus Ridge; NB: New Brunswick; NDA: Notre Dame arc (after Van Staal et al., 1998); NFL: Central Newfoundland; NI: Northwest Ireland; M: Maine; P: Precordillera; PA: Penobscot arc; WP: Western Puna arc. Modified from Benedetto, 1998b, figure 6. North Iapetus terranes after Harper et al. (1996).

the ‘Famatina-Puna-Avalonia system’ (Benedetto, 1998b). The brachiopod assemblage recently discovered in volcanosedimentary rocks of the western Puna belt (Benedetto, 2001b), which includes Tritoechia, Jaanussonites, Rugostrophia, Pinatoechia and probably Trigonostrophia, strongly suggests that Famatina volcanics continue to the north into the western Puna belt (‘Faja eruptiva occidental’). Finally it is interesting to remark that the Famatina fauna, as the same as other Celtic assemblages, is characterized by a number of ‘first records’ of genera that appear in younger rocks of other regions, generally Laurentia. Correlation between the chronologically well constrained faunas from the Famatina and Precordillera basins shows that the age of levels bearing Rugostrophia, Ahtiella, Productorthis, Camerella and Ffynnonia is midArenig in the Famatina basin and Early Llanvirn in the Precordillera, with the exception of Ffynnonia which first appears in late Arenig time (E. variabilis Zone) (Benedetto, 2001a). The absence of these genera in the mid-Arenig limestones of the Precordillera is hard to explain in terms of environmental preferences because they occur in both carbonate (San Juan Formation of the Precordillera) and clastic rocks (Suri and Molles Formations of the Famatina Range). Such a faunal diachronism strongly suggests that before the latest Arenig-Early Llanvirn time the Precordilleran carbonate platform was not in continuity with the Famatinian arcrelated basin. This evidence is consistent with the hypothesis that the Precordillera is an exotic terrane accreted to the Gondwana

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margin (Ramos and Keppie, 1999, and collection of papers therein). Although there is a general agreement that the Precordillera is a far-traveled, Laurentian-derived terrane, the timing and mechanisms of its accretion to the Gondwana margin are matters of debate. Current paleontological evidence from a number of different fossil groups supports a late Ordovician collision (Benedetto et al., 1999), even though a Late Silurian or younger amalgamation age has been suggested on the basis of tectonic and metamorphic data (Keller, 2000). In the frame of this complex south Iapetus geodynamic scenario it seems likely that the Famatina basin evolved as an independent terrane linked from its origin with other segments of a peri-Gondwanan island-arc system inhabited by Celtic province brachiopods (Fig. 4). By the Early- mid-Arenig the Precordillera was still faunally connected to the Laurentian platform, as the faunal similarity with the Toquima-Table Head assemblages indicates (Fig. 3). This time probably represents the transition between the ‘Laurentian’ and ‘isolation’ stages of the biogeographic evolution of the Precordillera (Benedetto et al., 1999). By the end of the Arenig and especially during the Early Llanvirn the Precordillera terrane became almost completely isolated from large paleocontinents such as Gondwana and Laurentia. The immigration of Celtic brachiopods at that time into the Precordillera (Cuyania) microcontinent may reflect some degree of approximation to other intra-Iapetus terranes during its movement from low to high latitudes. Independent evidence from K-bentonite records in the Precordillera basin also favors this hypothesis (Huff et al., 1998 and references therein). Although the oldest K-bentonites occur in mid-Arenig beds, they become more abundant toward the upper part of the San Juan Formation, within the variabilis and suecicus Zones interval. Huff et al. (1998) conclude that such volcanic ashes occur in proximity to volcanic arcs and that the Puna-Famatina terrane might have served as the source. On the other hand, the close correlation between the deepening-upward succession of facies at the top of the San Juan Formation (Canãs, 1999) and the sudden appearance of most of the Celtic forms (Ahtiella Assemblage Zone) suggests that sea level fluctuations played a complementary role enhancing dispersion of Celtic faunas. This transgressive event may be correlated to the well-documented latest Arenig-Early Llanvirn global sea-level highstand. SYSTEMATIC PALEONTOLOGY

All material used in the present study is deposited in the Paleontologic Collection of the Ca´tedra Estratigrafıá y Geologıá Historica, Department of Geology, Facultad de Ciencias Exactas, Fı´sicas y Naturales, Co´rdoba University, Argentina (prefix CEGHUNC). Subphylum RHYNCHONELLIFORMEA Williams et al., 1996 Class STROPHOMENATA Williams et al., 1996 Order BILLINGSELLIDA Schuchert, 1893 ¨ pik, 1934 Suborder CLITAMBONITIDINA O Superfamily CLITAMBONITOIDEA Winchell and Schuchert, 1893 Family TRITOECHIIDAE Ulrich and Cooper, 1936 As Popov et al. (2001) outlined, assignment of the genus Polytoechia to this group remains problematical because it possesses a well developed spondylium triplex. For this reason Ulrich and Cooper’s familial name Tritoechiidae instead of Polytoechiidae is preferred here. Genus TRITOECHIA Ulrich and Cooper, 1936 Type species.Deltatreta typica SCHUCHERT 1932.

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JOURNAL OF PALEONTOLOGY, V. 77, NO. 2, 2003 TRITOECHIA MOLLESENSIS new species Figure 5.1–5.13

Tritoechia sp., BENEDETTO, 1994, p. 236, pl. 4, figs. 4–11.

Diagnosis.Medium-sized, subquadrate shells. Ventral valve with moderately high interarea, inclined about 458 with respect to commissural plane. Dorsal valve gently and evenly convex, anteriorly sulcate. Ornamentation subequally parvicostellate, with 16–18 fine costellae per 3 mm. Teeth supported by high and long dental plates diverging anteriorly at 35–408. Ventral muscle field triangular occupying one-third of valve length. Vascula media initially broad and divergent. Vascula genitalia large, pouch-like, radially striated. Notothyrial platform anteriorly free supported by thick, short median septum. Description.Medium size, largest specimens 21 mm wide, commonly 12–15 mm wide, transversely subquadrate, with average length/width ratio of 0.87. Cardinal extremities nearly rightangled; maximum width at hinge or at posterior third. Ventral valve weakly convex in umbonal region becoming planar anteriorly. Ventral interarea moderately high, about one-fourth as high as wide, planar, apsacline, inclined about 458 with respect to the commissural plane. Pseudodeltidium strongly convex, slightly higher than wide. Dorsal valve gently and evenly convex, the maximum thickness at posterior third, with broad and shallow sulcus on the anterior half. Dorsal interarea anacline, about oneeighth as high as wide, with convex chilidial plates completely covering notothyrium. Ornament subequally parvicostellate, with 16–18 fine costellae per 3 mm; concentric growth lines closely and evenly spaced. Ventral interior with plate-like teeth supported by high and long dental plates diverging anteriorly at 35–408. Ventral muscle field triangular occupying one-third of valve length. Diductors elongately triangular with rounded anterior margins, slightly longer than narrow linear adductor scars, which are impressed on low median ridge. Mantle canal system weakly impressed, with broad and divergent proximal vascula media. Vascula genitalia large, pouch-like, radially ridged. Dorsal interior with narrow triangular notothyrial platform free anteriorly, supported by thick, short median septum confined to posterior part of valve; cardinal process simple, ridge-like. Socket ridges strong, moderately long, widely divergent, bounding long, semi conical sockets; brachiophores short, blunt, anteriorly directed, originating at anterior border of socket ridges. Muscle field large, radially striated, extending to near valve midlength and divided longitudinally by thin septum. Etymology.After the Los Molles River, the type section of the Molles Formation. Types.Holotype: Internal mold of ventral valve, CEGH-UNC 18970 (Fig. 5.5). Paratypes: three ventral valves and three dorsal valves with both internal and external molds; eight internal molds of ventral valves and nine internal molds of dorsal valves, CEGHUNC 18969, 18971–18984. Occurrence.Second member of the Molles Formation, levels Mo6, Mo7 and Mo8, Los Molles River section. Discussion.Of North American species of Tritoechia described by Ulrich and Cooper (1936, 1938), T. delicatula appear most similar to the new species T. mollesensis but differs in its more slender and divergent socket ridges and in possessing a broad and flat median ridge in the ventral valve; in addition, ridges bounding the anterior half of the ventral muscle field are nearly parallel to each another. The type species T. typica Ulrich differs in having discontinuous and hollow costellae. The new Famatinian species differs from T. billingsi Neuman, 1976 from the Summerford Group of Newfoundland, in the different morphology of ventral muscle field. The Kazakhstanian species T. tokmakensis Popov et al. (2001) is similar in external morphology

and shape of ventral muscle field, but has somewhat coarser ornamentation and possesses well-developed deposits into the apical region of the delthyrial chamber. T. florentinensis Laurie, 1980 from the Late Tremadoc-Early Arenig of Tasmania is comparable to T. mollesensis in ornamentation and shell proportions but differs in having deeply sulcate dorsal valve, a nearly orthocline ventral interarea and wider ventral adductor scars. Both T. azulensis Benedetto, 1987 and T. inaequicostata Benedetto, 1987 from the Argentine Precordillera can be distinguished from the Famatina specimens by its more strongly inclined and higher ventral interarea, which is approximately twice as wide as high, and in possessing a stronger median dorsal ridge and well differentiated dorsal muscle scars. The Precordilleran species T. gigas Benedetto, 2001a differs in being larger, in its proportionally lower ventral interarea, its coarser ornamentation and its deeply impressed ventral muscle field. The specimens referred to Tritoechia sp. from contemporaneous volcaniclastic rocks of the Chaschuil region in the northernmost part of the Famatina Range (Benedetto, 1994) are certainly conspecific with T. mollesensis. TRITOECHIA sp. Figure 5.14–5.21 Description.Small to medium-sized, ranging from 8 to 21 mm wide; cardinal extremities roundly obtuse to right-angled. Ventral valve hemipyramidal, about 90 percent as long as wide; ventral interarea high, about 37 percent as high as wide, planar, steeply apsacline or catacline, with proportionally broad delthyrium covered by an arched pseudodeltidium pierced apically by small foramen. Dorsal valve gently convex, subrectangular (average length/width ratio: 0.83), non sulcate. Dorsal interarea about 14 percent as high as wide, planar, anacline; notothyrium partially covered by arched chilidial plates. Ornamentation subequally multicostellate, with 11–14 costellae per 3 mm. Ventral interior with small, plate-like teeth supported by long, slightly divergent dental plates bounding laterally a narrow and elongate muscle field extending anteriorly up to 40 percent of valve length. Diductor muscle scars deeply impressed anteriorly, divided by a high, rounded median ridge. Vascula media broad, slightly divergent anteriorly; vascula genitalia suboval, radially striated. Interior of dorsal valve with narrow, triangular, posteriorly inclined notothyrial platform bounded by chilidial plates. Cardinal process simple, blade-like. Socket ridges strong, arched, forming a low angle with the posterior margin, bounding semi conical, transversely elongate dental sockets. High, thick median ridge confined to posterior part of valve, supporting the overhanging anterior margin of the notothyrial platform. Dorsal muscle field broadly triangular, traversed by four to six radially arranged transmuscle septa. Material examined.Five external molds and three internal molds of ventral valves; six internal molds and two external molds of dorsal valves, CEGH-UNC 15902–15916. Occurrence.Upper member of the Suri Formation, level CA, Cachiyuyo River section. Discussion.The rio Cachiyuyo specimens differ from Tritoechia mollesensis in having coarser ornamentation, a proportionally longer and deeply impressed ventral muscle field, and a more pronounced median ridge between the diductor scars. This form, however, is not assigned a formal specific name pending the recovery of additional specimens than would permit a more satisfactory diagnosis. Genus PINATOTOECHIA Benedetto, 2001b Type species.Pinatotoechia acantha BENEDETTO, 2001b.

BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA PINATOTOECHIA ACANTHA Benedetto, 2001b Figure 5.22–5.26 Description.Ventribiconvex shell, up to 12 mm wide. Ventral valve hemipyramidal, almost as wide as long. Lateral flanks evenly and slightly convex; anterior margin uniformly rounded. Ventral interarea one-third as high as wide, planar, strongly apsacline, with high, narrow, strongly arched pseudodeltidium. Dorsal valve subrectangular, slightly convex, with shallow median sulcus. Notothyrium entirely covered by convex chilidium. Ornamentation multicostellate, with 5–6 rounded costellae in 2 mm, each costella bearing a row of hollow, tubular spines ranging in diameter from 0.1 to 0.2 mm. Interior of ventral valve with small teeth supported by high, slightly divergent dental plates. Ventral muscle field subtriangular in outline, slightly longer than wide, strongly raised anteriorly forming a pseudospondylium, extending anteriorly for about onethird of valve length. Adductor tracks with subparallel sides, as wide as, and slightly shorter than diductor scars. Anterior margin of pseudospondylium supported by a short and prominent median ridge extending to about half valve length. Dorsal interior with narrow, slightly concave, posteriorly inclined notothyrial platform bounded laterally by high chilidial bases merging anteriorly with a narrow and high median septum confined to posterior third of the valve. Cardinal process simple, thickened, elongately oval in cross-section. Dental sockets semi conical bounded by strong socket ridges incurved towards posterior margin. Muscle field quadripartite, formed by subequal, elongately oval adductor scars arranged radially and separated by a pair of faint ridges. Material examined.One ventral valve and three dorsal valves with both internal and external molds, one incomplete external mold of dorsal valve, CEGH-UNC 15897–15901. Occurrence.Upper member of the Suri Formation, level CA, Cachiyuyo River section. Discussion.The genus Pinatotoechia was first described from Arenig volcaniclastic rocks of the Puna region, northwestern Argentina (Benedetto, 2001b). The external molds and ventral interiors from the Suri Formation are indistinguishable from the type material of P. acantha from the Vega Pinato locality, western Puna of Argentina. Associated dorsal valves in the Famatinian collection (absent in the type material) confirms that internally Pinatotoechia is closer to Tritoechia than any other representative of the Family Tritoechiidae. Pinatotoechia includes tritoechiids possessing a characteristic spinose ornament formed by simple rows of erect, tubular hollow spines on the top of the costellae. In other respects is very similar to the genus Tritoechia. The only other representative of this family bearing spinose ornament is Acanthotoechia Williams and Curry, 1985 from the Arenig Tourmakeady Limestone of Ireland. The latter genus, however, can be distinguished by having parvicostellate radial ornament and a concave dorsal valve that internally bears a well-defined peripheral rim. Class RHYNCHONELLATA Williams et al., 1996 Order PROTORTHIDA Schuchert and Cooper, 1931 Superfamily SKENIDIOIDEA Kozlowski, 1929 Family SKENIDIIDAE Kozlowski, 1929 Genus SKENIDIOIDES Schuchert and Cooper, 1931 Type species.Skenidioides billingsi SCHUCHERT AND COOPER, 1931. SKENIDIOIDES KAYSERI new species Figure 6.7–6.18 Skenidioides sp., BENEDETTO, 1994, p. 235, pl. 3, figs. 18–20, pl. 4, figs. 1–3.

