evolution of the late ordovician orthid brachiopod ... - GeoScienceWorld

J. Paleont., 74(6), 2000, pp. 983–991 Copyright q 2000, The Paleontological Society 0022-3360/00/0074-983$03.00

EVOLUTION OF THE LATE ORDOVICIAN ORTHID BRACHIOPOD GNAMPTORHYNCHOS JIN, 1989 FROM PLATYSTROPHIA KING, 1850, IN NORTH AMERICA JISUO JIN

AND

RENBIN ZHAN

Department of Earth Sciences, University of Western Ontario, London, Ontario, N6A 5B7, Canada, ,[email protected]., and Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing 210008, China

ABSTRACT—Gnamptorhynchos, a rhynchonellid-like orthid brachiopod, evolved from Platystrophia in Maysvillian (early Ashgill) time, and survived both the end-Richmondian and end-Hirnantian episodes of the latest Ordovician mass extinction. The name of the type species of Gnamptorhynchos, G. inversum Jin, 1989, is rejected and replaced by Gnamptorhynchos globatum (Twenhofel, 1928), which is a senior synonym. Gnamptorhynchos manitobensis new species is described here from the Selkirk Member (Maysvillian) of the Red River Formation, southern Manitoba. The new species is characterized by a transversely extended, strongly biconvex to globular shell with prominent umbones, relatively numerous costae and a notothyrial cavity supported dorsally by a short median ridge. It constituted part of a Late Ordovician epicontinental fauna that once spread widely in shallow, equatorial seas of North America. The new species is a morphological intermediate between Platystrophia and Gnamptorhynchos, with Platystrophia-like interareas, hingeline, and cardinal process, but Gnamptorhynchos-like shell posterior and notothyrial platform. Previously, Gnamptorhynchos was known from Hirnantian to mid-Aeronian rocks. The new species extends the lower range of the genus to Maysvillian.

INTRODUCTION

KING, 1850, is a long-ranging orthid brachiopod genus spanning from the Arenig (Early Ordovician, Neuman, 1964) to Wenlock (late Early Silurian, Cocks, 1978). In pioneer studies, the genus was confused with some spire-bearing brachiopods (e.g., Spirifer and Delthyris) because of its strophic outline and costate ribbing (Davidson, 1848). Platystrophia shows a high morphological diversity and great abundance in middle and upper Ordovician rocks of North America. McEwan (1919) recognized 60 species/subspecies of Platystrophia and divided them into three main morphological groups: uniplicate, biplicate, and triplicate, and each group was shown to have a specific geological range. Williams (1962) and Wright (1964), however, regarded McEwan’s classification of Platystrophia to be artificial. Alberstadt (1979) reviewed most of the North American Platystrophia species and recognized four species groups, each of which was shown also to have been dominant at different time intervals: colbiensis-elegantula group in Middle Ordovician, ponderosa group from Maysvillian to Richmondian, laticosta-cypha group in early and mid Richmondian, and annieana group in latest Richmondian. Many species of Platystrophia have a relatively short stratigraphic range and are useful for dating and correlating Upper Ordovician rocks in North America (Davis, 1985). Typical Platystrophia shells are strophic or mucronate (Spirifer-shaped), with a long, straight hingeline extending into ears at cardinal angles. The earliest known occurrence of Platystrophia was in the Arenig rocks in the Russian Platform located on the Baltica paleocontinent (Neuman, 1964) and it did not expand to Avalonia (British Isles) until Llanvirn time, as indicated by the occurrences of Platystrophia bailyana (Davidson, 1869) in southern Ireland (Cocks, 1978). In the North American (Laurentian) paleocontinent, the earliest occurrence of Platystrophia was in deep, cool-water deposits of Llanvirn age along the southeastern margin (Maine and Newfoundland). Platystrophia invaded inland seas of Laurentia by Trentonian time and experienced considerable morphological modifications. Some platystrophiinid orthids in Trentonian-Richmondian rocks display a striking morphological change toward a globular, rhynchonellid-like shell with a shortened hingeline, as shown by Platystrophia rhynchonelliformis McEwan, 1919, Platystrophia ponderosa auburnensis Foerste, 1909, and Platystrophia prayi Howe, 1966. Some latest Ordovician rhynchonellid-like platystrophiinids show a septaliumlike notothyrial platform that contains a cardinal process and is

