the late ordovician and early silurian pentameride ... - BioOne

J. Paleont., 78(2), 2004, pp. 287–299 Copyright q 2004, The Paleontological Society 0022-3360/04/0078-287$03.00

THE LATE ORDOVICIAN AND EARLY SILURIAN PENTAMERIDE BRACHIOPOD HOLORHYNCHUS KIAER, 1902 FROM NORTH CHINA JIA-YU RONG,1 REN-BIN ZHAN,1

AND

JISUO JIN2

Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China 210008, ,[email protected]; [email protected]. and 2Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada, ,[email protected]. 1

ABSTRACT—Holorhynchus giganteus Kiaer, 1902, a common Late Ordovician (mid-Ashgill) pentameride brachiopod in the Baltic region, Kazakhstan, and southern Tien Shan, is documented for the first time from the Badanjilin Formation (mid-Ashgill) of western Inner Mongolia (Alxa block), North China. Serial sections of the Chinese material confirm the presence of a vestigial ventral median septum in the early growth stage of H. giganteus, but the septum becomes embedded in the secondary shell thickening at the adult growth stage. A survey of the type material from Norway and additional material from other regions indicates that the incipient ventral median septum is a much more commonly developed structure than was previously believed. The presence of a well-developed pseudodeltidium in the Tien Shan material of H. giganteus and the absence of such a structure in conspecific material from many other regions require a systematic revision of the generic group. Holorhynchus has rodlike crura (5brachial processes) that do not form flanges at their junctions with the inner hinge plates (5outer plates 5 crural plates) and outer hinge plates (inner plates). This, together with the development of a crude spondylial comb structure, points to its affinity to the Virgianidae rather than to the Stricklandiidae. Holorhynchus can be regarded as a Lazarus taxon because of its absence during the crisis (Hirnantian) and survival (early-middle Rhuddanian) intervals associated with the Late Ordovician mass extinction and its reappearance in Kazakhstan and North China during the Early Silurian (late Rhuddanian-early Aeronian). The mid-Ashgill Holorhynchus fauna, typified by a number of large-shelled pentamerides, was common in the Baltic region, the Urals, Kazakhstan, Tien Shan, Alxa, Qaidam, Kolyma, and east-central Alaska, but largely absent from Laurentia and Siberia (except for Taimyr) in the ancient tropical-subtropical regions. This paleobiogeographic pattern agrees with the general pattern of the Late Ordovician brachiopod provincialism.

INTRODUCTION

KIAER, 1902 is a large and primarily smoothshelled pentameride genus that occurs commonly in the Baltic, Kazakhstan, and Tien Shan regions (e.g., St. Joseph, 1938; Rukavishnikova and Sapelnikov, 1973; Kovalevskii et al., 1991; Hints, 1993). Its distinctive shell morphology and relatively short stratigraphic range in the Late Ordovician make the genus a useful biostratigraphic tool for dating and correlating Rawthyan rocks in Europe and Kazakhstan (Kovalevskii et al., 1991; Brenchley et al., 1997). Its stratigraphic utility is hampered only to a minor degree by its limited recurrence in the Early Silurian (Llandovery) in the Kazakhstan paleoplate and its adjacent blocks (Sapelnikov and Rukavishnikova, 1975). Its large, smooth, and biconvex shell led some early workers to regard Holorhynchus as a transversely extended form of Pentamerus Sowerby, 1813. The lack of a typical pentameroid ventral median septum (usually clearly visible through the semitransparent shell of Pentamerus) and the presence of a minute dorsal cardinalia in Holorhynchus were first recognized by Kiaer (1902) as distinctive features among the Late Ordovician and Early Silurian pentameride brachiopods. St. Joseph (1938) was the first to study the shell internal structure of Holorhynchus by means of serial sectioning. Subsequently, Holorhynchus was treated as a virgianid (Amsden and Biernat, 1965) and found to be very common not only in the Upper Ordovician strata of the Baltic region but also in the Upper Ordovician and/ or Lower Silurian rocks of Kazakhstan and the Tien Shan region. Sapelnikov and his colleagues (e.g., Nikiforova and Sapelnikov, 1973; Sapelnikov and Rukavishnikova, 1975) provided further important information on the shell structures of Holorhynchus. Despite numerous previous studies, Holorhynchus requires further investigation partly because it is one of the small number of Late Ordovician-Early Silurian pentamerides that have been reported to lack a ventral median septum. In early works (e.g., St. Joseph, 1938), the shell structures in transverse sections tend to be illustrated using simple outlines or solid black fills, without distinction between the lamellar and the prismatic shell layers. As a result, it has been difficult to identify detailed or minute shell structures embedded in the thick prismatic layer or secondary thickening at the shell posterior. Serial sections and regular thin