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Diagnosis.Small, costate Skenidioides with about 14 costae on either flank, two weaker costae along dorsal sulcus, and a stronger, anteriorly dichotomized median costa in ventral valve; bifurcations and intercalations sporadically developed. Spondylium proportionally small, semicircular. Septalium long and narrow. Description.Shell small, 4.7 to 6.0 mm in length, ventribiconvex, semielliptical in outline (average length/width ratio: 0.72), maximum width at hingeline. Cardinal angles acute. Ventral valve hemipyramidal, with rounded carina and slightly curved lateral flanks; ventral interarea planar, proportionally high, about one-third as high as wide, strongly apsacline to catacline; delthyrium completely open. Dorsal valve moderately convex, flanks swollen, median sulcus narrow becomes deeper anteriorly. Dorsal interarea low, anacline. Ornament of rounded, subequal costae numbering 5–6 per mm with a mean of 14 costae on each flank; generally two weaker costae along dorsal sulcus and a coarser, anteriorly dichotomized median costa in ventral valve; bifurcations and intercalations sporadically developed on flanks, especially on anterior third of valve. Intercostal spaces covered by very fine, evenly spaced concentric growth lamellae. Interior of ventral valve with small semicircular spondylium confined to delthyrial chamber, mostly free but supported umbonally by a short septum extending anteriorly for one-sixth to oneeighth of valve length. Teeth small, rounded. Vascula narrow, widely divergent. Dorsal interior with slender erect brachiophores supported by high bases converging medially to form an anteriorly strongly elevated septalium that is supported by a thick ridge. Cardinal process long, blade-like, posteriorly expanded, occupying the entire length of septalium. Dental sockets small, suboval. Etymology.Named in honor of Emanuel Kayser who first described Ordovician fossils from the Sierra de Famatina in 1876. Types.Holotype: Internal mold of ventral valve (Fig. 6.11), CEGH-UNC 15894 from level Mo9, Molles Formation. Paratypes: One internal mold of ventral valve, CEGH-UNC 15802, from the Suri Formation; one exterior of ventral valve and one of dorsal valve; six internal molds of ventral valves; one internal mold of dorsal valve, CEGH-UNC 15889–15892, 15993a, 18894–15896 from the Molles Formation. Occurrence.Upper member of the Suri Formation, Cachiyuyo River section, level CA; upper part of the second member of the Molles Formation, levels Mo8 and Mo9, Los Molles River section. Discussion.The specimens from the Molles Formation are the same as those from coeval volcaniclastic rocks of the Chaschuil area, northern part of the Sierra de Famatina, referred to Skenidioides sp. (Benedetto, 1994). The Welsh specimens assigned by Bates (1968) to Skenidioides sp.1 are similar to the new Famatina species in having a well-developed septum supporting the spondylium, but differ in their coarser ribbing. The type species S. billingsi (Schuchert and Cooper, 1931), from Caradoc beds of Canada, has a comparable ornamentation but can be distinguished from S. kayseri in its larger size, in lacking enlarged median costa in the ventral valve, and in its proportionally wider and shorter septalium. Of the numerous mid-Ordovician species described from North America (Cooper, 1956), the new Famatinian species differs in having comparatively smaller spondylium and narrower septalium. In addition, in the majority of these North American species costae are more numerous and dichotomized distally, especially in the sulcus and over the adjacent regions. Genus PROTOSKENIDIOIDES Williams, 1974 Type species.Protoskenidioides revelata WILLIAMS, 1974.

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BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA PROTOSKENIDIOIDES cf. REVELATA Williams, 1974 Figure 6.1–6.6 Description.Shell very small, largest specimens 2.2 mm wide, ventribiconvex, semielliptical in outline, maximum width at hingeline. Ventral valve moderately and evenly convex, ventral interarea low, planar, about one-tenth as high as wide, apsacline, delthyrium open. Dorsal valve more transverse than ventral, sulcate; dorsal interarea low, linear, anacline. Ornamentation costate, with 5–6 simple rounded costae on either flank; ventral valve with a slightly enlarged median costa. Interior of ventral valve with small, semicircular, unsupported spondylium extended anteriorly for about one-tenth of valve length. Interior of dorsal valve with slender brachiophores pointed distally; notothyrial platform subrhomboidal, gently concave, supporting a long and narrow, ridge-like cardinal process. Dental sockets short, semi conical, defined by laminar fulcral plates. Larger specimens with brachiophore bases convergent onto high median ridge to form a well developed septalium extending anteriorly 35–40 percent of valve length; smaller specimens (about 1.5 mm width) with discrete brachiophore bases not forming a septalium. Material examined.Two internal molds of ventral valves and four internal molds of dorsal valves, CEGH-UNC 15729, 15801, 15877a, 15917, and 20075 from the Suri Formation and 20074 from the Molles Formation. Occurrence.Upper member of the Suri Formation, level CA, Cachiyuyo River section. Second member of the Molles Formation, level Mo7, Los Molles River section. Discussion.The small size of Famatinian specimens and the evidence of discrete brachiophore bases in one very small valve are suggestive of the genus Protoskenidioides Williams, 1974. In our material the septalium becomes fully developed in specimens up to 2 mm in width. In size, outline and ornamentation the adult specimens closely compare with the type species P. revelata Williams, from the Arenig of Shropshire (Wales), but differ in having fewer costae (in some Welsh specimens there are up to 18 costae, cf. Williams, 1974, pl. 14, fig. 11) and a proportionally longer and narrower septalium. The Irish species P. hibernicus Williams and Curry, 1985 shares with the Famatinian specimens a similar number of costae but it can readily be distinguished by its spinose extension on the median ridge and its transversely oval, mucronate shell outline. Genus CROSSISKENIDIUM Williams and Curry, 1985 Type species.Crossiskenidium spinosum WILLIAMS CURRY, 1985.

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CROSSISKENIDIUM? STELZNERI new species Figure 6.19–6.33 Diagnosis.External ornamentation multicostellate, with 7–8 rounded costellae in 1 mm and well-developed, uniformly spaced

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concentric growth lamellae, without spines. Spondylium semi conical, larger than wide. Cardinal process, when present, a very faint ridge. Dorsal median ridge broad and rounded. Description.Small, ventribiconvex shells, largest specimen 4.1 mm wide (3.8 mm in average); maximum width at hinge line, cardinal extremities right angled to slightly acute. Ventral valve strongly and evenly convex, with maximum thickness in posterior third; average length/width ratio of ventral valve 0.80; beak small, rounded, slightly incurved above hingeline. Ventral interarea proportionally high, strongly apsacline; delthyrium open. Dorsal valve moderately convex, sulcate, more transverse than ventral, average length/width ratio 0.66. Dorsal interarea curved, anacline, about one-third as high as ventral; notothyrium open. External ornament multicostellate, with 7–8 rounded costellae in 1 mm; growth lamellae well developed, in some specimens extending as flat-lying frills which form with costellae a reticulate pattern of ornamentation but without true spines. Interior of ventral valve with small, rounded, triangular teeth supported by short plates converging medially to form the lateral margins of a semiconical spondylium which varies from entirely sessile to free anteriorly, about 60 percent as wide as long, extending anteriorly one-third of valve length. Muscle attachment surface transversally striated. Interior of dorsal valve with slightly divergent brachiophores pointed distally; dental sockets small, subelliptical in outline, supported by discrete fulcral plates; brachiophore bases united to median ridge to form a concave septalium extended anteriorly for about 32 percent of valve length. Cardinal process generally absent, in one specimen represented by a very faint ridge. Median ridge broad and rounded, fading at the anterior third of valve. Dorsal muscle field large, formed by subelliptical anterior adductor scars and roundly triangular, smaller posterior adductor scars. Etymology.Named in honor of Alfred Stelzner who first discovered Ordovician fossils in the Sierra de Famatina in 1871. Types.Holotype: a dorsal valve with both external and internal molds, CEGH-UNC 15884, (Fig. 6.19–6.20, 6.30–6.31). Paratypes: one external mold of ventral valve and three external molds of dorsal valves, one ventral valve with both external and internal molds, five internal molds of ventral valves, and four internal molds of dorsal valves, CEGH-UNC 15732, 15733, 15875, 15876, 15877a, 15878–15883, 15885–15887, 15918. Occurrence.Upper member of the Suri Formation, level CA, Cachiyuyo River section. Discussion.The genus Crossiskenidium was erected by Williams and Curry (1985) to include small skenidiids from the Tourmakeady Limestone of Ireland characterized by a spiny lamellose external ornamentation. The Argentine specimens are similar to the Irish species C. spinosum Williams and Curry in shell size and proportions, as well as in most of internal features. Externally, both species are characterized by well-developed lamellose concentric growth lines. In the Famatina specimens, however, lamellae are not fringed with flattened spines, so that it assignation to

← FIGURE 5—1–13, Tritoechia mollesensis n.sp.; 1, incomplete external cast of ventral valve, CEGH-UNC 18984, 31.8; 2, external cast of dorsal valve, CEGH-UNC 18976, 32; 3, incomplete internal mold of ventral valve, CEGH-UNC 18982, 32.5; 4, dorsal view of conjoined valves, latex cast, CEGH-UNC 18977, 32.5; 5, internal mold of ventral valve, holotype, CEGH-UNC 18970, 31.8; 6, internal mold of ventral valve, CEGHUNC 18980, 32.5; 7, internal mold of ventral valve, CEGH-UNC 18978, 32.5; 8, 13, internal mold and cast of dorsal valve, CEGH-UNC 18981, 32.5; 9, internal mold of dorsal valve, CEGH-UNC 18969, 32.5; 10, internal mold of dorsal valve, CEGH-UNC 18983, 31.5; 11, internal mold of dorsal valve, CEGH-UNC 18973, 32.5; 12, internal mold of dorsal valve, CEGH-UNC 18979, 32; 14–21, Tritoechia sp.; 14, 15, 16, external mold, cast, and internal mold of dorsal valve, CEGH-UNC 15906, 34.5; 17, cast of exterior of ventral valve, CEGH-UNC 15908, 32.5; 18, internal mold of ventral valve, CEGH-UNC 15910, 32.5; 19, internal mold of ventral valve, CEGH-UNC 15907, 32.5; 20, internal mold of dorsal valve, CEGH-UNC 15735, 32.5; 21, internal mold of dorsal valve, CEGH-UNC 15902, 32. 22–26, Pinatotoechia acantha; 22, 23, 25, external mold (33.5), cast (33.5) and internal mold (32.5) of ventral valve, CEGH-UNC 15900; 24, external mold of dorsal valve showing hollow spines, CEGH-UNC 15901, 36; 26, internal mold of dorsal valve, CEGH-UNC 15897, 33.

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BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA Crossiskenidium remains in doubt. In this respect C.? stelzneri resembles C.? lamellosum Williams and Curry (1985), but in the latter lamellae extend as prominent peripheral flanges. The Famatinian specimens differ from the related genus Skenidioides in its well-developed concentric lamellae and finer costellae. Order ORTHIDA Schuchert and Cooper, 1932 Suborder ORTHIDINA Schuchert and Cooper, 1932 Superfamily ORTHOIDEA Woodward, 1852 Family ORTHIDAE Woodward, 1852 Genus PARALENORTHIS HavlIĆˇEK AND BRANISA, 1980 Type species.Paralenorthis immitatrix HAVLıĆˇ EK BRANISA, 1980.