P

LATYSTROPHIA

supported dorsally by one or two ridges. This led Jin (1989) to propose a new genus Gnamptorhynchos, which was initially assigned to the rhynchonellids. The ventral muscle field and dental plates of Gnamptorhynchos, however, are virtually identical to those of Platystrophia, and it now seems reasonable to treat Gnamptorhynchos as a platystrophiinid. During our study of the Late Ordovician brachiopod fauna of the northeastern Williston Basin, southern Manitoba (Fig. 1), another rhynchonellid-like platystrophiinid is recognized from the Selkirk Member (Maysvillian) of the Red River Formation. More interestingly, the shells from southern Manitoba show a Platystrophia-type unilobate cardinal process in a Gnamptorhynchos-type notothyrial platform supported by a median ridge. Like Gnamptorhynchos, their shells are typically globular at adult stage, but their hingelines are relatively long as in Platystrophia. The Manitoba shells, therefore, represent a morphological intermediate between Platystrophia and Gnamptorhynchos, and are important for understanding the evolution of the two genera in the Late Ordovician. In over 212 papers that dealt with Platystrophia (see Rex Doescher’s Brachiopod Database, Smithsonian International Brachiopod Information Centre), more than 80 percent of the Platystrophia species were documented from the Ordovician. The greatest species diversity of Platystrophia is found in rocks of Maysvillian-Richmondian age, as indicated by the systematic surveys of McEwan (1919) and Alberstadt (1979). Platystrophia evolution constitutes an integral part of the Late Ordovician radiation of brachiopods before the first episode of end-Ordovician mass extinction. Gnamptorhynchos represented one of the evolutionary branches produced through the morphological experimentation of Platystrophia to adapt to the tropical epicontinental seas of North America. In the present paper, we describe a new form of platystrophiinids from southern Manitoba and explore its significance to the Platystrophia-Gnamptorhynchos evolution. Platystrophia and Gnamptorhynchos are among the small number of brachiopod genera that survived the latest Ordovician mass extinction event, and their evolution and distribution are important in studying the Ordovician-Silurian faunal turnover of shelly benthos. Serial sections of brachiopod shells were prepared using a Croft Parallel Grinder and acetate films. The acetate peel sections were traced using a Kodak slide projector for outlines of shell structures and using a microscope for microscopic structures.

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FIGURE 1—Location map showing the Williston, Hudson Bay, Anticosti, and American mid-continental basins and Gnamptorhynchos occurrences.

GNAMPTORHYNCHOS MANITOBENSIS N.