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sections prepared for the present study indicate that Holorhynchus giganteus Kiaer, 1902, the type species of the genus, possesses a vestigial ventral median septum in small shells or at the early growth stage of adult shells. This has important implications for interpreting the evolution and taxonomic position of Holorhynchus in the family Virgianidae because it is not clear whether or not such an incipient ventral median septum is consistently developed in the type material of Holorhynchus from Norway. St. Joseph (1938, p. 296, footnote) insisted on the complete absence of a ventral median septum, whereas the structure appears to be present in a specimen illustrated by Cocks (1982, p. 776, pl. 83, fig. 13). Also, St. Joseph (1938) regarded Holorhynchus to be closely related to Stricklandia Billings, 1859 on the basis of their dorsal cardinalia, although he formally assigned Holorhynchus to the Pentameridae. Cocks (1982) carried the argument further by assigning Holorhynchus to the Stricklandiidae. As will be discussed in this paper, our data indicate that the genus has the closest affinity to the Virgianidae. Another area of confusion over the shell structure of Holorhynchus is the development of a pseudodeltidium, which has been found so far only in some Late Ordovician forms of H. giganteus reported from Tien Shan by Nikiforova and Sapelnikov (1973). Data available hitherto suggest that H. giganteus was the only large-shelled Late Ordovician pentameride that survived the Hirnantian mass extinction event to reappear in the Early Silurian. More detailed morphological and taxonomic study of Holorhynchus is critical for determining whether or not H. giganteus was a true Lazarus species and, if so, to locate a possible extinction refugium during the Hirnantian age. In this study, we attempt to contribute new data and observations regarding this interesting pentameride genus. MATERIAL AND STRATIGRAPHIC SETTINGS

The specimens used in this study were collected in two lots: one from the Upper Ordovician and the other from the Lower Llandovery (Fig. 1). The Upper Ordovician material of Holorhynchus giganteus was collected in 2002 from the Badanjilin Formation (mid-Ashgill) of Danmianshan in the Badanjilin Desert, western Inner Mongolia. The outcrop area, about 45 km south of

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FIGURE 2—Stratigraphic nomenclature of the Upper Ordovician and Lower Silurian successions of Danmianshan, Inner Mongolia, and Zhaohuajing, Ningxia, Northwest China.

FIGURE 1—Location maps of Holorhynchus occurrences in Upper Ordovician and Lower Silurian rocks of Northwest China. 1, Map of China, with inset enlarged as 2 to show the study area. 2, Detailed map of the two study areas, with the top left inset enlarged as 3, and the lower right inset enlarged as 4. 3, Upper Ordovician locality (solid triangle) at Danmianshan, Inner Mongolia. 4, Lower Silurian locality (solid square) at Zhaohuajing, Tongxin County, Ningxia.

Yagan and 135 km southeast of the town of Ejin County, is located in the northeastern part of the Paleozoic Alxa Block (Chen and Rong, 1992). The Badanjilin Formation (Fig. 2) consists mainly of a 300– 500 m thick succession of shallow marine, siliciclastic strata with carbonate interbeds, suggesting the Badanjilin Desert being an epeiric sea during the Late Ordovician. Holorhynchus occurs in the coquinitic-crinoidal wackestone and packstone facies of the formation, with well-preserved peloid grains common inside some of the shells (Fig. 3.5). Most shells of Holorhynchus giganteus in the Badanjilin Formation are broken and disarticulated, suggesting their deposition under high-energy water conditions. The species occurs in a low-diversity brachiopod assemblage, including H. giganteus (114 specimens), Altaethyrella Severgina, 1978 (108), Ovalospira Fu, 1982 (58), Leptellina Ulrich and Cooper, 1936 (9), Sowerbyella Jones, 1928 (6), Pectenospira Popov, Nikitin, and Sokiran, 1999 (1), atrypide (1), and orthide (1). Calculation of the above relative abundance data using the PAST software (Hammer et al., 2002) indicates that the brachiopod species diversity attains a notably low Shannon index value of 1.3.

O (Ni/N)ln(Ni/N)

Shannon index H 5 2

where Ni is the number of individuals of the ith species in a sample, N is the total number of individuals of all species in the sample; a high-diversity assemblage would have a Shannon index value greater than 2.5. Other fossil groups consist of relatively abundant solitary rugose corals and relatively small colonies of tabulate corals (e.g., Agetolites Sokolov, 1955), rare trilobites and nautiloids, and common crinoid ossicles (Zheng et al., 1987). The sample, however, was collected by means of hand-picking from weathered surfaces and probably carries a certain degree of collection bias. A combination of abundant shelly-crinoidal bioclastic grains, spherical peloid grains inside relatively well-preserved shells, common corals of relatively small size, and a low to moderate diversity of brachiopods suggests a depositional setting between normal and maximum storm wave bases (i.e., 30–70 m). The Holorhynchus giganteus assemblage from the Badanjilin Formation, therefore, can probably be assigned to Benthic Assemblage 3 (sensu Boucot, 1975). The Holorhynchus giganteus-bearing beds are correlative to the Badanjilin Formation in the No. 4 Section nearby (Zheng et al., 1987), where the occurrence of a Staurocephalus clavifrons Angelin, 1854 trilobite fauna indicates a mid-Ashgill age (Zhou and Zhou, 1982). Neither graptolites nor Holorhynchus have been found in the No. 4 Section. The type material of the Lower Silurian specimens of Holorhynchus sinicus Fu, 1982 was collected initially by Zhou Zhiqiang in 1978 from the upper part of the Zhaohuajing Formation (upper Rhuddanian–lower Aeronian) of central Ningxia, North China, and the specimens were subsequently described by Fu