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PARALENORTHIS SURIENSIS new species Figure 7.1–7.15 Diagnosis.Small, transversely semielliptical, alate shells with deeply sulcate dorsal valve. Ornament of 14–16 strong, rounded costae, interspaces capillate. Ventral muscle field well impressed, subpentagonal in outline, occupying about 35 percent of valve length. Dorsal valve with triangular, slightly concave notothyrial platform extending anteriorly in a broad rounded median ridge that tapers gradually to disappear at about two-thirds valve length. Description.Shell comparatively small, ranging from 8.5 mm to 12.5 mm in width, ventribiconvex, transversely semielliptical in outline; cardinal extremities acute, often alate. Ventral valve moderately and uniformly convex, thickness ranging from onethird to one-fourth valve length; average length/width ratio: 0.77; ventral interarea proportionally low, apsacline, gently concave; delthyrium open, triangular, slightly wider than long. Dorsal valve gently convex, deeply sulcate, lateral areas uniformly convex becoming flattened toward the ears. Average length/width ratio 0.71. Dorsal interarea planar, steeply anacline, about a half as long as ventral interarea. Ornament costate, with strong, high costae with rounded crests, interspaces deep, of similar width than ribs. Commonly 15–16 costae on mature ventral valves; dorsal valves with 7–8 costae on either flank and two finer ribs in sulcus. Interspaces and ribs finely capillate; concentric growth fila inconspicuous. Ventral valve interior with bluntly triangular teeth bearing deep, small, oval crural fossettes on their inner sides, supported by short and thick dental plates. Delthyrial chamber large, triangular, with a short, well-defined pedicle callus; ventral muscle field moderately impressed, slightly wider than long, subpentagonal in outline, bounded laterally by low, medially convergent ridges occupying about 35 percent of valve length. Diductor muscle scars elongately rectangular, mostly impressed on inner sides of dental plates; adductor scars undifferentiated, located on a flat rectangular median area unbounded anteriorly. Ventral vascular system with broad, strongly divergent vascula media. Dorsal valve interior with triangular, slightly concave notothyrial platform extending anteriorly in a broad rounded median ridge

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that tapers gradually to disappear at about two-thirds of valve length. Cardinal process blade-like, somewhat thickened anteriorly in large specimens. Brachiophores relatively slender, bluntly triangular distally, diverging at about 100–1058, supported by thick bases converging to median ridge. Dental sockets transversely oval to widely divergent, mostly excavated into shell deposits below hinge line. Muscle field weakly impressed. Etymology.From its occurrence in the Suri Formation. Types.Holotype: Internal mold of ventral valve, CEGH-UNC 15715 (Fig. 7.7–7.8). Paratypes: ten external molds of both valves, eight ventral valves with both external and internal molds, one internal mold of ventral valve, two dorsal valves with both external and internal molds and six internal molds of dorsal valves, from the Rıó Cachiyuyo section, level CA, CEGH-UNC 18993–19013; twelve exteriors of both valves, three internal mold of ventral valves and two internal molds of dorsal valves from rıó Saladillo Chico, level QS 22, CEGH-UNC 18985–18992; three ventral valves and two dorsal valves with both external and internal molds from Saladillo Grande River section, level QS 39, CEGH-UNC 19014–19017. Occurrence.Upper member of the Suri Formation, Cachiyuyo River section, level CA; Saladillo Chico River, levels QS22 and QS39. Discussion.The new Famatinian species closely resembles the Arenig Welsh species P. proava (Salter) (Bates, 1968; Jaanusson and Bassett, 1993) from which it differs in shell proportions, shape and details of external ornamentation, being virtually indistinguishable in internal features. According to the measurements provided by Bates (1968), the specimens from the Carmel Formation are slightly less transverse, the dorsal valves having a mean length/width ratio of 0.74 (0.85 in ventral valves), while in the Famatinian specimens that ratio is 0.71 (0.77 in ventral valves). Other differences between them are that P. suriensis has more acute, often auriculate cardinal extremities, and a deeper and sharp dorsal sulcus bearing two instead of four costae. Some prominently alate specimens of P. suriensis are similar in outline to P. alatus (Sowerby), from the Early Arenig of Wales (Bates, 1969), but differ in having fewer ribs (15–17 as a mean, rather than more than 30 in the Welsh form), stronger cardinalia, more prominent median ridge, and deeply impressed dorsal muscle field. P. robustus (Neuman, 1964) from the Shin Brook Formation of Maine, is similar in rib number and proportions but differs in its non-alate outline, in having shallow dorsal sulcus bearing three costae, and in having a proportionally longer ventral muscle field. The type species P. immitatrix Havlıćˇek and Branisa, 1980, from Arenig beds of Bolivia, differs in having right-angled or slightly acute cardinal extremities, shallower dorsal sulcus, finer ornamentation and a comparatively shorter notothyrial platform. The Argentine species P. altiplanicus Benedetto, 1998a, is readily distinguished by its shorter and wider ventral muscle field and its

← FIGURE 6—1–6, Protoskenidioides cf. revelata; 1, internal mold of ventral valve, CEGH-UNC 15801, 316; 2, internal mold of ventral valve, CEGHUNC 15917, 316; 3, internal mold of dorsal valve, CEGH-UNC 20074, 313; 4, internal mold of dorsal valve, CEGH-UNC 20075, 315; 5, internal mold of dorsal valve, CEGH-UNC 15729, 310; 6, internal mold of dorsal valve, CEGH-UNC 15877b, 312. 7–18, Skenidioides kayseri n. sp.; 7, 8, external mold and cast of ventral valve, CEGH-UNC 15895a, 36; 9, internal mold of ventral valve, CEGH-UNC 15895b, 36; 10, internal mold of ventral valve, CEGH-UNC15889, 37; 11, 12, internal mold and cast of ventral valve, holotype, CEGH-UNC 15894, 39; 13, internal mold of ventral valve, CEGH-UNC 15893a, 37; 14, internal mold of dorsal valve, CEGH-UNC 15890, 310; 15, 16, internal mold (36) and cast of dorsal valve (38), CEGH-UNC 15892; 17, internal mold of ventral valve, CEGH-UNC 15891, 36; 18, internal mold of ventral valve, CEGH-UNC 15802, 312. 19–33, Crossiskenidium? stelzneri n. sp.; 19–20, 30–31, dorsal valve, holotype CEGH-UNC 15884, external mold and cast, and internal mold and cast, all 39; 21, Cast of dorsal exterior, CEGH-UNC 15887, 310; 22, external mold of dorsal valve, CEGH-UNC 15877, 310; 23, external mold of dorsal valve, CEGH-UNC 15888, 38; 24, cast of ventral exterior, CEGH-UNC 15918, 310; 25, internal mold of ventral valve, CEGH-UNC 15918, 310; 26, 27, internal mold and cast of ventral valve, CEGH-UNC 15883, 38; 28, internal mold of ventral valve, CEGH-UNC 16885, 310; 29, internal mold of ventral valve, CEGH-UNC 15877a, 312; 32, internal mold of dorsal valve, CEGH-UNC 15882, 310; 33, internal mold of dorsal valve, CEGH-UNC 15880, 315.

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BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA much smaller notothyrial platform. The slightly younger Famatinian species P. riojanus (Levy and Nullo, 1973) from the overlying Molles Formation, differs from P. suriensis in being larger and in having up to 25 costae, but especially in the proportions of ventral muscle field, which becomes unusually longer and deeply impressed in mature individuals. PARALENORTHIS

(Levy and Nullo, 1973) Figure 7.16–7.27

RIOJANUS

Orthis caligramma Dalman. KAYSER, 1876, pl. 3, figs. 12–18. Orthambonites riojanus LEVY AND NULLO, 1973, p. 142, la´m. 1, figs. 1– 10. Orthambonites mollesensis LEVY AND NULLO, 1973, p. 174, la´m. 1, figs. 11–19. Orthambonites mollesensis Levy and Nullo. ACENÕLAZA AND TOSELLI, 1977, p. 67, la´m. 1, figs. 8–10. Paralenorthis riojanus (Levy and Nullo). BENEDETTO, 1994, p. 228, la´m. 2, figs. 1–17.

Material examined.Five conjoined valves and 40 external and internal molds of ventral and dorsal valves, CEGH-UNC 15933–15950, 18952–18968, 19014–19017 (level Mo2) and one internal mold of ventral valve, CEGH-UNC 18967 (level Mo6). Occurrence.Molles Formation, Los Molles River section, levels Mo2 and Mo6. Remarks.This large and distinctive species of Paralenorthis, which forms monospecific assemblages in the lower third of the Molles Formation, was originally referred by Kayser (1876) to Orthis caligramma Dalman. Later Levy and Nullo (1973) assigned this material to the species Orthambonites riojanus and O. mollesensis, which were considered conspecific by Herrera and Benedetto (1989) and reassigned to the genus Paralenorthis, the specific name riojanus being retained. On the basis of a large collection of well-preserved specimens of P. riojanus from coeval volcaniclastic rocks exposed in the Chaschuil area, the author redescribed and illustrated the external and internal features of this species (Benedetto, 1994). Examination of some exceptionally well-preserved specimens of P. riojanus from the Molles Formation adds the following new morphological information: 1) The total number of costellae ranges from 15 to 25 in largest shells; 2) in the interspaces there are fine capillae, numbering 4–7; growth fila are well developed over the costellae, especially on the anterior third of the valve; 3) a well-developed triangular pedicle callus is always present; 4) with growth the ventral muscle field becomes unusually longer and deeply impressed; 5) during the ontogeny the cardinal process varies from a slender ridge to a high, robust, anteriorly thickened ridge, in some specimens erected on an oval, enlarged base; 6) the mantle canal system is saccate; in the dorsal valve the vascula media are constituted by broad, strongly divergent external pair of trunks, and a weakly divergent pair of internal vascula media; in both valves the follicular embayments are strongly sculptured and comparatively large, occupying most of posterolateral areas of valves.

PARALENORTHIS

RIOJANUS BREVIS

223 new subspecies

Figure 8.1–8.14 Diagnosis.Medium-sized, semielliptical to subcircular shells with cardinal extremities right-angled or slightly acute. Costate ornament with 18–23 rounded costae. Ventral valve interior with short, thickened dental plates, a proportionally short and narrow, triangular muscle field extending anteriorly 20–25 percent of valve length. Strongly sculptured vascula genitalia. Dorsal muscle field with ovate to subrectangular posterior adductor scars, larger than the subtriangular anterior ones. Description.Medium-sized, up to 18 mm wide, semielliptical to semicircular in outline (average length/width ratio 0.86 in ventral valve and 0.81 in dorsal valve), ventribiconvex, with maximum width at the hinge; cardinal extremities right-angled or slightly acute; anterior margin uniformly rounded. Ventral valve moderately and evenly convex in lateral and anterior views. Ventral interarea planar, slightly concave below umbo, about onefourth as high as wide, apsacline. Dorsal valve gently convex, faintly sulcate. Costate ornament with 18–23 rounded costae and equally rounded, finely capillate interspaces crossed by fine, evenly spaced concentric growth fila. Ventral valve interior with large, triangular teeth and short, thickened dental plates. Muscle field proportionally short and narrow, triangular in outline, in most specimens 20–25 percent of valve length. Adductor scars long and narrow, as long as the centrally located subrectangular adductor field. In some specimens the ventral muscle field terminates in a rounded callosity. Vascula media broad, well-impressed, proximally subparallel or slightly convergent becoming widely divergent anteriorly. Vascula genitalia subovate with strongly sculptured genital areas. Dorsal valve interior with strong, rounded brachiophores supported by thick, short bases converging to a narrowly triangular notothyrial platform extending anteriorly into a broad, rounded median ridge tapering to disappear slightly beyond the valve midlength. Cardinal process ridge-like, usually somewhat thickened anteriorly, becoming bulbous in some specimens. Sockets deep, suboval in outline. Muscle field usually well impressed; posterior adductors ovate to subrectangular, larger and more widely separated each another than the subtriangular anterior adductors. Etymology.Refers to the proportionally short ventral muscle field. Types.Holotype: An internal mold of dorsal valve, CEGHUNC 19018, from level Mo9 of the Los Molles River section (Fig. 8.10, 8.11). Paratypes: Forty-eight external and internal molds of both valves, CEGH-UNC 19019–19066. Occurrence.All material is from the second member of the Molles Formation, Los Molles River section, levels Mo6, Mo7, Mo8 and Mo9. Discussion.This specimens have in common with P. riojanus (Levy and Nullo), from immediately underlying beds of same formation, a similar number of costae and large and strongly

← FIGURE 7—1–15, Paralenorthis suriensis n. sp.; 1, cast of ventral exterior, CEGH-UNC 18989b, 33; 2, exterior of ventral valve, CEGH-UNC 18992, 33; 3, exterior of dorsal valve, CEGH-UNC 19008, 33; 4, exterior of dorsal valve, CEGH-UNC 18980, 33; 5, detail of ornamentation showing capillae, dorsal valve, CEGH-UNC 18989a, 39; 6, cast of conjoined specimen, dorsal view, CEGH-UNC 15712, 33; 7, 8, internal mold and cast of ventral valve, holotype, CEGH-UNC 15715, 33; 9, internal mold of ventral valve, CEGH-UNC 18996, 33; 10, internal mold of ventral valve, CEGH-UNC 18993, 32.5; 11, 12, internal mold of dorsal valve (34) and detail of cardinalia (36), CEGH-UNC 18988; 13, internal mold of dorsal valve, CEGH-UNC 19011, 33; 14, 15, internal mold and cast of dorsal valve, CEGH-UNC 18996, 33. 16–27, Paralenorthis riojanus; 16, internal mold of ventral valve, CEGH-UNC 18956, 32; 17, cast of dorsal exterior, CEGH-UNC 15949, 32; 18, internal mold of ventral valve, CEGH-UNC 15939, 32; 19, cast of ventral exterior, CEGH-UNC 18966, 32; 20, 21, internal mold and cast of ventral valve, CEGH-UNC 18961a, 31.6; 22, internal mold of ventral valve, CEGH-UNC 18959, 31.6; 23, internal mold of dorsal valve, CEGH-UNC 18961b, 32; 24, internal mold of dorsal valve, CEGH-UNC18967, 32; 25, internal mold of dorsal valve, CEGH-UNC 18961c, 32; 26, cast of dorsal interior, CEGH-UNC 18961d, 32; 27, internal mold of dorsal valve, CEGH-UNC 15939b, 32.