SP. AND THE

RED RIVER FAUNA

The benthic shelly fauna of the Red River Formation, mainly from the Selkirk Member, previously referred to as the ‘‘Arctic Ordovician Fauna’’ (see Nelson, 1959; Jin et al., 1995), is characteristic of the Late Ordovician equatorial epicontinental shelly fauna of North America (Jin, 1999). Diagnostic features of the fauna include 1) a relatively high level of diversity and abundance, 2) gigantism, and 3) provincialism with pan-continental distribution. In the past, the age of the Red River Formation has been dated variously as Trentonian, or Edenian, or Maysvillian by different authors (Ludvigsen, 1975; Le Fevre et al., 1976; Nelson, 1981; Barnes et al., 1981; Sanford, 1987; Norris, 1993). Jin et al. (1997) assigned a Maysvillian age to the Selkirk Member of the Red River Formation on the basis of its correlation with the Surprise Creek Formation of the Hudson Bay Lowlands, as both stratigraphic units contain the Tetraphalerella churchillensis—Kjaerina hartae brachiopod Zone (Fig. 2). In the Williston Basin, the upper Red River Formation (particularly the Selkirk Member) of southern Manitoba and the coeval upper Bighorn Formation of Wyoming (Macomber, 1970) contain a great variety of corals, brachiopods, cephalopods, gastropods, etc. The common shelly biota includes calcareous algae (Fisherites, Receptaculites), stromatoporoids (Cystostroma, Beatricea), corals (Saffordophyllum, Calapoecia, Catanipora, Manipora, Protrochiscolithus, Chaetetes, Palaeophyllum, Grewingkia, Lobocorallium), brachiopods (Hesperorthis, Platystrophia, Gnamptorhynchos, Glyptorthis, Plaesiomys, Dinorthis, Diceromyonia, Thaerodonta, Strophomena, Tetraphalerella, Kjaerina, Oepikina, Megamyonia, Rhynchotrema, Hiscobeccus, Hypsiptycha), cephalopods (Cyclendoceras, Orthoceras, Armenoceras, Lambeoceras, Narthecoceras, Wilsonoceras, Diestoceras), gastropods (Maclurites, Hormotoma, Loxonema, Ectomaria), and ichnofossils (Thalassinoides, Trypanites). Many shelly organisms, particularly some brachiopods, gastropods, and cephalopods, show pronounced gigantism. In brachiopods, the gigantism is reflected in two aspects: 1) the shells show dramatic increase in length, width and globosity, and 2) very large strophomenid and rhynchonellid shells tend to have strong rugae and growth lamellae. Gigantic species of strophomenids (e.g., Strophomena, Tetraphalerella, Oepikina, and Kjaerina) may have shells of 60–80 mm in width, with thick shell walls and various types of rugae (Jin et al., 1997). In orthids and rhynchonellids, the shells are not only longer and wider, but also

become globular, often with the dorsal valve more convex than the ventral valve, as in most species of Plaesiomys and Hiscobeccus. Some Late Ordovician species of Platystrophia show a similar evolutionary trend toward a large, strongly convex to globular, rhynchonellid-like shell (with shortened hingeline). This is particularly evident in Platystrophia ponderosa Foerste, 1909, Platystrophia rhynchonelliformis McEwan, 1919, and Platystrophia prayi Howe, 1966. The shells of Platystrophia ponderosa auburnensis Foerste, 1909, from the Arnheim Formation of Ohio, for example, are the largest and most globose known for the genus, with an equidimensional shell size 39 mm long, 39 mm wide, and 39.5 mm thick (Fig. 3). As indicated by the shell shape (shortened of hingeline and interareas, and strongly convex dorsal valve), Gnamptorhynchos evolved from such rhynchonellid-like Platystrophia by evolving a septalium-like structure and eventually a trilobate cardinal process. The evolution of Platystrophia and Gnamptorhynchos toward a homeomorph of rhynchonellids may have been an adaptation to relatively high-energy substrate conditions. Typical Platystrophia shells are strophic, with a long, straight hingeline extended into prominent ears. Like spiriferids, strophic platystrophiinid shells were well adapted to living on a muddy, relatively soft but stable (due to flocculation under low-energy conditions) substrate because the long hingeline had a ski-effect to prevent the shells from sinking into the substrate. This interpretation appears supported by the predominant occurrence of Platystrophia in soft-weathering micrite and calcareous mudstone. Rhynchonelliform shells, however, have a much wider range of substrate distribution from high-energy rocky shore to low-energy muddy bottom. The tapered, variously rostrate posterior of a rhynchonellid shell is attached to the substrate by the pedicle. This enables the shells to rotate, which increases their flexibility in feeding radius and direction and their ability to avoid smothering by sediments. A rhynchonelliform shell, therefore, has an adaptive advantage over strophic shells in higher-energy environments, where substrate sediments tend to have a high degree of mobility. Gnamptorhynchos globatum (Twenhofel, 1928), for example, is most abundant in the sand-rich mudstone and wackestone beds of the Prinsta Member of the Ellis Bay Formation, Anticosti Island. The sandy units are interpreted by Long and Copper (1987) as deposits of longshore drift of siliciclastics driven by relatively strong currents. The Selkirk Member of the Red River Formation, where Gnamptorhynchos manitobensis n. sp. occurs, is intensely bioturbated and contains abundant skeletal sands and in places skeletal grainstones. This is supporting evidence that these homeomorphic rhynchonellid shells represent a morphological adaptation to living in relatively turbulent environment with an unstable substrate. Biogeographically, Gnamptorhynchos is a typical element of the North American epicontinental fauna of the Late Ordovician. The majority of brachiopod taxa that evolved in North America during Edenian-Richmondian times can be traced throughout the continent, from the western margin (Rocky Mountains, Mackenzie Mountains) to the eastern margin (Anticosti Island), and from New Mexico to northern Greenland. From a global perspective, the Ashgill brachiopod fauna of Laurentia shows a high degree of provincialism (Sheehan and Coorough, 1990; Jin, 1996, 1999). The majority of the taxa evolved in Laurentia during the Ashgill rarely occurs in the adjacent paleocontinents. The combination of pan-continental distribution and provincialism is especially apparent in the paleogeographic occurrences of many large-shelled taxa, such as Platystrophia, Plaesiomys, Dinorthis, Tetraphalerella, Megamyonia, Hiscobeccus, Lepidocyclus, and Hypsiptycha. Presently, Gnamptorhynchos is known from both marginal (Anticosti) and inland (Williston) basins. As discussed below in the section of Systematic Paleontology, however, the genus may be common also in the American mid-continental basins (e.g., Ohio,