RONG ET AL.—ORDOVICIAN-SILURIAN HOLORHYNCHUS FROM NORTH CHINA (1982). In 1995, two of the present authors (JYR and RBZ) carried out further field work in the area and collected topotype material (field collection NX6). The Holorhynchus sinicus-bearing strata of the Zhaohuajing Formation are exposed in the Zhaohuajing area (Fig. 1), north of the town of Tongxin County, southern Ningxia. In addition, Holorhynchus sinicus occurs in the largely coeval strata at Yezhugou, 20 km south of the town of Zhongning County in central Ningxia. The Zhaohuajing Formation is composed of dark gray to black, argillaceous, nodular, bioclastic limestone with calcareous sandstone and siltstone in the basal part. Gao (1987) and He et al. (1987) reported an association of Holorhynchus sinicus with rugose corals (e.g., Brachyelasma irregulare He in Kong and Huang, 1978 and Tongxinophyllum tongxinensis Gao, 1987), tabulate corals, and a few stropheodontid and atrypide brachiopods. Below the Holorhynchus sinicus-bearing horizon, a stratigraphic unit of dark gray, yellowish gray, or purple gray, argillaceous, bioclastic or nodular limestones yielded a low-diversity and highabundance brachiopod assemblage characterized by Eospirifer dasifiliformis Fu, 1982, Dolerorthis Schuchert and Cooper, 1931, and Meifodia Williams, 1951. Other fossil groups include diverse rugose corals, some tabulate corals, nautiloids, gastropods, and trilobites. At the Yezhugou section (about 40 km north of the type area of Holorhynchus sinicus; Fig. 1) of the Zhaohuajing Formation, H. sinicus occurs in the argillaceous limestone of Bed 7 (31.4 m, upper Zhaohuajing Formation). In a thin bed (25 cm) of intercalated calcareous shale of this stratigraphic unit, the presence of a graptolite association, represented by Pseudoclimacograptus hughesi (Nicholson, 1867), Normalograptus miserabilis (Elles and Wood, 1906), and N. minutus (Carruthers, 1868), suggests a late Rhuddanian-early Aeronian age (Gao, 1987; He et al., 1987). The Holorhynchus sinicus-bearing horizon is overlain by the Hanxia Formation of siliciclastic facies (sandstone and siltstone) and is underlain by the lower Zhaohuajing Formation of carbonate rocks with interbedded siliciclastic mudstone. A total of 14 broken ventral valves and a single incomplete dorsal valve of Holorhynchus sinicus was collected, and the shells range from very small (8 mm in width) to very large (78 mm). The Holorhynchus sinicus brachiopod assemblage is virtually monospecific, except for a single additional specimen of eostropheodontid and a trilobite. TAXONOMIC SIGNIFICANCE OF THE VENTRAL MEDIAN SEPTUM IN PENTAMEROIDS

In the majority of pentameride genera, the spondylium is supported by a median septum and, in comparison, the lack of a ventral median septum is unusual and considered a distinctive feature for some of the pentamerides. The archetypal pentameride, Pentamerus, has a median septum extending between one-third to two-thirds of the shell length, as do many other genera (e.g., Pentameroides Schuchert and Cooper, 1931, Brooksina Kirk, 1922, and Tcherskidium Nikolaev and Sapelnikov, 1969). In many other pentameride groups, however, the length of the median septum is highly variable. Several genera (e.g., Kirkidium Amsden, Boucot, and Johnson, 1967 and Pentamerifera Khodalevich, 1939) possess the longest median septum, which extends almost to the anterior shell margin. (For examples of various median septum lengths, see Kirk, 1922, 1925; Nikolaev and Sapelnikov, 1969; and Sapelnikov, 1976, 1985a.) There are also pentamerides (e.g., Kulumbella Nikiforova, 1960, Microcardinalia Boucot and Ehlers, 1963, and Galeatellina Sapelnikov and Rukavishnikova, 1976) that have a short spondylium and median septum restricted to the posterior one-fifth to one-fourth of the shell. Towards the other end of the spectrum, several genera (e.g., Eoconchidium Rozman, 1967, Harpidium Kirk, 1925, and Vosmiverstum Breivel and Breivel, 1970) bear a short median septum confined mainly