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BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA sculptured genital areas, but differ in its smaller size, and in having much shorter and shallow ventral muscle field and lower interareas than in P. riojanus. Although is difficult to assess to what extent such differences correspond to a specific or subspecific level, the latter is preferred taking into account that similarities between riojanus and brevis are greater than any other species. P. brevis is always found above the last occurrence of P. riojanus, with the exception in the level Mo6 (the lowermost occurrence of P. brevis) in which both forms coexist. Similarities in some external and internal features suggest a phylogenetic relationship between the subspecies riojanus and brevis. The new subspecies P. riojanus brevis differs from P. suriensis in its non-alate, less transverse shell outline, and its somewhat finer ornamentation. Internally can be differentiated in having proportionally smaller ventral muscle field. Genus SURIORTHIS new genus Type species.Suriorthis depressus new species. Diagnosis.Shell small, dorsibiconvex, alate; dorsal valve sharply sulcate. Ornamentation costate to sparsely costellate, with subangular ribs lacking fila and capillae. Ventral muscle field short, triangular, with rounded or straight anterior margin, situated on callus. Vascula media strongly divergent. Notothyrial platform continued anteriorly into a thick rounded median ridge reaching anterior margin. Cardinal process simple, blade-like. Dorsal muscle field with rounded posterior adductor scars and larger, suboval anterior scars. Etymology.Named for its occurrence in the Suri Formation. Discussion.The small and triangular notothyrial platform bearing a ridge-like cardinal process, the rod-like brachiophores and the ventral muscle field confined to the delthyrial cavity indicate that the new genus is a representative of the Orthidae. Hesperonomiella, which is included here among the Orthidae following the argumentation of Laurie (1991), is probably most closely related to Suriorthis than any other member of this family. Suriorthis resembles Hesperonomiella in shell outline, non-capillate ornament, shape of ventral muscle field and cardinalia structure, but differs in its flattened ventral valve and its coarsely and sparsely costellate ornament. Orthis, the type genus of the family, can be distinguished from Suriorthis by its planoconvex profile, whereas Orthambonites, Paralenorthis, Sivorthis and Sinorthis differ in having typically ventribiconvex shells. Although some species of Paralenorthis, such as P. orbicularis (Pander) and P. alatus J. de C. Sowerby, approximate to Suriorthis in having alate cardinal extremities, they differs externally in having typically costate ornament, lower and less inclined ventral interarea, and evenly convex rather than nearly planar ventral valve. Furthermore, the non-capillate and non-filate external ornament of Suriorthis serves to distinguish it from Paralenorthis. The straight and strongly divergent vascula media tracks of Suriorthis are similar to Paralenorthis and Sinorthis and contrast with the subparallel course of the proximal portions of the vascula media in Orthis, Orthambonites and Sivorthis. In summary, the Famatina

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specimens display a combination of external and internal features that have not been previously recorded in other members of the Orthidae, justifying the erection of a new genus. SURIORTHIS DEPRESSUS new species Figure 8.15–8.30 Diagnosis.As for the genus. Description.Small (width ranges from 4.4 mm to 12.3 mm; average 7.8 mm), dorsibiconvex, transversally semielliptical in outline (average length/width ratio 0.53), widest at hinge line, with alate cardinal extremities at all growth stages. Ventral valve nearly planar in small specimens becoming weakly convex in adults, maximum convexity at posterior third, alae flattened or slightly concave. Ventral interarea planar, proportionally high, about one-sixth valve length, strongly apsacline to almost catacline; delthyrium open, equilateral. Dorsal valve gently and evenly convex, with sharp median sulcus; dorsal interarea anacline, planar, shorter than ventral. Ornamentation varying from costate to sparsely costellate, with 28–34 high subangular costae and angular interspaces, numbering 7–8 per 2 mm, becoming finer and crowded toward alae; most of costae simple, some of them bifurcate at valve midlength; in ventral valve a median costa slightly stronger than laterals; in dorsal valve, two, occasionally four, thinner costae in sulcus. External surface non capillate. Ventral interior with small teeth bluntly triangular in outline, supported by short, receding, subvertical dental plates. Ventral muscle field triangular with rounded or nearly straight anterior margin, situated on a low platform, extending anteriorly for about one-third valve length, bounded laterally by dental plates and their anterior prolongations as low ridges. Adductor and diductor scars undifferentiated. Pedicle callus small. Vascula media straight, divergent anterolaterally and distally bifurcate; vascula genitalia radially striated. Dorsal interior with triangular, slightly concave notothyrial platform, moderately raised above the valve floor, occupying onefifth to one-sixth of valve length and extending anteriorly into a thick, rounded median ridge reaching anterior margin. Cardinal process simple, blade-like. Brachiophores bluntly triangular, diverging anteriorly at about 808 to one another; sockets deep, suboval, excavated below the interarea. Muscle field weakly impressed in small individuals becoming strongly impressed with growth, with rounded posterior adductor scars and larger, suboval anterior scars. Etymology.Named for its plane to gently convex ventral valve. Types.Holotype: A ventral valve with both external and internal molds (Fig. 8.16, 8.16), CEGH-UNC 15758. Paratypes: 12 external molds of both valves, eight ventral valves with both external and internal molds, one internal mold of ventral valve, two dorsal valves with both external and internal molds, and six internal molds of dorsal valves, CEGH-UNC 15734, 15735, 15759– 15768, 15801–15803, 15884b, 15919–15932.

← FIGURE 8—1–14, Paralenorthis riojanus brevis n. subsp.; 1, mold of ventral exterior, CEGH-UNC 19029, 32.5; 2, cast of ventral exterior, CEGHUNC 19022, 32.5; 3, cast of ventral exterior, CEGH-UNC 19021, 32; 4, cast of dorsal exterior, CEGH-UNC 19041, 32; 5, internal mold of ventral valve, CEGH-UNC 19024, 32.5; 6, internal mold of ventral valve, CEGH-UNC 19026, 32; 7, internal mold of ventral valve, CEGHUNC 19030, 33; 8, internal mold of ventral valve, CEGH-UNC 19019, 32.5; 9, internal mold of ventral valve, CEGH-UNC 19028, 33; 10, 11, cast and internal mold of dorsal valve, holotype, CEGH-UNC 19018, 32; 12, internal mold of dorsal valve, CEGH-UNC 19027, 32.5; 13, internal mold of dorsal valve, CEGH-UNC 19023, 32.5; 14, internal mold of dorsal valve, CEGH-UNC 19020, 34. 15–30, Suriorthis depressus n. gen. and sp.; 15, 16, ventral valve, exterior cast and internal mold, holotype, CEGH-UNC 15758, 34; 17, 24, ventral valve, exterior cast and internal mold, CEGH-UNC 15919, 36; 18, cast of dorsal exterior, CEGH-UNC 15921, 35; 19, 20, cast of ventral exterior (36) and ventral internal mold (35), CEGH-UNC 15924, 35; 21, cast of dorsal exterior, CEGH-UNC 15920, 35; 22, internal mold of dorsal valve, CEGH-UNC 15768, 36; 23, 25, cast and internal mold of ventral valve, CEGH-UNC 19801, 35; 26, internal mold of dorsal valve, CEGH-UNC 15929, 35; 27, 28, dorsal internal mold and cast, CEGH-UNC 15762, 35; 29, 30, dorsal internal mold (36) and cast showing cardinalia (310), CEGH-UNC 15922.

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Occurrence.Upper member of the Suri Formation, level CA, Cachiyuyo River section. Genus HESPERONOMIELLA Ulrich and Cooper, 1936 Type species.Protorthis porcias WALCOTT, 1924. Remarks.The genus Hesperonomiella differs from typical members of the Family Hesperonomiidae in having multicostellate rather than unequally parvicostellate ornament and in lacking notothyrial ridges or chilidial plates fused posteriorly with the cardinal process. In addition, it has a shorter ventral muscle field and a saccate vascular system. On the basis of these features Laurie (1991) call in question its inclusion within the hesperonomiids and suggests closer relationships with the Orthidae. Of the six genera assigned to the hesperonomiids by Williams and Harper (2000) it seems that, besides the type genus, only Murjukiana Severgina, Protohesperonomia Williams and Curry, and the new genus Mollesella described below can be confidently included in this family. HESPERONOMIELLA ARCUATA new species Figure 9.13–9.30 Diagnosis.Small to medium-sized, semielliptical, ventribiconvex, with cardinal extremities alate, anterior commissure unisulcate. Ventral valve moderately to strongly convex, arcuate; dorsal valve deeply sulcate, median sulcus deflecting ventrally near anterior margin. Ornamentation evenly multicostellate, with 8–10 rounded costellae per 2 mm. Ventral muscle field short, rounded anteriorly, occupying one-fourth to one-fifth valve length. Ventral vascula media widely divergent. Description.Small to medium-sized shell ranging from 5.3 mm to 14.8 mm in width, commonly 6–8 mm wide, semielliptical (average length/width ratio 0.58), ventribiconvex, with cardinal extremities acute, often auriculate or mucronate. Anterior commissure unisulcate. Ventral valve moderately to strongly convex, arcuate, with prominently elevated medial portion and flattened to gently concave slopes. Ventral interarea proportionally low, planar, apsacline; delthyrium open, unmodified. Dorsal valve moderately convex with a deep, initially subangular median sulcus that widens and becomes rounded anteriorly, deflecting ventrally near anterior margin. Dorsal interarea lower than ventral, anacline, notothyrium open, unmodified. Ornamentation evenly multicostellate, with 8–10 rounded costellae per 2 mm; costellae finer toward cardinal extremities. Ventral interior with bluntly triangular teeth bearing well-defined oval crural fossettes on their inner sides, supported by short, slightly divergent dental plates that laterally bound muscle field, which is short, confined to delthyrial cavity, occupying one-fourth to one-fifth valve length, subtriangular in outline and with rounded anterior margin often defined by a low ridge. Diductor scars narrow located on sides of delthyrial cavity, adductor scars occupying a wider median area. Vascula media originate at the end of diductors and widely divergent anteriorly. Genital areas large, radially ridged. Dorsal interior with moderately raised, triangular notothyrial platform bearing a long, ridge-like cardinal process. Brachiophores blade-like, blunt, supported by secondary shell, divergent anteriorly at about 1008 to one another, flanking deep, sockets. Dorsal muscle field quadripartite, generally extending up to the valve midlength and divided longitudinally by a broad, rounded median ridge; anterior adductor scars roundly triangular, equal or slightly larger than suboval posterior scars. Etymology.From the arcuate profile of ventral valve. Types.Holotype: A dorsal valve with both internal and external molds, CEGH-UNC 15759 (Fig. 9.15, 9.27–9.28) from the Cachiyuyo River section, level CA. Paratypes: 26 ventral and dorsal valves CEGH-UNC 15726–15731, 15737–15754, 15761–

15762 from the same precedence as the holotype, and several complete and fragmentary exteriors of both valves, eight internal molds of ventral valves, and seven of dorsal valves from the Saladillo Chico River section, CEGH-UNC 15734–15736 from level QS38 and CEGH-UNC 19077–19088 from level QS37. Occurrence.Upper member of the Suri Formation, level CA of the Cachiyuyo River section, and levels QS 37 and QS 38 of the Saladillo Chico River section. Discussion.In shell size and general appearance the Suri Formation specimens resemble the type species H. porcias (Walcott) but differ in having alate rather than roundly acute cardinal extremities, a more convex ventral valve profile and a slightly coarser ornamentation (4–5 costellae per mm instead of seven in the North American species). H. quebecensis Cooper, 1956, from the Mystic Conglomerate of Quebec, differs in its non-mucronate shell outline, less convex dorsal valve and coarser ornamentation. This latter feature also serves to distinguish the Famatinian species from both H. kuznetskiana Severgina, 1967 and H. helanshanensis Fu, 1982. The Welsh species H. carmelensis Bates, 1968 differs from H. arcuata in its subquadrate shell outline and its unequally multicostellate ornament. Hesperonomiella sp. from the Treiorwerth Formation of Anglesey (Neuman and Bates, 1978), is distinguished by its larger shell and proportionally wider ventral muscle field. The Tasmanian species H. jurikae Laurie, 1991 is similar in ornamentation but differs in its higher ventral interarea, non-alate outline, weaker dorsal sulcus, lower median dorsal ridge and proportionally longer ventral muscle field. The new Famatinian species most closely resembles H. sulcata Benedetto, 2001a, from contemporaneous limestones of the Argentine Precordillera, especially in its acute, often mucronate cardinal extremities and well-developed dorsal sulcus. The Precordilleran species, however, can be differentiated by its coarser ornamentation, anteriorly thickened cardinal process, deeply impressed dorsal muscle field and broader and flattened dorsal median ridge. Genus MONORTHIS BATES, 1968 Type species.Monorthis typis BATES, 1968. MONORTHIS TRANSVERSA new species Figure 9.1–9.12 Orthis vespertilio J. de C. SOWERBY KAYSER, 1876, pl. 3, figs. 22–23 (non Sowerby, 1839). Hesperonomia sp., ACENÕLAZA AND TOSELLI, 1977, lam. 1, fig. 7. Monorthis aff. menapiae (Davidson). BENEDETTO, 1994, p. 227, lam. 1, figs. 1–15.

Diagnosis.Shell convexoplane, medium sized, strongly transverse with cardinal extremities mucronate in early growth stages, acute in larger specimens. Ventral valve with narrow angular carina. Dorsal valve moderately convex with deep, angular median sulcus. Ornamentation multicostellate with 7–8 costellae in 2 mm. Ventral muscle field subtriangular, narrow. Cardinal process ridgelike, slightly swollen anteriorly. Dorsal median ridge high arising in front of notothyrial platform and extending anteriorly to near the valve midlength. Description.Shell medium-sized (maximum width 15 mm), convexoplane in profile, strongly transverse, (average length/ width ratio: 0.38), alate, cardinal extremities mucronate in early growth stages, acute in larger specimens. Ventral valve planar with narrow, carinate median fold reaching the anterior margin; lateral slopes generally flat, slightly concave in some specimens. Ventral interarea low, planar, strongly apsacline to almost catacline. Delthyrium wide, open. Dorsal valve moderately convex with deep, angular median sulcus. Dorsal interarea very low, anacline or orthocline. Ornamentation multicostellate with 7–8 rounded costellae per 2 mm, increasing mainly by bifurcation in ventral valve and interpolation in dorsal valve.

BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA Ventral interior with small teeth supported by short, robust, widely divergent dental plates. Ventral muscle field confined to delthyrial cavity, proportionally narrow, triangular in outline, anterior margin rounded, extending anteriorly for about one-fourth of valve length. Diductor scars as wide as the central adductor field. Ventral mantle canal system indistinct. Dorsal interior with moderately high, triangular notothyrial platform bearing a ridge-like cardinal process which in some specimens becomes slightly swollen anteriorly; brachiophores slender, rod like, diverging anteriorly at about 908, bounding deep sockets. High, rounded median ridge arising in front of notothyrial platform and extending anteriorly to near the valve midlength. Dorsal muscle field and mantle canal system indistinct. Etymology.Refers to the transverse shell outline. Types.Holotype: A ventral valve with both internal and external molds, CEGH-UNC 19098 (fig. 9.1, 9.2) from level Mo6. Paratypes: three external molds, five ventral internal molds and one dorsal mold, CEGH-UNC 19089–19097 from level Mo3; nine external molds, one ventral internal mold, and five dorsal internal molds, CEGH-UNC 19621–19630 from level Mo6; one ventral internal mold and one dorsal internal mold, CEGH-UNC 19099–19100, from level Mo7; three external molds, one ventral internal mold, and three dorsal internal molds, CEGH-UNC 19631–19635 from level Mo9. Occurrence.Second member (levels Mo3, Mo6, Mo7, Mo9) of the Molles Formation, Los Molles River section. Discussion.Bates (1968) erected Monorthis based on specimens from the Arenig Treiorwerth Formation of Wales. The type material of Monorthis typis, although showing the diagnostic features of the genus, is too poorly preserved to make a detailed description of the ornamentation, teeth and muscle fields. The other Welsh species, M. menapiae (Davidson) (Bates, 1969), from the Ogof Heˆn Formation, Ramsey Island, is represented by better preserved internal and external molds, but knowledge of shell proportions and outline is difficult as the specimens are all distorted. The presence of Monorthis in the central Famatina Range was first documented by Kayser in its 1876’s monograph, who described and figured isolated specimens assigned to Orthis vespertilio Sowerby. Extensive collections made by the author in the Chaschuil area revealed that Monorthis is a fairly common component of Famatinian faunas. The Chaschuil specimens, formerly referred to M. aff. menapiae (Davidson) (Benedetto, 1994), are undoubtedly conspecific with those from the Molles Formation described here. Material from both collections seems to be closely related to the Ramsey Island species in features independent of deformation such as costellae density, cardinalia and muscle fields morphology, but distortion makes very difficult to compare the shape, outline and proportions of shells. For these reasons, and until new best preserved material is described, I recommend restricting the name M. menapiae to the type material. The Famatina specimens are assigned herein to a new species on the basis of their transverse outline and long, auriculate cardinal extremities. These serve to distinguish it from the Precordilleran species M. cumillangoensis Benedetto, 2001b, that has a larger subrectangular shell. Internally, the Precordilleran species differs in having a proportionally larger and wider ventral muscle field and more robust cardinalia. Monorthis transversa also differs from M. coloradoensis Benedetto, 1998a, from the Sepulturas Formation of northwestern Argentina, in its transverse rather than subrectangular shell outline, its smaller subtriangular ventral muscle field and in lacking peripheral thickening in the ventral valve interior. Family PRODUCTORTHIDAE Schuchert and Cooper, 1931 Genus PANDERINA Schuchert and Cooper, 1931 Type species.Productus abscissus PANDER, 1830.

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PANDERINA? AMBIGUA new species Figure 10.1–10.17 Diagnosis.Medium-sized, alate, subequally biconvex shells with deflected anterior margins in later growth stages. Notothyrium partially closed by a chilidium. Ramicostellate ornament with coarse angular primary ribs and additional costellae forming well defined bundles, numbering 30–35 costellae along anterior margin. Growth lamellae prominent. Ventral muscle field triangular, becoming progressively larger and deeply impressed with growth. Cardinal process ridge-like, gradually thickening with growth. Dorsal muscle field large, subquadrate, divided longitudinally by broad, rounded ridge, with posterior adductor scars much smaller than the subrectangular anterior ones. Description.Medium-sized, transversely elliptical in outline, subequally biconvex or ventribiconvex, with alate cardinal extremities. Shell more convex anteriorly, the maximum thickness located beyond the midlength. Anterior commissure rectimarginate. Ventral valve moderately convex, carinate, becoming relatively flat toward the cardinal extremities; anterior margin deflected dorsally in later growth stages. Small, pointed beak not incurved over the low, apsacline ventral interarea; delthyrium wide and open. Dorsal valve initially planar or gently convex, increasing the convexity in later growth stages, with narrow, angular median sulcus fading anteriorly. Dorsal interarea lower than ventral, anacline; notothyrium partially covered by chilidial plates. Ornamentation ramicostellate, in some specimens costellae forming well defined bundles. Primary ribs coarse, angular, separated by angular interspaces, usually numbering 6–8 on dorsal valve (3–4 on either side) and 7–9 on ventral valve; in the latter a slightly stronger angular median rib forms the ventral carina; new finer costellae originate by lateral branching but intercalations sporadically present. Bifurcations start at one third of valve length so that small specimens tend to have few simple coarse costae dichotomized distally, whereas in full-grown specimens splitting is more evident, originating 30–35 costellae along anterior margin; radial ornament becomes gradually weaker towards ears. Growth lamellae strong and regularly spaced, more accentuated and crowded near margins. Ventral interior with small teeth bearing oval crural fossettes in their inner sides, supported by short dental plates almost completely masked by deposits infilling lateral apical chambers. Delthyrial chamber deep, concave. Ventral muscle field triangular, with rounded front margin; in young specimens small (about onefourth of valve length), weakly impressed, unbounded anteriorly, becoming progressively larger (occupying up to 40 percent of valve length) and deeply impressed with growth; in medium-sized and large individuals muscle field situated on a slightly elevated platform simulating a pseudospondylium; in one gerontic individual this platform is strongly thickened anteriorly to form an incipient premuscular callus. Diductor scars linear; adductor scars occupying a wide, slightly raised central area. Vascula media narrow, strongly divergent antero-laterally. Dorsal interior with erect, ridge-like cardinal process increasingly thickened with growth, flanked by well-defined chilidial plates and extended anteriorly with thick brachiophores diverging anteriorly at 110–1208. Notothyrial platform triangular in outline, proportionally short, extending one-fifth–one-sixth of valve length, varying with ontogeny from low to moderately elevated. Sockets large, suboval. Dorsal muscle field large, subquadrate in outline, slightly longer than wide, reaching about the valve midlength and bisected longitudinally by a broad rounded ridge. Posterior adductor scars roundly triangular or transversely oval, much smaller than subrectangular anterior adductor scars, which in some large individuals are surrounded by antero-lateral ridges. Pallial markings not preserved.

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BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA Etymology.Refers to the ambiguous morphology. Types.Holotype: A ventral valve with both internal and external molds, CEGH-UNC 20066 (Fig. 10.6–10.7, 10.11), from level S2 of the Saladillo Grande section. Paratypes: Thirty-eight external and internal molds of both valves and three conjoined specimens CEGH-UNC 16653–15672, 20046–20065 from level CA, Cachiyuyo River section; eight specimens from the Saladillo Grande River, CEGH-UNC 20067–20073. Occurrence.Upper member of the Suri Formation, Cachiyuyo River section (level CA) and Saladillo Grande River section (level S2). Discussion.During ontogeny the Famatina specimens display morphological variations affecting some features considered taxonomically significant at generic level within the family Productorthidae, such as the degree of branching of primary costae, thickening of cardinal process and degree of impression of ventral muscle field. On the other hand, adult shells show a particular combination of diagnostic features of different genera of productorthids, making their classification quite problematical. The strongly lamellose ornamentation and the presence of a chilidium approximate our specimens to Productorthis, but its well delimited ventral muscle field simulating a pseudospondylium is more like that of Panderina. In the latter genus concentric lamellae are few and sparse, especially in the type species Panderina abcissa (Pander), but in P. tetragona (Pander) lamellae are restricted to the periphery of the valves, and in P. pakriensis Rubel are almost absent (Rubel, 1961). The simple, thickened-ridge cardinal process of the Famatina specimens is similar to that of Prantlina but also reminiscent of certain species of Panderina such as P. lata (Pander) and P. pakriensis Rubel. In Panderina, however, the cardinal process tends to be more prominent and bulbous, like in Productorthis. The small to medium-sized specimens of the Famatina sample are reminiscent of Nicolella, especially in having an unbounded and weakly impressed ventral muscle field and broad, distally dichotomized primary costellae. With growth, however, the muscle field becomes more strongly impressed, an incipient pseudodeltidium gradually develops, and the ornament becomes costellate, such a combination of features rending questionable its assignment to Nicolella. The Bohemian genus Ferrax (Havlıćˇek, 1977; Mergl, 1991) shares with the Famatina species a well-developed chilidium, but differs in having sparsely lamellose ornamentation, straight anterior margin of ventral muscle field and ridge-like, not thickened cardinal process. In shape, ornamentation and internal features of ventral valve our specimens compare with Styxorthis, from the Arenig of the Czech Republic, but in this genus, according to Mergl (1991), the cardinal process is trilobed. On balance, the combination of a lamellose ornamentation, a well-delimited and commonly not strongly elevated ventral muscle field, the absence of a premuscular callus and the simple,

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thickened cardinal process is more suggestive of the genus Panderina, to which the Famatinian material is questionably assigned. A single dorsal valve of a Productorthid collected from the Acoite Formation of northwestern Argentina (Benedetto, 1998a) is probably conspecific with P.? ambigua. Genus PRODUCTORTHIS Kowlowski, 1927 Type species.Productus obtusus PANDER, 1830. PRODUCTORTHIS ANGULENSIS new species Figure 10.17–10.29 Diagnosis.Medium-sized ventribiconvex shells with alate cardinal extremities. Ornament of subangular costellae numbering 5–6 per 2 mm. Ventral muscle field subpentagonal to subrectangular, bounded anteriorly by a prominent boss-like callus. Cardinal process gradually thickening with growth, becoming a bulbous ridge in larger specimens. Dorsal muscle field quadripartite, subrectangular, extending anteriorly to valve midlength; anterior adductor scars suboval in outline with indented anterior margin, larger than the narrowly rectangular posterior ones. Description.Medium-sized, largest specimens 14 mm wide, generally 10–12 mm in width, strongly ventribiconvex. Outline strongly transverse in small shells (length/width ratio ranging from 0.52 to 0.65) becoming progressively equidimensional because of allometric growth (fig. 10.18, 10.20), large shells being slightly wider than long (average length/width ratio 0.86). Maximum width at hinge. Cardinal extremities alate; anterior margin uniformly rounded. Ventral valve initially moderately convex becoming strongly convex in larger specimens, about 30–40 percent as deep as long, with maximum height at the posterior third of valve; lateral profile uniformly curved with minute convex umbo. Ventral interarea apsacline, relatively low, slightly curved and transversally striated; delthyrium wide and open. Dorsal valve gently and evenly convex, shallowly sulcate; umbo inconspicuous; dorsal interarea linear, about one-fourth as high as ventral, anacline. Notothyrium partially closed by small chilidium. Radial ornamentation ramicostellate with subangular costellae increasing in number by interpolation and principally by branching forming well defined bundles, numbering 5–6 costellae per 2 mm counted on anterior third of valve, with a total number of 38–42 costellae along anterior margin. Regularly spaced concentric growth lamellae over entire external surface, becoming more prominent and closely spaced near margins. Ventral interior with stout teeth, their inner sides bearing deep, suboval crural fossettes. Dental plates thick and short, subvertical, bounding deep delthyrial chamber. Muscle field subpentagonal to subrectangular, extending anteriorly for about one-third of valve length, limited laterally by ridge-like prolongations of dental plates and anteriorly by a prominent boss-like thickening of valve floor; such premuscular thickening is less developed in small to

← FIGURE 9—1–12, Monorthis transversa n. sp.; 1, 2, ventral valve, cast of exterior and internal mold, holotype, CEGH-UNC 19098, 33.5; 3, external ventral mold, CEGH-UNC 19628, 33.5; 4, 5, internal mold (33) and cast (34) of ventral valve, CEGH-UNC 19100; 6, cast of ventral exterior, CEGH-UNC 19631, 33; 7, cast of ventral exterior, CEGH-UNC 19630, 33; 8, cast of dorsal exterior, CEGH-UNC 19628, 33; 9, internal mold of ventral valve, CEGH-UNC 19095, 32.5; 10, internal mold of dorsal valve, CEGH-UNC 19099, 33; 11, 12, internal mold of dorsal valve (33) and cast (35), CEGH-UNC 19635. 13–30, Hesperonomiella arcuata n. sp.; 13, cast of ventral exterior, CEGH-UNC 15741, 35; 14, cast of ventral exterior, CEGH-UNC 19078, 33; 15, 27, 28, cast of dorsal exterior (35), cast of dorsal interior (36) and internal mold of dorsal valve (35), holotype, CEGH-UNC 15759; 16, cast of ventral exterior, CEGH-UNC 15752, 34; 17, cast of dorsal exterior, CEGH-UNC 15746, 35; 18, internal mold of ventral valve, CEGH-UNC 19726, 36; 19, internal mold of ventral valve, CEGH-UNC 19079a, 33; 20, internal mold of ventral valve, CEGH-UNC 15740, 35; 21, internal mold of ventral valve, CEGH-UNC 15754, 35; 22, internal mold of ventral valve, CEGH-UNC 15730, 35; 23, internal mold of dorsal valve, CEGH-UNC 15744, 35; 24, internal mold of dorsal valve, CEGH-UNC 15743, 35; 25, internal mold of dorsal valve, CEGH-UNC 15737, 36; 26, internal mold of dorsal valve, CEGH-UNC 15738, 36; 29, 30, internal mold and cast of dorsal valve, CEGHUNC 15732, 35.