JIN AND ZHAN—EVOLUTION OF LATE ORDOVICIAN BRACHIOPOD GNAMPTORHYNCHOS

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FIGURE 2—Stratigraphic ranges of Gnamptorhynchos and other selected species of associated brachiopods in the Red River Formation of southern Manitoba and the Ellis Bay and Gun River formations of Anticosti Island (member names in italics are informal).

Kentucky, Tennessee), hidden under the generic name Platystrophia. Gnamptorhynchos has not been found on other paleocontinents. Provincialism of the North American brachiopod fauna has been largely attributed to the inability of shallow-water epicontinental forms to cross deep oceanic barriers during the CaradocAshgill eustatic sea-level rise, when oceanic islands (as faunal migration ‘‘stepping stones’’) became submerged (Fortey, 1984;

Jin, 1996). Evolutionary isolation of epicontinental faunas was also a probable factor (Sheehan and Coorough, 1990). SYSTEMATIC PALEONTOLOGY

Figured specimens are deposited in the Type Collection of Invertebrate and Plant Fossils of the Geological Survey of Canada

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TABLE 1—Statistics of shell dimensions (mm) of Gnamptorhynchos manitobensis new species. Specimen

L

W

T

W1

W2

L/W W1/W W2/W

GSC 117739 GSC 117740 GSC 117741 GSC 117742 C-205935* C-205935*

10.2 15.4 20.5 19.4 14.3 16.3

12.2 19.2 26.1 22.8 18.6 18.6

8.3 12.7 20.3 17.4 11.3 12.5

8.9 15.2 19.5 17.7 12.6 14.1

5.1 8.3 12.4 10.8 7.8 8.1

0.83 0.8 0.79 0.85 0.77 0.87

0.73 0.79 0.75 0.78 0.68 0.75

0.42 0.44 0.48 0.47 0.42 0.44

* GSC field locality number, non-type specimens.