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to the umbonal area and the spondylium is largely free anteriorly (Nikiforova and Sapelnikov, 1971; Rong and Yang, 1981). Within the 80 or so genera of the suborder of Pentameridina, there are eight genera that have been reported to lack a ventral median septum (Boucot et al., 2002). These genera are scattered across two superfamilies: the Stricklandioidea and the Pentameroidea. Within the former there occur three such genera: Aenigmastrophia Boucot, 1971, Spondylostrophia Sapelnikov and Rukavishnikova, 1975, and Stricklandiella Sapelnikov and Rukavishnikova, 1973, all of Llandovery age. Several pentameroid genera also lack a ventral median septum (e.g., Eokirkidium Khodalevich and Sapelnikov, 1970, Vagranella Sapelnikov, 1960, and Subriana Sapelnikov, 1960), all of which are Late Silurian (Ludlow) in age except for Holorhynchus (Boucot et al., 1971; Sapelnikov, 1985b; Kovalevskii et al., 1991). Data presented in this paper, of course, preclude Holorhynchus from this group (see more detailed discussion under systematic description of the genus). Distribution of the genera without a ventral median septum across different pentameride superfamilies suggests that a free spondylium is polyphyletic in origin. The first occurrence of such a septumless condition is found in the three stricklandioid genera during the Llandovery Epoch, and the second occurrence in the pentamerids and subrianids mainly during the Ludlow. So far, a completely free spondylium has not been found in the Clorindoidea and Gypiduloidea, although some genera in these two superfamilies may have a very short median septum. In terms of autecology, those genera that lack a ventral median septum almost invariably have a transversely extended, subelliptical to subtriangular shell (especially the dorsal valve), usually with broad costae or plicae (e.g., Spondylostrophia, Vagranella, Eokirkidium, and Subriana). It can be postulated that such shells required a weaker muscular force for opening or closing than did smooth and elongate shells (longer leverage from the hinge axis to the shell gravity center) with a rectimarginate commissure (which would require a wider opening to achieve the same open area compared to shells with a zigzag commissure). In other words, shells with a reduced shell length probably did not need a median septum to fortify the muscle attachment site on the spondylial floor. It should be pointed out that, in some taxa, such as Vagranella diversoplicata Sapelnikov, 1960 and Stricklandiella robusta (Rong and Yang, 1981), the spondylium is supported posteriorly by thickened shell substance, even though the median septum is absent. The shell thickening would have served the same supporting function in a weakly biconvex shell as would a median septum in moderately to strongly convex shells. In a few pentameroids, the spondylium may be either free or supported by a very short (apical) median septum. The presence or absence of a short or incipient median septum can often be observed in the same population of a species. As will be discussed later in this paper, Holorhynchus appears to fall into this category. Similar examples of Silurian age include Mariannaella Sapelnikov and Rukavishnikova, 1975 (Llandovery); Virgianella Nikiforova and Sapelnikov, 1971 (middle Llandovery); Pseudoconchidium Nikiforova and Sapelnikov, 1971 (middle Llandovery); and Lissidium Lenz, 1989 (Wenlock). To determine the presence or absence of a ventral median septum, it is crucial to investigate carefully the lamellar shell layer in the apical areas of both young and adult forms, especially using the technique of serial sectioning. CHARACTERISTICS OF THE SPONDYLIUM IN HOLORHYNCHUS

Compared to other genera of the Virgianidae, the spondylium in Holorhynchus is rather short and shallow, which, together with the lack or poor development of a median septum, makes a uniform, single-chambered umbonal cavity (Fig. 3.1–3.3). Also, the

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FIGURE 3—Holorhynchus sinicus Fu, 1982 from the Zhaohuajing Formation, lower Llandovery, Tongxin, Ningxia; 1, 2, NIGP135533, thin section of arched apical area of ventral valve showing U-shaped spondylium and lamellar layer of median septum (msl) embedded in shell thickening (msp—lateral boundary of prismatic layer of median septum). Note median septum not long enough to divide umbonal cavity; 3, NIGP135534, slightly oblique longitudinal section cutting through both side walls of spondylium and apical median septum (msl); 4, 5, 6, NIGP135535, thin