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BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA medium-sized individuals. Diductor scars narrow and long, confined to margins of muscle field; adductor field occupying a broad, flattened central area, strongly raised anteriorly. Vascula media poorly impressed consisting of broad, initially adjacent trunks. Dorsal interior with high, subtriangular notothyrial platform limited laterally by well developed chilidial ridges united posteriorly to form the chilidium and prolonged anteriorly into short, rounded, widely divergent brachiophores. Cardinal process gradually thickened with growth, varying from a stout but slender ridge in small specimens to a prominent, bulbous ridge in larger specimens. Sockets subtriangular, deeply excavated into posterior shell wall. Dorsal muscle field quadripartite, subrectangular, extending anteriorly to mid length of valve, divided medially by strong, rounded ridge and transversally by faint, slightly oblique ridges. Anterior adductor scars large, suboval in outline with indented anterior margin; posterior adductor scars narrow, transversely rectangular, one-third–one-fourth the length of anterior scars, located immediately in front of notothyrial platform. Etymology.From the Angulos village, situated to the east of fossiliferous localities. Types.Holotype: An internal mold of dorsal valve, CEGHUNC 20019 (Fig. 10.26–10.28) from level Mo8. Paratypes: twenty-six external and internal molds of both valves, CEGH-UNC 20020–20045 from levels Mo6, Mo7, Mo8 and Mo9; a ventral internal mold from level Mo8, CEGH-UNC 15893b. Occurrence.All material is from the second member of the Molles Formation, Los Molles River section, levels Mo6, Mo7, Mo8 and Mo9. Discussion.The combination of a relatively inflated shell, low interareas, strong concentric growth lamellae, well developed chilidium, thickened cardinal process and prominent premuscular callus supports the assignment of the Famatina specimens to the genus Productorthis. The new species resembles the type species P. obtusa (Pander), from the Arenig of Russian platform (re-described by Alichova, 1953 and Rubel, 1961) in both size and shell proportions, but differs in having finer ornamentation. Internally, the Russian species can be differentiated by its less developed premuscular callus and more prominent dorsal median ridge. Of the other Russian species, P. parallela (Pander) differs by having coarser ribbing and a proportionally more elongated shell, P. eminens (Pander) can be differentiated by its strongly inflated ventral valve, and P. aculeata (Pander) differs in having few stronger angular ribs and an unusually large cardinal process. The new Famatinian species P. angulensis closely compares in both external and internal features with P. mainensis Neuman from Maine and Newfoundland (Neuman, 1964, 1976) which subtly differs from the Famatinian species in that growth lamellae are projected as scalloped frills and the premuscular thickening tend to be less prominent. P. angulensis is also very similar to P. americana Cooper, 1956, but in the North American species the ventral valve

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is more inflated and the umbo is higher and recurved on the interarea. The other North American species P. agilera (Willard) differs in the larger size, proportionally smaller ventral muscle field and much higher cardinal process. The only previously described species from Argentina is P. cienagaensis Herrera and Benedetto (1989) from the San Juan Limestone. It can readily be distinguished by its subspherical, non-alate shell, incurved ventral beak and higher, posteriorly trilobate cardinal process. Finally, it is interesting that the overall morphology of juvenile specimens of P. angulensis is very similar to that of larger individuals of Panderina? ambigua from the underlying Suri Formation so as to suggest a phylogenetic relationship between them. Family HESPERONOMIIDAE Ulrich and Cooper, 1936 Genus HESPERONOMIA Ulrich and Cooper, 1936 Type species.Hesperonomia planidorsalis ULRICH AND COO1936.

PER,

HESPERONOMIA ORIENTALIS Benedetto, 1998a Figure 11.1–11.12 Material examined.About 70 specimens preserved as external and internal molds of both valves, CEGH-UNC 15673–15699, 15726, 15727, 15736, 15808–15856. Occurrence.Upper member of Suri Formation, Cachiyuyo River section, level CA. Remarks.Specimens from the Famatina Range are similar in all substantial respects to the type material from the upper part of the Acoite Formation of the Cordillera Oriental in northwestern Argentina (Benedetto, 1998a). Abundant material collected from the Suri Formation shows that this species experiences important changes with growth. Small shells (Fig. 11.4 and 11.8) are more transverse in outline, the ventral interarea is comparatively low and the ventral muscle field, which is remarkably broader and shorter than in adults, is reminiscent of that of the hesperonomiid Treioria Neuman and Bates, 1978. In full-grown dorsal valves the adductor muscle scars become deeply impressed, a well-defined short chilidium is developed, and the cardinal process becomes a thick ridge. Genus MOLLESELLA new genus Type species.Mollesella planiventralis new species. Etymology.After the Los Molles River. Diagnosis.Medium sized to large shells with planar ventral valve and moderately convex sulcate dorsal valve. Ornamentation finely multicostellate. Ventral valve with triangular muscle field and proximal half of vascula media subparallel. Notothyrial platform strongly raised continued anteriorly as high, rounded median ridge bearing simple, anteriorly thickened cardinal process, which becomes bulbous and prominent in full-grown specimens.

← FIGURE 10—1–16, Panderina? ambigua n. sp.; 1, exterior of dorsal valve, CEGH-UNC 20059, 34; 2, cast of dorsal exterior, CEGH-UNC 20053, 34; 3, cast of ventral exterior, CEGH-UNC 20046, 34; 4, 5, conjoined specimen, dorsal and lateral views, CEGH-UNC 20055, 34; 6, 7, 11, ventral valve, holotype, external mold, cast and internal mold, CEGH-UNC 20006, 34; 8, internal mold of juvenile ventral valve, CEGH-UNC 15657, 35; 9, internal mold of juvenile ventral valve, CEGH-UNC 15659, 35; 10, interior of ventral valve, CEGH-UNC 20065, 35; 12, internal mold of ventral valve, CEGH-UNC 20063, 34; 13, 14, internal mold of dorsal valve (35) and cast showing cardinalia (38), CEGH-UNC 15670; 15, internal mold of dorsal valve, CEGH-UNC 15664, 34; 16, internal mold of dorsal valve, CEGH-UNC 20061, 35. 17–29, Productorthis angulensis n. sp.; 17, cast of ventral exterior, CEGH-UNC 20032, 33; 18, conjoined specimen, dorsal view, CEGH-UNC 20021, 32.5; 19, cast of dorsal exterior, CEGH-UNC 20031, 33; 20, internal mold of ventral valve, juvenile specimen, CEGH-UNC 20028, 34.5; 21, internal mold of ventral valve, CEGH-UNC 20031, 33; 22, internal mold of ventral valve, CEGH-UNC 15893b, 33; 23, cast of ventral interior, 20037, 33; 24, internal mold of ventral valve, CEGH-UNC 20025, 33; 25, internal mold of ventral valve, CEGH-UNC 20039, 33; 26, 27, 28, dorsal valve, internal mold (33), cast (33) and detail of cardinalia (36), holotype CEGH-UNC 20019; 29, internal mold of dorsal valve, CEGH-UNC 20022, 33.5.

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BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA Discussion.The size and proportions of valves, its finely multicostellate ornament, the triangular and proportionally short ventral muscle field, and the simple cardinal process are all features reminiscent of the Hesperonomiidae. Furthermore, the inclusion of Mollesella in this family is supported by the shell morphology of early growth stages, which are characterized by planar to gently convex dorsal valves whose internal features are virtually identical to those seen in the adult stages of Hesperonomia. The vascular system and muscle pattern of both valves also indicate affinities with hesperonomiids. However, some uncertainty exists about its suprageneric affinities because chilidial plates (‘notothyrial ridges’), which are considered a diagnostic feature of this family (Ulrich and Cooper, 1938; Laurie, 1991; Williams and Harper, 2000), are lacking in the new genus. It can readily be distinguished from Hesperonomia by its flattened ventral valve and bulbous cardinal process. In the latter feature Mollesella is comparable to the Siberian genus Murjukiana Severgina, 1967 from which differs in having convexoplane profile and in lacking dorsal peripheral rim. The genus Monorthis, which is considered herein more closely related to the orthids than the hesporonomiids, shares with Mollesella the convexoplane profile but differs in having a more transverse shell, carinate median fold in the ventral valve, smaller ventral muscle field confined to the delthyrial chamber, anteriorly divergent vascula media and slender, ridge-like cardinal process. Mollesella is of interest in the context of the phylogeny of hesperonomiids and the early diversification of orthid families. It may have originated from Hesperonomia orientalis Benedetto, recorded at the top of the underlying Suri Formation, by gradual flattening of the ventral valve and increase in dorsal valve convexity, which in turn caused the rising of the notothyrial platform. The ontogeny of Mollesella, showing a complete transition from small flattened dorsal valves bearing a blade-like cardinal process to large, convex valves bearing high notothyrial platform and prominent cardinal process, strongly supports the phylogenetic relationship between the two genera. MOLLESELLA PLANIVENTRALIS new species Figure 11.13–11.28 Diagnosis.The same as the genus. Description.Medium sized to large shells, maximum width 34 mm, usually 22–28 mm wide, convexoplane, semielliptical in outline (average length/width ratio 0.69), cardinal extremities acute. Ventral valve relatively flat with slightly convex umbonal area. Ventral interarea moderately high, curved below the umbo, steeply apsacline, with large triangular, unmodified delthyrium. Medium to large dorsal valves moderately and evenly convex in longitudinal profile, small valves (less than 15 mm) tend to be planar or gently convex. Dorsal sulcus angular originating at umbo, becoming larger and shallow towards anterior margin. Dorsal interarea low, planar, about a half as high the ventral, anacline; notothyrium open. Ornament finely multicostellate with rounded,

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subequal ribs numbering 8–9 in 2 mm in the central part of valve, crossed by fine, closely spaced growth fila. Ventral interior with stout, triangular teeth supported by short, curved, widely divergent dental plates. Ventral muscle field triangular in outline, about as long as wide, extending anteriorly onethird to one-fourth valve length, bounded laterally and anterolaterally by low ridges representing prolongations of dental plates. Diductor muscle scars triangular, impressed on the inner sides of dental plates and occupying about one-third of muscle field width; adductor scars rectangular, impressed in a shallow flat groove. Ventral vascular system with broad vascula media arising at the terminations of diductor scars, initially subparallel, their distal parts poorly preserved in available specimens, apparently curved anterolaterally and bounding large, radially ridged gonadal sacs. Dorsal interior with wide, triangular, strongly raised, gently concave notothyrial platform which continues anteriorly as high, rounded median ridge to fade at about one-third of valve length. Cardinal process simple, blade-like, slightly thickened anteriorly, in some large specimens becoming bulbous and prominent expanding on anterior slope of notothyrial platform (Fig. 11.24, 11.26). Sockets, excavated into posterior wall of the valve. Brachiophores rod-like, strongly divergent. Muscle field quadripartite, extending anteriorly about 40 percent of valve length, with a pair of subcircular, deeply impressed, longitudinally striated posterior adductor scars and a pair of smaller, V-shaped anterior ones. Dorsal vascular system generally well-impressed, formed by anteriorly divergent vascula media arising from anterior end of adductor scars and a pair of vascula antemyaria subparallel to the former, originating at lateral sides of anterior adductors. Etymology.Refers to the almost planar ventral valve. Types.Holotype: An internal mold of ventral valve, CEGHUNC 19649 (Fig. 11.22). Paratypes: 56 external and internal molds of ventral and dorsal valves, CEGH-UNC 19636–19648 (level Mo6), CEGH-UNC 19650–19661 (level Mo7), CEGHUNC 15895c, 19662–19670 (level Mo8), CEGH-UNC 19671– 19677 (level Mo9), CEGH-UNC 19678–19691 (level Mo4). Occurrence.Molles Formation (second member), levels Mo4, Mo6, Mo7, Mo8, and Mo9, Los Molles River section. Suri Formation, Cachiyuyo River section, level CA. Superfamily PLECTORTHOIDEA Schuchert and LeVene, 1929 Family PLECTORTHIDAE Schuchert and LeVene, 1929 Genus DESMORTHIS Ulrich and Cooper, 1936 Type species.Desmorthis nevadensis ULRICH AND COOPER, 1936. DESMORTHIS? BIFURCATA new species Figure 12.25–12.34 Diagnosis.Small sized, ventribiconvex shells with gently sulcate dorsal valve and evenly convex ventral valve. Ornamentation sparsely costellate with 16–18 rounded costae most of them bifurcate anteriorly. Ventral muscle field short, bounded by slender,

← FIGURE 11—1–12, Hesperonomia orientalis; 1, cast of dorsal exterior, CEGH-UNC 15828, 32; 2, cast of dorsal exterior, CEGH-UNC 15682, 32; 3, exterior of ventral valve, CEGH-UNC 15851, 32; 4, internal mold of ventral valve, juvenile specimen, CEGH-UNC 15808, 35; 5, conjoined specimen, dorsal view, CEGH-UNC 15699, 31.5; 6, internal mold of ventral valve, CEGH-UNC 15683, 31.75; 7 internal mold of ventral valve, CEGH-UNC 15680, 3 1.75; 8, internal mold of ventral valve, juvenile specimen, CEGH-UNC 15816, 34; 9, internal mold of ventral valve, CEGH-UNC 15681, 31.5; 10, internal mold of dorsal valve, CEGH-UNC 15674, 31.5; 11, 12, internal mold of dorsal valve (32) and cast (33), CEGH-UNC 15845. 13–28, Mollesella planiventralis; 13, 14, 18, external mold of dorsal valve (31.2), and dorsal (31.2) and posterior (31) views of cast, CEGH-UNC 19662; 15, cast of dorsal exterior, CEGH-UNC 19654, 31.2; 16, cast of ventral exterior, CEGH-UNC 19665, 31.2; 17, cast of ventral interior, CEGH-UNC 19673, 31.5; 19, 20, internal mold of dorsal valve and cast, CEGH-UNC 19652, 31.2; 21, internal mold of ventral valve, CEGH-UNC 19646, 31.5; 22, internal mold of ventral valve, holotype, CEGH-UNC 19649, 31.5; 23, internal mold of dorsal valve, CEGH-UNC 19690, 31.75; 24–26, interior of dorsal valve, cast showing cardinalia (33.75), internal mold (31.5) and detail (33.75); 27, internal mold of dorsal valve, CEGH-UNC 19644, 31.5; 28, internal mold of dorsal valve, CEGH-UNC 15895c, 31.5.