(GSC), Peabody Museum of Yale University (YPM), or the Department of Earth Sciences, University of Western Ontario (W). Shell size (usually measured by shell length) of orthids, strophomenids, and rhynchonellids: small, 0–10 mm; medium, 10–20 mm; large, 20–30 mm; giant, .30 mm. Phylum BRACHIOPODA Order ORTHIDA Schuchert and Cooper, 1932 Family PLECTORTHIDAE Schuchert and LeVene, 1929 Subfamily PLATYSTROPHIINAE Schuchert and LeVene, 1929 Genus GNAMPTORHYNCHOS Jin, 1989 Type species.Gnamptorhynchos regularis var. globata Twenhofel, 1928, Ellis Bay Formation (latest Ashgill), Anticosti Island, Que´bec. Platystrophia regularis var. globata Twenhofel, 1928 is proposed here to be a senior synonym of Gnamptorhynchos inversum Jin, 1989 originally designated as the type species of Gnamptorhynchos Jin, 1989. Other species assigned.Gnamptorhynchos selliseptalicium Jin, 1989, Gun River Formation (Aeronian), Anticosti Island, Que´bec. Gnamptorhynchos manitobensis new species, Selkirk Member (Maysvillian), Red River Formation, southern Manitoba. Species questionably assigned.Platystrophia ponderosa auburnensis Foerste, 1909, Mount Auburn Member (upper Maysvillian), McMillan Formation, Ohio (Alberstadt, 1979 mentioned the presence of a cruralium-like structure which is similar to the cardinalia of Gnamptorhynchos, but serial sectioning of topotypes are needed for more detailed comparison). Platystrophia rhynchonelliformis McEwan, 1919, Trenton Limestone, Ellisburg, New York. Platystrophia camerata Twenhofel, 1928, Ellis Bay Formation, Prinsta Member (Hirnantian), Anticosti Island, Que´bec. Platystrophia prayi Howe, 1966, Aleman Limestone (Maysvillian), Trans-Peco Texas. Platystrophia orbiculoidea Jin and Chatterton, 1997, Whittaker Formation, latest Ordovician (persculptus Zone), Avalanche Lake, Mackenzie Mountains, northwestern Canada (small, silicified shells with very short hingeline and poorly preserved cardinalia). Species rejected.Gnamptorhynchos inversum Jin, 1989 (5 Platystrophia regularis var. globata Twenhofel, 1928). Diagnosis (emended herein).Medium to relatively large, strongly convex to globular shells with shortened hingeline, strong, angular to subangular costae, well-defined ventral sulcus and dorsal fold. Interareas prominent, apsacline. Delthyrium open. Dental plates well developed. Septalium-like notothyrial platform

supported dorsally by one or two ridges; cardinal process bladelike, uni- or trilobate. Occurrence.Early Ashgill (Maysvillian)–Early Llandovery (Aeronian), North America. Discussion.Jin (1989) originally assigned Gnamptorhynchos to the rhynchonellid family Orthorhynchulidae on the basis of its rhynchonellid-like shell shape and septalium-like notothyrial platform in the dorsal valve. As shown by the present study, Gnamptorhynchos shows a greater degree of similarity to the platystrophiinid orthids than to the orthorhynchulids, especially in the configuration of its dental plates and ventral muscle field and the presence of short brachiophores. It is proposed here to transfer Gnamptorhynchos from the Orthorhynchulidae to the Platystrophiinae (this suggestion appears to have been accepted by Watkins, 1999 as a result of his personal communications with Jin). Gnamptorhynchos can be distinguished from Platystrophia in having a combination of the following characteristics: (1) a strongly biconvex to globular shell with a tapered, variously rostrate posterior and shortened hingeline, giving the shell a rhynchonellid-like (particularly orthorhynchulid-like) appearance; (2) a well-developed notothyrial platform (in the dorsal valve) that is elevated well above the valve floor and supported dorsally by one or two ridges; and (3) late forms of Gnamptorhynchos (G. globatum and G. selliseptalicium) have a trilobate cardinal process, although the older species, G. manitobensis n. sp., has a unilobate cardinal process that is similar to the weak, blade-like cardinal process in Platystrophia. Platystrophia ponderosa auburnensis Foerste, 1909, Platystrophia rhynchonelliformis McEwan, 1919, and Platystrophia prayi Howe, 1966 (see also Macomber, 1970) have a globular, rhynchonellid-like shell, which suggests their affinity to Gnamptorhynchos. The internal structures of these species, however, are poorly known and need further investigation. GNAMPTORHYNCHOS GLOBATUM (Twenhofel, 1928) new combination Figures 3.1–3.5, 4.16–4.20, 5 Platystrophia regularis var. globata TWENHOFEL, 1928, p. 177, pl. 15, figs. 10–12. Gnamptorhynchos inversum JIN, 1989, p. 75, pl. 10, figs. 6–15; pl. 11, figs. 1–10; pl. 27, figs. 1–4.