RONG ET AL.—ORDOVICIAN-SILURIAN HOLORHYNCHUS FROM NORTH CHINA anterior portion of the spondylium in Holorhynchus curves strongly to the dorsal direction, with the distal end bending slightly backward to the posterior direction in relatively large specimens. Cross sections through the distal end of the spondylium, therefore, become an elongate oval structure (Fig. 3.8; see also St. Joseph, 1938; Fu, 1982). A similar configuration of the spondylium can be found in other virgianids (e.g., Pleurodium, Wang, 1955 and Plicidium Rong and Yang, 1981). In the serial sections prepared for the present study, the spondylial comb structure (s. Jin and Copper, 2000 5 spondylial filament of Rong and Yang, 1977) is observed for the first time in Holorhynchus (Fig. 3.8). The structure is crude and restricted to the spondylial floor at the distal end resembling that of Virgiana Twenhofel, 1914. In terms of shell microstructure, the lack of spondylial saw structure, median groove (common in Pentamerus and Pentameroides), and hook structure (common in stricklandiids; Jin and Copper, 2000) can be regarded as supporting evidence for assigning Holorhynchus to the Virgianidae. The comb structure was originally observed by Rukavishnikova and Sapelnikov (1973) and, subsequently, Rong and Yang (1981) found the comb structure to be best developed near the anterior end of the spondylium in some stricklandiids. The structure typically consists of 10 to 30 parallel ridges along the spondylial floor. Rong and Yang (1981) further pointed out that the comb structure (as small denticles in the transverse sections) is distinctive from the spondylium in its optical property, and they postulated that the structure may have served as attachment sites of dorsal diductor muscles, similar to the filamentous structures in the cardinal pit of certain spiriferides, atrypides, and rhynchonellides. Jin and Copper (2000) noted that the comb structure originated from the secondary thickening layer (myotest) and was commonly buried in this layer. The comb structure has been found in Pleurodium, Clorinda Barrande, 1879, Plicidium (Rong and Yang, 1981), Stricklandia Billings, 1859 (Jin and Copper, 2000), Prostricklandia Rukavishnikova and Sapelnikov, 1973, Proconchidium Rukavishnikova and Sapelnikov, 1973, Microcardinalia, Tcherskidium (Zhan and Cocks, 1998), and Virgiana Twenhofel, 1914 (Jin and Copper, 2000). All these genera appear to be confined stratigraphically to rocks of mid-Ashgill to Llandovery age. The comb structure is associated mainly with taxa that have a shallow spondylium. More work is needed to determine the taxonomic value of this structure, which is absent from the family Pentameridae, although it seems to occur randomly in several other pentameride families. EVOLUTIONARY, PALEOECOLOGICAL, AND PALEOBIOGEOGRAPHICAL IMPLICATIONS

Holorhynchus appears to have spread widely in the warm water regions during the mid-Ashgill. It was commonly associated with abundant corals of relatively high diversity, as recorded in the Badanjilin Formation of Inner Mongolia. So far, Holorhynchus has not been found to associate with the Hirnantia Lamont, 1935 Fauna in the Kosov Province during Hirnantian time (Rong and Harper, 1988; Brenchley et al., 1997). Stratigraphically, Holorhynchus occurs almost invariably immediately below the Hirnantia Fauna (e.g., see Kovalevskii et al., 1991), suggesting that the genus inhabited shallow and warm waters immediately before the Hirnantian mass extinction event.

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After its absence from the early Hirnantian to middle Rhuddanian, Holorhynchus reappeared during the late Rhuddanian-early Aeronian, represented by two species, H. cinghizicus Borissiak, 1955, from Kazakhstan and H. sinicus from North China. The absence of Holorhynchus during the crisis and survival intervals across the Late Ordovician mass extinction event indicates that the genus is a Lazarus taxon (s. Jablonski, 1986). Survival of large or gigantic brachiopod taxa across the glacially induced Late Ordovician mass extinction event (Sheehan, 2001) was a very rare phenomenon because Late Ordovician brachiopod gigantism was common only in deposits of shallow tropical waters and the largeshelled species usually became extinct at the onset of the Hirnantian glaciation and sea level drawdown (Jin, 2001). In future research, it would be interesting to locate the refugium of Holorhynchus and to determine its environmental characteristics during the crisis (Hirnantian) and survival (early-middle Rhuddanian) intervals. During the Late Ordovician (mid-Ashgill, pre-Hirnantian), a pentameride brachiopod fauna characterized by one or more of such relatively large forms as Holorhynchus, Proconchidium, Tcherskidium, and Eoconchidium, existed in a group of paleoplates including Baltica, Kazakhstan, South China, and their adjacent blocks (e.g., Kolyma, Alaska, Alxa). Holorhynchus was perhaps the most distinctive and most common member of this fauna and it would be appropriate to call this fauna the ‘‘Holorhynchus fauna,’’ although Holorhynchus itself could be absent in some regions (e.g., South China). In contrast, this fauna is virtually absent from the large, tropically located paleoplate Laurentia, except for the occurrence of Holorhynchus, Tcherskidium, and Eoconchidium in the suspect terranes of eastcentral and northeastern Alaska (Blodgett et al., 1987, 1988) and Proconchidium in the Canadian Arctic (Rong et al., 1989). In Siberia, only one occurrence of a Holorhynchus-Tcherskidium association is known from the Taimyr region (Nikiforova, 1989; Cocks and Modzalevskaya, 1997). The complete absence of the Holorhynchus fauna from Gondwana as well as peri-Gondwana blocks further supports the interpretation that the fauna lived mainly in tropical warm waters (Fig. 4). The pattern of Ashgill brachiopod provincialism can be traced back to the early Caradoc (Nemagraptus gracilis Biozone) during the major global sea level rise and marine transgression (Jin, 1996). A similar pattern of provincialism has been observed also in some other benthic faunas. The Agetolites coral fauna, for example, has been found to be common in Kazakhstan, Kolyma, Alaska, North China, South China, Qaidam, Zhungar, and other regions, but the coral fauna is similarly missing from Laurentia and Siberia (Lin in Lai et al., 1982). The provincialism was probably not the result of different depositional environments because the Foliomena Havlicek, 1952 Fauna, which was partly coeval to the Holorhynchus fauna but lived in deeper water (BA4–BA6; Sheehan, 1979; Cocks and Rong, 1988; Rong and Harper, 1988), is also absent from Siberia and Laurentia (except for two marginal areas; see Rong et al., 1999). The mid-Ashgill Holorhynchus fauna of shallow and warm tropical waters was replaced in several regions (e.g., Kazakhstan and the Baltic region) by the Hirnantia Fauna of relatively shallow and cool water environment during the Hirnantian glacial interval. The similar absence in Laurentia and Siberia of these three faunas of widely disparate geological