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BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA almost parallel dental plates. Brachiophore bases converging toward valve floor, their anterior ends curving medially and anteriorly bounding the notothyrial chamber. Cardinal process a slender, long ridge. Fulcral plates incipient. Description.Small, ventribiconvex, subrectangular to semielliptical in outline, slightly wider than long (average length/width ratio 0. 75), maximum width near shell midlength. Larger specimen 9 mm wide, holotype 5.4 mm wide and 3.6 mm long. Cardinal extremities right-angled or slightly obtuse. Ventral valve moderately and evenly convex, interarea proportionally low, apsacline. Dorsal valve almost planar in small specimens, gently convex in adults, bearing a shallow and broad sulcus, interarea very low, anacline. Ornament sparsely costellate with 16–18 rounded, 3 per mm primary ribs, most of them bifurcating at midlength. Aditicules, growth fila and capillae absent. Interior of ventral valve with short, subrectangular muscle field extending anteriorly for about one-fourth–one-third valve length. Teeth small, triangular, supported by slender almost parallel dental plates. Impressions of diductor muscles parallel-sided, narrower than the central slightly raised adductor area. Dorsal interior with small, rounded brachiophores supported by bases converging toward the valve floor forming the sides of the subrectangular notothyrial chamber, which extends anteriorly up to one-fourth of valve length; anterior ends of brachiophore bases curving medially, in some specimens they meet and bound the front of the notothyrial chamber. Cardinal process a slender ridge occupying entire length of notothyrial chamber. Sockets small, suboval, erected on incipient fulcral plates. Dorsal muscle field not discernible. Etymology.Refers to the bifurcation of primary costae. Types.Holotype: A dorsal valve with corresponding internal and external molds, CEGH-UNC 19848 (Fig. 12.32, 12.33). Paratypes: two external and three internal molds of ventral valves; one external mold and three internal molds of dorsal valves, CEGHUNC 19840–19847, 19849. Occurrence.Upper part of the Suri Formation, level CA, Cachiyuyo River section; level QS38, Saladillo Chico River section. Discussion.The Famatinian specimens are similar to Desmorthis in outline, shell proportions and cardinalia morphology but present some features atypical of the genus, especially in ornament and ventral valve interiors. In D. nevadensis Ulrich and Cooper, 1936, the type species of Desmorthis, costellae are relatively fine and arise by intercalation, whereas in other North American species such as D. crassus Ross, 1970, and D. costata Cooper, 1956, the ribs are coarser and commonly simple. Moreover, the external surface of these species is capillate and aditicules are well developed at least in the type species. In the Famatina specimens, by contrary, ribs number increases by bifurcation and capillae and aditicules are lacking. The ventral muscle field in Desmorthis, on the other hand, tends to be proportionally larger than in our material, generally extending forward to almost

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the valve midlength. The nearly coeval Bolivian and northwestern Argentine species D. segnis Havlıćˇek and Branisa, 1980, differs from D.? bifurcata in having coarser and dichotomized costae and shorter and less impressed ventral muscle field. This taxon probably represents a new genus; however too few specimens are available to give a precise diagnosis and consequently it is questionably assigned to Desmorthis. Family GIRALDIELLIDAE Williams and Harper, 2000 Genus FAMATINORTHIS Levy and Nullo, 1973 Type species.Famatinorthis turneri LEVY AND NULLO, 1973. FAMATINORTHIS TURNERI Levy and Nullo, 1973 Figure 12.1–12.13 Mimella nova LEVY AND NULLO, 1973, p. 148, la´m. 2, figs. 1–6. Famatinorthis turneri Levy and Nullo. BENEDETTO, 1994, p. 231, la´m. 3, figs. 1–17.

Material examined.Three external molds, four internal molds of ventral valves and eight of dorsal valves, CEGH-UNC 19692–19700, 19805–19810 (level Mo2); one specimen with conjoined valves, five external molds, four internal molds of ventral valves and nineteen internal molds of dorsal valves, CEGH-UNC 19811–19839. Occurrence.Molles Formation, Los Molles River section, level Mo2; Quebrada La Lenã section, levels QF7 and QF8. Diagnosis (revised).Medium to large sized, dorsibiconvex, transversely subelliptical in outline; ventral valve gently convex, dorsal valve moderately to strongly convex, sulcate. Ornament multicostellate with aditicules. Ventral muscle field deeply impressed, subpentagonal or pear-shaped, raised anteriorly. Ventral vascula media broad, arcuate. Convergent brachiophore plates forming welldefined septalium. Cardinal process a long, faint ridge. Remarks.Famatinorthis is one of the commonest and distinctive brachiopods of the Molles Formation. This monospecific genus was assigned by Levy and Nullo (1973) to the plectorthids on the basis of its similarities with Mimella, to which they referred specimens from the same collection possessing anteriorly elevated ventral muscle field. As the author noted, such specimens (Mimella nova Levy and Nullo) correspond to fully-grown shells of Famatinorthis (Benedetto, 1994). In the revised edition of the Treatise, Williams and Harper (2000) erected the new family Giraldiellidae including Famatinorthis. Although this assignment seems to be logical, the morphologic criteria to differentiate giraldiellids from plectorthids appear somewhat diffuse. For example, the Moroccan genus Irhirea, which shares with Famatinorthis several features (relative convexity, cardinalia), was included among the plectorthids. Paterorthis and probably Atlantida also could be better classified among the giraldiellids. It seems likely that Famatinorthis, Irhirea and probably Paterorthis and Tazzarinia represent a distinctive west-Gondwanan stock of plectorthoideans characterized by large shells dorsibiconvex or convexoplane in profile, with a well-differentiated septalium, a simple,

← FIGURE 12—1–13, Famatinothis turneri; 1–5, conjoined specimen, ventral, dorsal, lateral, posterior and anterior views, CEGH-UNC 19812, 33.2; 6, external mold, detail of ornamentation showing aditicules, CEGH-UNC 19824, 35; 7, internal mold of ventral valve, CEGH-UNC 19700, 31.75; 8, internal mold of ventral valve, CEGH-UNC 19814, 31; 9, 10, internal mold of dorsal valve (32) and cast showing cardinalia (34), CEGH-UNC 19699; 11, 12, internal mold of dorsal valve, dorsal and posterior views, CEGH-UNC 19698, 33; 13, internal mold of dorsal valve, CEGH-UNC 19816, 32. 14–24, Ffynnonia famatinensis n. sp.; 14, internal mold of dorsal valve, juvenile specimen, CEGH-UNC 15867, 32.5; 15, 17, 19, ventral valve, internal mold, external mold and cast, holotype, CEGH-UNC 15722, 32; 16, internal mold of ventral valve, CEGHUNC 15881b, 3; 18, 20, internal mold of ventral valve and cast of exterior, CEGH-UNC 15857, 34; 21, internal mold of ventral valve, CEGHUNC 15757, 32; 22, 23, cast of dorsal interior showing cardinalia (33) and internal mold (32), CEGH-UNC 15725; 24, internal mold of dorsal valve, detail of muscle field, CEGH-UNC 15870, 34. 25–34, Desmorthis? bifurcata n. sp.; 25, external cast of ventral valve, CEGH-UNC 19847, 33; 26, internal mold of ventral valve, CEGH-UNC 19846, 32.5; 27, 28, internal mold of ventral valve and cast, CEGH-UNC 19845, 33.5; 29, internal mold of ventral valve, CEGH-UNC 19849, 35; 30, 31, internal mold of dorsal valve and cast, CEGH-UNC 19840, 35; 32, 33, dorsal valve, cast of exterior (6) and internal mold (37), holotype, CEGH-UNC 19848; 34, internal mold of dorsal valve, CEGH-UNC 19844, 37.

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long, ridge-like cardinal process and a proportionally large and deeply impressed ventral muscle field. Family PLATYSTROPHIIDAE Schuchert and LeVene, 1929 Genus FFYNNONIA Neuman and Bates, 1978 Type species.Pleurorthis costatus BATES, 1968. FFYNNONIA FAMATINENSIS new species Figure 12.14–12.24 Diagnosis.Medium-sized, subequally biconvex, subrectangular to semi-elliptical shells. Ventral valve weakly sulcate anteriorly, dorsal valve uniformly convex or with a very weak fold. Ornament fascicostellate, with 12–14 rounded primary costae and 40–50 costellae along the anterior margin. Ventral muscle field subquadrate, strongly raised anteriorly. Fulcral plates well-developed only in larger specimens. Dorsal muscle field quadripartite with posterior adductor scars transversely rectangular, smaller than subtriangular, heart-shaped, anterior adductor scars. Description.Shell commonly medium-sized (width range 13.5 mm–18.4 mm), subequally biconvex, subrectangular to semielliptical in outline (average length/width ratio 0.60). Hingeline long, in most specimens representing the greatest shell width. Ventral valve moderately convex, weakly sulcate on anterior half. Interarea slightly curved, moderately high, apsacline; delthyrium triangular, open. Dorsal valve with narrow median sulcus becoming poorly defined or absent anteriorly, where a very weak fold is occasionally present to accommodate ventral sulcus. Dorsal interarea narrow, parallel-sided, steeply anacline to orthocline. External ornament fascicostellate, with 12–14 rounded primary costae that divide into 40–50 costellae along anterior margin. Ventral interior with large teeth, triangular in section, supported by receding, slightly divergent dental plates. Muscle field deeply impressed, strongly raised anteriorly, subquadrate in outline, extending anteriorly to one-third of valve length, laterally bounded by ridge-like extensions of dental plates. Diductor muscle scars long, extending slightly beyond but not enclosing subrectangular medially located adductor scars. Vascula media arising from anterior margins of diductor scars, initially subparallel and then divergent. Dorsal interior with broadly subtriangular, slightly concave notothyrial platform. Cardinal process thin, ridge-like, extending full length of platform. Brachiophore stout, diverging anteriorly at about 1008, their bases converging medially to sides of notothyrial platform. Sockets relatively small, deep, limited by fulcral plates that are well-defined only in larger specimens. Dorsal muscle field quadripartite, divided transversely by a faint ridge and longitudinally by a low and rounded median ridge starting in front of notothyrial platform; posterior adductor scars transversely oval, about one-third the length of subtriangular, heart-shaped anterior adductor scars. Etymology.Named after the Sierra de Famatina. Types.Holotype: a ventral valve with both external and internal molds, CEGH-UNC 15722, (Fig. 12.15, 12.17, 12.19); paratypes: ten ventral valves and nine dorsal valves, CEGH-UNC 16721, 15723–15725, 15755–15757, 15857–15874. Occurrence.All from the upper member of the Suri Formation, level CA, Cachiyuyo River section. Discussion.The type species F. costata (Bates) from the Arenig of Wales (Bates, 1968; Neuman and Bates, 1978) is very similar to the Famatinian species. The main differences are that in F. famatinensis the dorsal fold is undifferentiated or poorly developed and the ventral muscle field is proportionally wider and its anterior margin tend to be rectimarginate rather than arcuate. In addition, in the Famatinian species a true pseudospondylium is lacking. Of the three species of Ffynnonia described from the Argentine Precordillera, F. famatinensis is externally quite similar to the late Arenig species F. spondyliformis Benedetto, 2001a, except for the weaker fold and sulcus, but the latter differs in

having a strongly elevated, spoon-shaped pseudospondylium and larger and shorter notothyrial platform. Both the Early Llanvirn Precordilleran species F. minuta (Benedetto and Herrera, 1987) and F. fasciculata (Benedetto and Herrera, 1987) can also be distinguished from F. famatinensis by having stronger fold and sulcus. In addition, F. minuta has only a few branched costellae and a suboval ventral muscle field. Order PENTAMERIDA Schuchert and Cooper, 1931 Suborder SYNTROPHIDIINA Ulrich and Cooper, 1936 Superfamily PORAMBONITOIDEA Davidson, 1853 Family TETRALOBULIDAE? Ulrich and Cooper, 1936 Genus RUGOSTROPHIA Neuman, 1971, emend. Neuman, 1976 Type species.Rugostrophia sylvestris NEUMAN, 1971. RUGOSTROPHIA PROTOANDINA new species Figure 13.5–13.15 Diagnosis.Medium-sized, dorsibiconvex shells with sulcus and fold starting at about midlength. Multicostellate ornament with 9–10 costellae in 2 mm. Ventral muscle field on spatulate, anteriorly raised pseudospondylium. Notothyrial platform subpentagonal, raised anteriorly, bounded by short, discrete brachiophore bases. Dorsal adductor scars nearly equidimensional, deeply impressed posteriorly, arranged in semicircle around notothyrial platform and separated by radial ridges. Description.Medium-sized, dorsibiconvex shells, larger specimens 23 mm wide, with average length/width ratio 0.75. Ventral valve moderately and evenly convex in posterior view, maximum convexity in posterior third. Sulcus starting at about valve midlength, rounded in section, deepening rapidly towards the margin, in larger specimens forming U-shaped tongue. Ventral interarea curved, apsacline, about one-fifth valve length, delthyrium open. Dorsal valve slightly more convex than ventral, with a rounded fold becoming higher anteriorly. With ontogeny the commissure varies from almost non-plicate in small shells to strongly plicate in larger specimens. Dorsal interarea anacline lower than ventral one. Ornament multicostellate formed by subequal, rounded costellae numbering 9–10 in 2 mm, separated by interspaces bearing a single row of circular pits, about 12 in 2 mm. Growth lamellae over entire shell stronger near margins. Ventral interior with strong teeth triangular in section, supported by short, thick dental plates subperpendicular or slightly converging with valve floor, bounding large, spoon-shaped delthyrial chamber. Ventral muscle field strongly impressed, tripartite, suboval in outline, extending anteriorly up to one-third of valve length. Diductor scars large, occupying entire delthyrial chamber, adjacent medially along a linear suture. Adductor field subcircular to subtriangular, located immediately anterior to diductor field, raised on valve floor occupying the anterior part of a spatulate pseudospondylium. Posterolateral areas with large, radially septate vascula genitalia. Dorsal interior with short, discrete, gently concave brachiophore bases sloping down to valve floor to form lateral margins of subpentagonal, strongly raised notothyrial platform. Brachiophores rounded, widely divergent. Sockets, partially excavated on fulcral plates. Cardinal process absent. Muscle field quadripartite extending for about one-third of valve length. Adductor scars nearly equidimensional, deeply impressed posteriorly, forming a semicircle around a thickening in front of the notothyrial platform and separated by radial ridges. Etymology.Refers to the pre-Andean margin of Gondwana. Types.Holotype: Internal mold of dorsal valve, CEGH-UNC 19867 (Fig. 13.11, 13.12). Paratypes: Five external molds of ventral valves and three of dorsal valves, four ventral internal molds, and seven dorsal internal molds, CEGH-UNC 19861–19866, 19868–19880.

BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA Occurrence.Upper member of the Suri Formation, Cachiyuyo River section, level CA; Saladillo Grande section, level S6. Discussion.The suprageneric assignment of Rugostrophia remains problematical, the genus was originally attributed to the Huenellidae (Neuman, 1971) and later reassigned to the Porambonitidae (Neuman, 1976; Neuman and Bates, 1978). On the other hand, the genus Cuparius, which was placed in synonymy with Rugostrophia (Neuman and Bates, 1978), was considered as a syntrophopsid by Ross (1971). The chief morphological character to classify Rugostrophia as a porambonitid is its pitted or ‘porambonitid’ ornament. This feature, however, seems to be recurrent in the suborder Syntrophiidina, being developed in the tetralobulid Punctolira and probably in certain syntrophopsids, and so it does not itself indicate phylogenetic relationships. Dorsal interiors of Rugostrophia, with short and discrete brachiophore supporting plates and without evidence of a cardinal process, are quite different from those of Porambonites in which the brachiophore bases are typically longer and there is an incipient cardinal process. The ventral valves of the two genera are also different: Porambonites has longer and subparallel dental plates that do not form a true pseudospondylium. As Neuman (1971) noted, the internal morphology of Rugostrophia is reminiscent of certain huenellids, such as the Late Cambrian Huenellina that possesses a very similar tripartite ventral muscle field and short and discrete brachiophore bases (Schuchert and Cooper, 1932). The peculiar arrangement of dorsal muscle scars in Rugostrophia forming a semicircle in front of the notothyrial platform, however, is more closely comparable to that of Tetralobula. This feature, along with their brachiophore plates disconnected medially and its well-defined pseudospondylium suggest closest affinities with the tetralobulids, to which it is tentatively assigned here. Although Rugostrophia shares with the syntrophopsids a similar pseudospondylium, representatives of this family differ in having coalescent brachiophore plates. However, the real significance of many characters used in the family-level classification of porambonitoideans is still poorly understood and further investigation on the phylogenetic relationships of genera of this group is needed. From the two previously described species from eastern North America volcaniclastic rocks, R. sylvestris Neuman, 1971, and R. latireticulata Neuman, 1976, the new species differs in having finer ornament and more closely spaced pits. In the latter the ventral muscle field is proportionally longer. In size, outline and ornament R. protoandina is closer to R. cardilatus (Ross, 1971) but differs in that both notothyrial platform and pseudospondylium are strongly raised anteriorly and in the more deeply impressed dorsal muscle field. The unnamed species of Rugostrophia described by Neuman and Bates (1978) from the Treiorwerth Formation of Wales has larger and more inflated shells, widest at the anterior third rather than at midlength. Ventral interiors of both species are essentially identical but dorsal valves of R. protoandina have stronger septa separating adductor scars. Specimens assigned to Rugostrophia sp. from contemporaneous volcaniclastic rocks of the Puna region (Benedetto, 2001b) differ from the Famatina species in that the fold begins at the umbo, the pseudospondylium is longer and narrow, like that of R. latireticulata, and the dorsal adductor scars are indistinct. Family RECTOTROPHIIDAE Bates, 1968 [nom transl. herein (ex Rectotrophiinae Bates, 1968, p. 176)] Diagnosis (revised).Shells subtriangular in outline, lacking well-defined fold and sulcus, with posterolateral sides steeply sloping. Exterior surface smooth or with very faint radial striations. Anterior prolongations of dental plates long, subparallel or slightly convergent defining elongate pseudospondylium. Cardinal process absent or rudimentary. Notothyrial platform slightly

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raised anteriorly, merged with subparallel, receding brachiophore bases. Species assigned.Rectotrophia globularis Bates, 1968; Trigonostrophia reversa Benedetto, herein. Discussion.Bates (1968) created a new subfamily of huenellids, the Rectotrophiinae, to include the monotypic genus Rectotrophia from the Treiorwerth Formation of Wales. However, as Neuman and Bates (1978) stated, a close relationship with the Huenellidae seems unlikely, as the representatives of this family possess typical wide-hinged, strongly plicate shells. The new genus Trigonostrophia shares with Rectotrophia a distinctive triangular, non-syntrophioid shell and a combination of internal features that have not been recorded in any group of porambonitoideans. This quite different stock of porambonitoideans is considered herein as having familial rank. It is characterized, as pointed out by Neuman and Bates (1978) in discussing the affinities of Rectotrophia, by a combination of ‘syntrophid’ and ‘camerellid’ features. Similarities with the former lie principally in the shell form. Internally the camerelloideans strongly differ in having well developed septalium and spondylium. Representatives of the family Rectotrophiidae are only known from Arenig beds of Wales and Argentina (Famatina and western Puna basins). Genus TRIGONOSTROPHIA new genus Type species.Trigonostrophia reversa new species. Diagnosis.Small to medium-sized, dorsibiconvex, subtriangular shells. Lateral sides straight. Ventral valve with broad shallow sulcus bearing anteriorly a low and rounded median swelling; lateral slopes narrow, steeply inclined. Anterior margin of valve strongly deflected dorsally. Dorsal valve domed, anteriorly sulcate, bounded by rounded, swollen carinas. Both interareas low and narrow. External surface smooth except for very fine radial striations. Ventral muscle field long and narrow, parallel-sided, extending for about 40 percent of valve length, raised above valve floor to form a sessile pseudospondylium. Notothyrial platform short, subpentagonal, slightly raised anteriorly, merged with subparallel or gently convergent receding brachiophore bases. Cardinal process lacking. A pair of suboval well-defined protuberances toward valve midlength. Dorsal mantle canal system formed by two pairs of straight trunks and several shorter, closely spaced minor trunks. Etymology.Refers to the subtriangular or trigonal shell outline. Discussion.Externally, the new genus Trigonostrophia differs from Rectotrophia principally in its more definitely triangular outline, in having a broad median swelling in the ventral sulcus, in lacking wave-like crenulations at the front of the valves, and in having a shallow sulcus on the anterior part of the dorsal valve. With respect to the latter feature, an incipient sulcus also is present on the dorsal valves of R. globularis figured by Neuman and Bates (1978, pl. 67, figs. 26–28). That was probably the reason why Bates (1968) considered as ventral rather than dorsal the valve designated as holotype (see discussion in Neuman and Bates, 1978, p. 606). Dorsal interiors of both genera are relatively similar. The rudimentary cardinal process mentioned by Neuman and Bates (1978) correspond to the linear suture dividing the muscle bundles attached on the notothyrial platform, and may or may be not considered as a ‘true’ cardinal process. The ventral interiors, however, markedly differ in that the pseudospondylium in Trigonostrophia is much longer and tends to be parallel-sided. TRIGONOSTROPHIA REVERSA new species Figure 13.16–13.31 Porambonitidae gen. et sp. indet., BENEDETTO, 2001b, p. 31, fig. 4L.

Diagnosis.As for the genus.

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BENEDETTO—ORDOVICIAN BRACHIOPODS FROM ARGENTINA Description.Small to medium-sized shells, the largest specimen 16 mm wide and 12.8 mm long, (generally 13–14 mm wide), dorsibiconvex, almost equilaterally triangular in outline, slightly wider than long (average length/width ratio 0.83). Maximum width near anterior margin. Lateral sides straight forming an apical angle of 80–858. Ventral valve gently convex in posterior third, becoming flattened or slightly concave anteriorly. Sulcus broad and shallow, flat in profile, originating at about one-third valve length, its median region swollen from which originate a low and rounded median fold becoming more prominent near anterior commissure. Sulcus bounded by swollen areas continuous with narrow, steeply inclined, planar lateral slopes. Anterior margin of valve strongly deflected dorsally. Dorsal valve moderately convex, anteriorly sulcate, approximately twice as deep as ventral, evenly convex and domed in posterior view with swollen, steeply inclined lateral slopes. Median sulcus beginning at about onefourth of valve length from umbo, deepening and widening slowly to accommodate the opposite ventral median swelling, bounded by rounded, swollen carinas. Interareas of both valves low and narrow. External surface smooth except for very fine radial striations barely visible on the posterolateral sides of valves. Ventral interior with small triangular teeth supported by thin, short dental plates. Muscle field long and narrow, parallel-sided, extending forward for about 40 percent of valve length, bounded laterally by prolongations of dental plates, subparallel to each another or slightly convergent anteriorly, raised above valve floor forming a sessile pseudospondylium which in some specimens is supported anteriorly by a low median septum. Dorsal interior with short, subpentagonal, slightly raised anteriorly notothyrial platform merged laterally with short, subparallel or gently convergent brachiophore bases. Notothyrial platform extending anteriorly to one-seventh–one-eighth of valve length. Cardinal process absent, its place occupied by a barely visible median suture dividing diductor scars. Adductor muscle field not impressed. Towards valve midlength a pair of suboval protuberances of unknown function. Dorsal mantle canal system well impressed formed by an inner pair of straight, subparallel trunks and an external pair of anterolaterally divergent trunks. In between of inner trunks as well as outside the external pair there are four or five shorter, closely spaced trunks. Etymology.Refers to the presence of a sulcus instead of a fold in the dorsal valve Types.Holotype: A ventral valve with both internal and external molds, CEGH-UNC 19881 (Fig. 13.20–13.22). Paratypes: eight incomplete external molds, fifteen internal molds of ventral valves and fourteen internal molds of dorsal valves, CEGH-UNC 19882–19900 and 19991–20018. Occurrence.Upper member of the Suri Formation, level CA of the Cachiyuyo River section; level S2 of the Saladillo Grande River section.

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Superfamily CAMERELLOIDEA Hall and Clarke, 1894 Family CAMERELLIDAE Hall and Clarke, 1894 Genus CAMERELLA Billings, 1859 Type species.Camerella volborthi HALL AND CLARKE, 1894. CAMERELLA sp. Figure 13.1–13.4 Description.Shell small for genus, largest specimen 4.1 mm long and 3.7 mm wide, pear-shaped in outline, slightly longer than wide, with maximum width at anterior third of shell, subequally biconvex. Posterolateral margins subtending an angle of about 75–858. Ventral valve gently convex in umbonal region becoming flattened anteriorly, sulcus indistinct. Dorsal valve moderately and evenly convex, without evidence of fold. Shell surface of posterior two-thirds smooth, anterior third bearing 6–8 coarse, rounded costae. Interior of ventral valve with narrow and slender spondylium extending anteriorly for about one-fifth of valve length, supported by a blade-like septum occupying the posterior third of valve. Interior of dorsal valve with very small, slender cruralium extending for almost one-third of valve length. Material examined.Ten ventral and dorsal valves and several fragmentary valves, CEGH-UNC 19850–19860. Occurrence.Upper member of the Suri Formation, rio Cachiyuyo River section, level CA; Saladillo Chico River section, level QS37; Saladillo Grande River section, level S2. Discussion.The Suri Formation specimens are assigned to Camerella on the basis of their small, globular, anteriorly costate shells and the short spondylium and cruralium. Of the numerous North American species described and illustrated by Cooper (1956), they are most similar to C. breviplicata Billings from the Mystic Conglomerate of Quebec. This Canadian species resembles the Famatinian form in number of costae, relative convexity of valves and especially in the indefiniteness of sulcus and fold, but differs in its larger and almost equidimensional shells. External features of the Famatina material are nearly identical to those of Camerella sp. from the Acoite Formation of northwestern Argentina (Benedetto, 1998a), and are probably conspecific. Presently available material, however, is too limited for an accurate comparison with other species. ACKNOWLEDGMENTS

The present research was made possible through financial support from the Agencia Nacional de Promocioń Cientı´fica y Tecnolo´gica (Grants Foncyt PICT 2301 and PICT 5387), the Consejo Nacional de Investigaciones Cientı´ficas y Tećnicas (CONICET, Grant PIP 4874/96) and CONICOR (Agencia Co´rdoba Ciencia). I would like to thank R. Astini and F. Da´vila for assistance in the field and collection of paleontological material. R. Cocks (The Natural History Museum, London) and E. Villas (Universidad de

← FIGURE 13—1–4, Camerella sp.; 1, internal mold of dorsal valve, CEGH-UNC 19853, 33.5; 2, internal mold of dorsal valve, CEGH-UNC 19854, 32.5; 3, internal mold of ventral valve, CEGH-UNC 19857, 34; 4, internal mold of ventral valve, CEGH-UNC 19856, 35. 5–15, Rugostrophia protoandina n. sp.; 5, cast of dorsal exterior, CEGH-UNC 19869, 32.5; 6, exterior of ventral valve, CEGH-UNC 19868, 37; 7, detail of specimen illustrated in figure 5 showing external ornamentation, 37; 8, exterior of dorsal valve, CEGH-UNC 19873, 31.5; 9, 10, internal mold of ventral valve (32) and cast (32.5), CEGH-UNC 19880; 11, 12, internal mold of dorsal valve (32) and cast (33), holotype, CEGH-UNC 19867; 13, internal mold of ventral valve, CEGH-UNC 19876, 32; 14, internal mold of ventral valve, CEGH-UNC 19877, 32; 15, internal mold of dorsal valve, CEGH-UNC 19862, 32.5. 16–31, Trigonostrophia reversa n. gen and sp.; 16, 18, 19, cast of dorsal exterior and internal mold (dorsal and antero-dorsal views), CEGH-UNC 19894, 32; 17, cast of dorsal exterior, CEGH-UNC 19897, 32.5; 20–22, cast of ventral exterior (32.5), mold of ventral interior and cast (32), holotype, CEGH-UNC 19881; 23, internal mold of dorsal valve, CEGH-UNC 20019, 32.5; 24, internal mold of ventral valve, CEGH-UNC 19889, 32.5; 25, 26, 29, internal mold of dorsal valve (32), cast showing cardinalia (35) and posterior oblique view (32), CEGH-UNC 20010; 27, two internal molds of ventral valves, CEGH-UNC 19888a, b (33); 28, detail of dorsal internal mold, posterior view, CEGH-UNC 20012, 35; 30, internal mold of dorsal valve, CEGH-UNC 20011, 32.5; 31, internal mold of dorsal valve, CEGH-UNC 20013, 33.

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Zaragoza, Spain) critically reviewed the manuscript and greatly helped to improve this work. This is a contribution to the IGCP project 410, The Great Ordovician Biodiversification Event. REFERENCES

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