Description.Shell medium-sized, transverse, subtriangular to subcircular, unequally biconvex, with deeper dorsal valve. Average shell length 13.3 mm, width 16.1 mm, thickness 11.3 mm. Shell convexity, measured by thickness/length ratio, averaging 0.84. Hingeline straight, reaching between one-third to one-half of shell width. Anterior commissure denticulate (Fig. 4.19, 4.20). Ventral umbo low, moderately convex, with slightly incurved to suberect beak. Interarea sharply defined, apsacline, occupying one-third to one-half of maximum shell width. Delthyrium open. Sulcus beginning near apex, becoming deep and angular towards anterior, occupied by one costa in umbonal area, with two intercalating costae flanking median one 3–4 mm anterior of beak. Dorsal umbo more convex than ventral umbo, protruding posteriorly to same level as, or even higher than, ventral umbo. Fold high, steep-sloped, beginning nearly at beak, marked on umbo by

→ FIGURE 3—1–5, Gnamptorhynchos globatum (Twenhofel, 1928), YPM10420, holotype, dorsal, ventral, lateral, posterior, and anterior views, Prinsta Member, Ellis Bay Formation, Prinsta Bay, Anticosti Island, 32. 6–10, Gnamptorhynchos? camerata (Twenhofel, 1928), YPM10344, lectotype (herein selected), dorsal, ventral, lateral, posterior, and anterior views, Prinsta Member, Ellis Bay Formation, Prinsta Bay, Anticosti Island, 31.5. 11–15, Platystrophia regularis Shaler, 1865, GSC 117744, dorsal, ventral, lateral, posterior, and anterior views of a strophic/mucronate shell typical of the genus, lower Ellis Bay Formation, Junction Cliff (A439), Anticosti Island, 32. 16–20, Gnamptorhynchos? auburnensis Foerste, 1909, W1946, dorsal, posterior, ventral, anterior, and lateral views of a shell showing characteristic features of gigantism: large size, strong globosity, and enlarged and strongly convex dorsal valve, Arnheim Formation (earliest Richmondian), Ohio, 31.5.


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FIGURE 5—Transverse serial sections of Gnamptorhynchos globatum (Twenhofel, 1928), GSC 102449 (original paratype of G. inversum Jin, 1989), Prinsta Member, Ellis Bay Formation, Lousy Cove (A466b), Anticosti Island, showing the characteristic cardinal process, septalium, and accessory teeth.

FIGURE 6—Shell dimensions of Gnamptorhynchos manitobensis n. sp. Sample from the Selkirk Member of the Red River Formation, Garson Quarry, southern Manitoba. Note that shells become more transversely extended and more strongly convex with ontogeny.