← section of dorsal valve showing smooth junctions of crus with inner hinge plate (ihp) and outer hinge plate (ohp); note inner hinge plates fusing smoothly (not wedged in) with inner surface of valve floor; 7, NIGP135536, cross section showing broad V-shaped spondylium near its midlength; 8, NIGP135537, thin section through distal end of spondylium; note comb structure near recurved tip (top of spondylium).

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FIGURE 4—Paleogeographic distribution of Holorhynchus during midAshgill and early Llandovery times (see Appendix). The genus is largely absent from the large, tropically located Laurentia and Siberia paleoplates as well as from the coldwater Gondwana and peri-Gondwana regions.

and paleoecological settings implies that the Late Ordovician brachiopod provincialism was probably not controlled simply by environmental factors (e.g., water temperature) but rather affected by the relative positions of the paleoplates, sea level stand, ocean currents, and availability of oceanic islands as stepping stones of faunal migration (e.g., Fortey, 1984; Jin, 1996). SYSTEMATIC PALEONTOLOGY

Figured specimens are deposited in the Nanjing Institute of Geology and Palaeontology (NIGP), Nanjing or in the Xi’an Institute of Geology and Mineral Resources (XIGM), Xi’an, China. Order PENTAMERIDA Schuchert and Cooper, 1931 Superfamily PENTAMEROIDEA M’Coy, 1844 Family VIRGIANIDAE Boucot and Amsden, 1963 Subfamily VIRGIANINAE Boucot and Amsden, 1963 Genus HOLORHYNCHUS Kiaer, 1902 Type species.Holorhynchus giganteus Kiaer, 1902. Langa˚ra Formation (5Langa˚ra Shale and Limestone), mid-Ashgill, Oslo region, Norway (St. Joseph, 1938; Cocks, 1982; Brenchley et al., 1997). Diagnosis (emended herein).Shell medium-sized to very large, transversely to longitudinally ellipsoidal to subrhomboidal in outline, nearly equibiconvex to ventribiconvex; usually smooth or with weak, irregular costae; trilobate in some large shells. Spondylium entirely free or supported by minute median septum in apical area (Fig. 3.1–3.3); median septum may be embedded in prismatic layer or shell thickening at adult growth stage. Inner hinge plates (5outer plates 5 crural plates) short, discrete, subparallel to each other; outer hinge plates (5inner plates) slightly longer than inner hinge plates (Figs. 3.4–3.6, 7); crura (5brachial processes) rodlike, circular in cross section, forming smooth junctions with outer and inner hinge plates without flanges. Species assigned.Holorhynchus giganteus, Langa˚ra Formation, mid-Ashgill, Oslo region, Norway (Brenchley et al., 1997);

Holorhynchus giganteus latisulcifer Rukavishnikova and Sapelnikov, 1973, Tolen beds, central and eastern Kazakhstan; Nondia canadensis Boucot and Chiang, 1974, Nonda Formation, Aeronian, Rocky Mountains, northern British Columbia, Canada; Holorhynchus cinghizicus Borissiak, 1955, Alpeis horizon, lowermiddle Llandovery, eastern and southern Kazakhstan. Holorhynchus sinicus Fu, 1982, Zhaohuajing Formation, upper Rhuddanian-lower Aeronian, Ningxia, North China. Species excluded.Holorhynchus lalaensis Sapelnikov, 1963, Wenlock beds, eastern slope of central Urals; excluded from Holorhynchus by Rong and Boucot (1998) because of its triangular and strongly dorsibiconvex shell with laterally directed crural flanges. Holorhynchus tuberosus Sapelnikov, 1963, Wenlock beds, eastern slope of central Urals; trilobate shells with extremely wide and shallow spondylium and prominent, laterally directed crural flanges preclude it from Holorhynchus (Rong and Boucot, 1998). Virgianella sogdianica Nikiforova and Sapelnikov, 1973, from the upper Archalyk Formation (lower-middle Llandovery) of the Zeravshan Range, was assigned to Holorhynchus (Nondia) by Rong and Boucot (1998), but it may be best to retain this species in Virgianella because of its strongly convergent inner hinge plates (forming a U-shaped cruralium) in V. sogdianica and other congeneric species (e.g., Rong and Yang, 1981; Sapelnikov, 1985a). Occurrences.Mid-Ashgill to lower Llandovery. Balto-Scandia, Urals, Kazakhstan, Tien Shan, North China, Alaska, and Kolyma. Discussion.The lack of a ventral median septum has been regarded as one of the diagnostic features of Holorhynchus (Boucot et al., 2002). The common presence of an incipient median septum and the variability of other features of Holorhynchus from the Badanjilin Formation, however, requires emendation of the generic diagnosis. For the Norwegian type material of Holorhynchus giganteus, St. Joseph (1938, p. 296) emphasized the total absence of a ventral median septum, despite that a structure in his serial sections of the apical area superficially resembles a median septum. Cocks (1982, p. 776, pl. 83, fig. 13) illustrated an internal mould of H. giganteus from the Langa˚ra Formation, which showed a minute median septum. A rudimentary ventral median septum in this taxon appears to be present in the apical area of a specimen figured by Boucot et al. (1971, pl. 5, fig. 2), although these authors stated that a ventral median septum is absent. Sapelnikov (1985b, pl. 14, fig. 2) noted a rudimentary ventral median septum embedded in the apical shell thickening in a transverse section of a shell from the Archalyk Formation (Ashgill) of the Zeravshan-Gissar Range of southern Tien Shan. Detailed reinvestigation in the future may prove that a rudimentary median septum is a much more consistent feature in Holorhynchus than was believed in the past. Holorhynchus giganteus from the type area of Norway and most other localities appears to have a completely open delthyrium and lack a pseudodeltidium (e.g., St. Joseph, 1938; Sapelnikov and Rukavishnikova, 1975). Shells of Holorhynchus giganteus reported in this paper also lack this structure. The conspecific material from the Zeravshan Range of southern Tien Shan (Nikiforova and Sapelnikov, 1973; Sapelnikov, 1985a), however, has a well-developed, ventrally concave pseudodeltidium. It is not clear at the present whether or not the presence or absence of a pseudodeltidium is of taxonomic significance at the specific level. In other groups of pentameroids, however, the pseudodeltidium is a consistent structure. Pentamerus and Pentameroides, for example, invariably have a pseudodeltidium similar to that of Holorhynchus giganteus from Tien Shan (Jin and Copper, 2000). Nondia Boucot and Chiang, 1974, from the lower Nonda Formation (upper Rhuddanian-lower Aeronian) of the Canadian Rocky Mountains, resembles Holorhynchus in having a smooth

RONG ET AL.—ORDOVICIAN-SILURIAN HOLORHYNCHUS FROM NORTH CHINA shell and a free spondylium. Boucot and Chiang (1974) distinguished Nondia from all other genera of the Virgianinae by its subrhomboidal shell outline and a median ridge in the ventral valve (mistakenly stated to be in the dorsal valve in Rong and Boucot, 1998). These morphological similarities and differences led Rong and Boucot (1998) to treat Nondia as a subgenus of Holorhynchus. Our investigation of Holorhynchus and Nondia suggests that the ventral median ridge is not sufficiently consistent to be regarded as a diagnostic feature at the generic or subgeneric levels. Thus, it seems that the only differences between Holorhynchus and Nondia are their shell size and outline—Nondia has a medium shell size and an elongate, subrhomboidal shell outline, whereas a large shell size and a transversely extended outline typify Holorhynchus. At the present, Nondia is treated provisionally as a subgenus of Holorhynchus (s. Rong and Boucot, 1998) because the presence or absence of a ventral median septum is not clearly known in the poorly preserved type material of Nondia. Holorhynchus is also similar to Virgianella Nikiforova and Sapelnikov, 1971 in possessing a large and smooth shell, but Virgianella differs in its strongly arched ventral umbo and beak and its inner hinge plates being variously convergent to each other towards the valve floor or forming a cruralium (Sapelnikov, 1985a, pl. 15, figs. 2, 3). An incipient ventral median septum is also visible (wedged into the thickened prismatic layer) in the apical area in shells of Virgianella (Nikiforova and Sapelnikov, 1971, p. 50), although these authors did not regard this structure as a true median septum. The presence of a median septum in Virgianella is corroborated by the presence of a short and strong median septum in the serial sections of V. glabella (Rong and Yang, 1981) from the upper Xiangshuyuan Formation (Aeronian, Llandovery) of northeastern Guizhou, South China. This led Rong and Yang (1981) to emend the diagnosis of Virgianella to include forms with a smooth shell and a short median septum. It should be noted also that each of the crura (brachial processes) in Virgianella forms a laterally directed flange at its junction with the inner hinge plate (similar to the crura of Pseudoconchidium), whereas in Holorhynchus the junctions of crura with the outer or inner hinge plates are relatively smooth without flanges (similar to those of Virgiana). HOLORHYNCHUS GIGANTEUS Kiaer, 1902 Figures 5.1–5.18, 5.22–5.25, 6, 7 Holorhynchus giganteus KIAER, 1902, p. 63, figs. 1–7; SCHUCHERT AND COOPER, 1932, pl. 27, fig. 30; ST. JOSEPH, 1938, p. 292, pl. 4, figs. 1– 5; BORISSIAK, 1955, p. 44, pl. 4, figs. 1, 2; BORISSIAK, 1964, p. 72, pl. 1, figs. 1–4; GAURI AND BOUCOT, 1968, p. 106, pl. 11, figs. 3, 4; BOUCOT ET AL., 1971, p. 277, pl. 5, figs. 1–9; pl. 6, figs. 1–5; NIKIFOROVA AND SAPELNIKOV, 1973, p. 68, pl. 1, figs. 8–10; pl. 2, figs. 1–5; pl. 3, figs. 1, 2; pl. 4, fig. 1; pl. 5, fig. 1; SAPELNIKOV AND RUKAVISHNIKOVA, 1975, p. 47, pl. 7, fig. 1–4; pl. 43, fig. 1; NIKITIN IN APOLLONOV ET AL., 1980, p. 64, pl. 20, fig. 1; COCKS, 1982, p. 774, pl. 83, figs. 12–17; ORADOVSKAYA IN KOREN ET AL., 1983, p. 53, pl. 9, fig. 1; HINTS, 1993, p. 121, pl. 1, figs. 5–7; BEZNOSOVA, 1994, p. 61, pl. 2, figs. 1–7. Holorhynchus cf. giganteus; SAPELNIKOV AND BEZNOSOVA, 1980, p. 4, pl. 1, figs. 1–5. Holorhynchus ex gr. giganteus; NIKIFOROVA, 1989, p. 80, pl. 2, figs. 1–3. Holorhynchus giganteus latisulcifer RUKAVISHNIKOVA AND SAPELNIKOV, 1973, p. 90, pl. 1, figs. 1–14; pl. 2, figs. 9, 10; SAPELNIKOV AND RUKAVISHNIKOVA, 1975, p. 49, pl. 8, figs. 1–14; pl. 9, figs. 1–5.