two costae increasing to four through bifurcation 2–4 mm anterior of apex. Each shell flank with seven to eleven strong, angular costae. Growth lines very fine, clearly visible only near anterior margin. Dental plates moderately high, thick, extending anterior of hingeline (Fig. 5). Teeth small, slender, supplemented by nodular, laterally positioned accessory teeth. Hinge fossettes deep, bounded by inner ridges. Muscle field shallow. Principal sockets in dorsal valve supplemented by accessory sockets. Notothyrial platform septalium-like, broad, deep, rectangular in cross-section. Floor of notothyrial platform free from valve floor, supported dorsally by two low ridges. Inner socket ridges strong, ventrally supporting brachiophores. Etymology.Gnamptorhynchos is a noun in the neuter, singular, nominative form (Jin, 1989) and the feminine specific name globata is changed to globatum accordingly. Type.YPM104840, holotype of Platystrophia regularis var. globata Twenhofel, 1928 (Fig. 3.1–3.5), Prinsta Bay, Anticosti Island, Prinsta Member, Ellis Bay Formation. Occurrence.Prinsta Member, Ellis Bay Formation, latest Ordovician (Hirnantian), Anticosti Island. Discussion.Examination of the type specimens reveals that Platystrophia regularis var. globata Twenhofel, 1928, is identical to Gnamptorhynchos inversum Jin, 1989. The type specimens of P. globata are from the Prinsta Bay and those of G. inversum from the adjacent Lousy Cove, both on Anticosti Island. They are all from the siliciclastic-rich micritic mudstone beds of the Prinsta

Member, Ellis Bay Formation, within 3 m of strata. The two species are considered synonymous on the basis of their rhynchonellid-like shell, strong globosity, reversed lateral profile (i.e., the dorsal valve being larger and more convex than the ventral valve), and relatively strong, angular and simple costae (usually three in the sulcus and four on the fold). GNAMPTORHYNCHOS MANITOBENSIS new species Figures 4.1–4.15, 6, 7, 8 Diagnosis.Medium to relatively large, transversely extended, strongly dorsibiconvex shell of Gnamptorhynchos with relatively long hingeline and interareas, relatively fine and numerous costae, septalium-like notothyrial platform supported by median septum; cardinal process blade-like. Description.Shell medium to large, transversely subquadrate to subelliptical (Fig. 6), with average length 16.0 mm (maximum 20.5 mm), width 19.6 mm (maximum 26.0 mm), and thickness 13.8 mm (maximum 20.3 mm). Lateral profile biconvex to slightly dorsibiconvex. Dorsal fold and ventral sulcus well-developed, both originating from the umbo, becoming wider anteriorly to occupy slightly more than two-fifths of shell width. Hingeline straight, about three-fourths of shell width (Fig. 7). Cardinal extremities rounded. Greatest shell width attained near mid-length. Both ventral and dorsal umbones strongly arched, with curved beaks. Ventral interarea sharply defined, apsacline, attaining maximum height of 2 mm, with open delthyrium; dorsal interarea low

← FIGURE 4—1–15, Gnamptorhynchos manitobensis n. sp., from the Selkirk Member, Red River Formation, Garson Quarry, Manitoba; 1–5, GSC 117739, paratype. dorsal, ventral, lateral, posterior and anterior views of conjoined valves, 33; 6–10, GSC 117740, paratype, dorsal, ventral, lateral, posterior and anterior views of conjoined valves, 32.5; 11–15, GSC 117741, holotype, dorsal, posterior, lateral, ventral and anterior views of conjoined valves, 32.5. 16–20, Original paratype of Gnamptorhynchos inversum Jin, 1989 (5Gnamptorhynchos globatum), GSC 102445, dorsal, ventral, posterior, lateral and anterior views of conjoined valves, 33, Prinsta Member, Ellis Bay Formation, Lousy Cove, Anticosti Island.

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FIGURE 8—Transverse serial sections of Gnamptorhynchos manitobensis n. sp. GSC 117743, paratype, Selkirk Member, Red River Formation, Garson Quarry, southern Manitoba. Note well-developed parallel dental plates, weak septiform cardinal process, and thick secondary deposits at shell posterior. FIGURE 7—Plot of ratios of hingeline width/shell width (W1/W) for Gnamptorhynchos manitobensis n. sp. Sample from the Selkirk Member of the Red River Formation, Garson Quarry, southern Manitoba. W1/W ratio is about 0.9 in typical Platystrophia and falls to 0.4–0.6 in Gnamptorhynchos, but ranges intermediate between the two genera (0.7–0.8) in the new species.

(generally not exceeding 1 mm), orthocline. Costae simple, without intercalation or bifurcation, evenly distributed, with subangular to subrounded crests. Each shell flank bearing 10–12 costae in adult forms but 7–8 in immature ones. Number of costae on fold and in sulcus variable (independent of shell size), ranging from 4–6 and 3–5 respectively. Concentric growth lines well developed, evenly distributed on entire shell surface; imbricating growth lamellae present only near anterior margin of relatively large shells. Teeth small; dental plates thick, subparallel, buried apically into secondary shell thickening to fuse with lateral shell walls, extending anteriorly to become lateral bounding ridges of muscle field (Fig. 8). Ventral muscle field elongate, subquadrate, well impressed in its posterior portion, becoming slightly elevated anteriorly and confined by high bounding ridges. Cardinal process low, thin, ridge-like, confined to posterior part of notothyrial cavity. Secondary shell deposits well developed in umbonal area. Median ridge short, formed by secondary shell thickening, dorsally supporting small, septalium-like notothyrial platform. Brachiophore bases massive, extending anteriorly into platy brachiophore processes. Adductor scars sharply impressed, becoming wider anteriorly, bearing low median ridge, and confined by conspicuous lateral bounding ridges. Etymology.After Manitoba, from where the type specimens came. Types.Five specimens are selected as types: holotype, GSC 117741 (Fig. 4.11–4.15); paratypes, GSC 117739 (immature shell), GSC 117740, GSC 117742 (serially sectioned), and GSC 117743 (serially sectioned). All from the Selkirk Member, Red River Formation, Garson Quarry (508069N, 968429W), Garson, southern Manitoba. Other material examined.Five individuals with conjoined

valves, two dorsal and one ventral valves from the same locality and horizon as those type specimens. Occurrence.Selkirk Member, Red River Formation, Late Ordovician (Maysvillian), southern Manitoba. Discussion.The new species differs from the other two previously known congeneric species, Gnamptorhynchos globatum and G. selliseptalicium, in having a wider hingeline and a greater number of costae on the fold and in the sulcus. The hingeline width/shell width ratios of the new species fall intermediate between those of typical Gnamptorhynchos and Platystrophia (Fig. 7). In the studied collection, six of the 12 well-preserved specimens have six costae on the fold (five in the sulcus), three shells have four on the fold (three in the sulcus), two have five on the fold (four in the sulcus), and one ventral valve has seven costae in the sulcus. G. globatum and G. selliseptalicium usually have four (rarely three or five) costae on the fold and three (rarely two or four) in the sulcus. Internally, Gnamptorhynchos manitobensis differs from the type species, G. globatum, in lacking accessory teeth and sockets and in having a unilobate cardinal process. The cardinal process of the new species is incipient, being a low and short ridge, much like the cardinal process of the type species of Platystrophia, P. biforatus (von Schlotheim, 1820). The new species is assigned to Gnamptorhynchos rather than to Platystrophia on account of its strongly globular shell at adult stage and septalium-like notothyrial platform. Gnamptorhynchos? camerata (Twenhofel, 1928) from the Ellis Bay Formation of Anticosti Island has a relatively large shell with rather numerous costae, but it differs from the new species in having a considerably less transverse shell, more prominently tapering ventral umbo, and intense bifurcation and intercalation of the costae. The internal structures of P.? camerata are not clearly known. ACKNOWLEDGMENTS

R. Elias of the University of Manitoba kindly provided the specimens of Gnamptorhynchos manitobensis from Garson Quarry, southern Manitoba. T. White arranged the loan of types of Gnamptorhynchos globatum and G.? camerata in his custody. The critical comments of P. Sheehan and an anonymous reviewer

JIN AND ZHAN—EVOLUTION OF LATE ORDOVICIAN BRACHIOPOD GNAMPTORHYNCHOS greatly helped improve the clarity of the presentation. This study was funded by a research grant (JJ) from the Natural Sciences and Engineering Research Council of Canada. RZ wishes to acknowledge the partial support by the Innovation Project of Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. REFERENCES

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