Description (Inner Mongolian material).Shell small to very large, with many small to medium-sized specimens being transversely subelliptical in outline, weakly to moderately ventribiconvex (Fig. 5); outer shell surface usually smooth, but faint costae maybe present in relatively large shells. Concentric growth lines

293

commonly developed, irregular in strength and spacing, interrupted by irregular growth lamellae in anterior and lateral areas of large individuals (Fig. 5.25). Narrow and shallow medial furrow clearly defined in both valves (Fig. 5.13, 5.16, 5.17). Ventral palintrope uniformly curved and well developed; ventral umbo and beak relatively low, strongly curved, arched above dorsal umbo. Delthyrium open, without pseudodeltidium. Anterior margin rectimarginate. Spondylium broadly V- to U-shaped (Fig. 5.7, 5.8), confined to posterior one-fourth of shell length; distal portion of spondylium bending strongly toward dorsal direction, with its distal end curving slightly backward to posterior. Median septum present, well-defined in small shells, dividing umbonal cavity into two minute chambers (Fig. 6, 0.2–0.3 mm from apex), becoming embedded in apical shell thickening in large shells. Inner hinge plates short, subparallel to each other in cross section (Figs. 5.4, 7), extending anteriorly for one-seventh to onefifth of shell length. Outer hinge plates triangular in shape, robust, equal to or slightly longer than inner hinge plates in length. Crura rodlike, subcircular in cross section, forming smooth junctions (without flanges) with outer and inner hinge plates, with anterior ends unsupported by outer or inner hinge plates (Fig. 7). Low and thin median ridge present in some specimens, extending to midlength of valve in some large specimens. Material examined.One hundred twenty-four specimens from the Badanjilin Formation (mid-Ashgill), Danmianshan, Yagan, Ejin Banner, western Inner Mongolia. Most specimens are incomplete and disarticulated, and shell dimensions can be measured for only seven specimens (Table 1). There are many larger specimens that cannot be measured because of their incompleteness. Figured specimens: NIGP135526, NIGP135527, NIGP135528, NIGP135529, NIGP135530, NIGP135531, NIGP135532, NIGP135546, and NIGP135547 (last two serially sectioned; see Table 1 and Figs. 6, 7). Discussion.Our investigation of Holorhynchus giganteus from the Baltic region, Kazakhstan, Tien Shan, and North China revealed a number of morphological variations that may have taxonomic implications: 1. Ventral median septum. As discussed in previous sections, the ventral median septum is present at least in some specimens of the mid-Ashgill Holorhynchus giganteus from Norway, Tien Shan, and North China and in the Llandovery H. sinicus from North China. 2. Pseudodeltidium. So far, a well-developed pseudodeltidium has been found only in the Tien Shan material assigned to H. giganteus by Nikiforova and Sapelnikov (1973), although the type material from Norway appears to lack this structure (St. Joseph, 1938). A restudy of the Norwegian forms by detailed serial sectioning will be crucial for assessing the variability of this feature among different populations of Holorhynchus giganteus. 3. Median ridge. A median ridge may be present on the inner surface of some large dorsal valves of the Inner Mongolian material and in some specimens from the Norwegian type area (see Boucot et al., 1971, pl. 6, fig. 1; Cocks, 1982, pl. 83, fig. 14). A reassessment of the consistency of this structure in the Norwegian type material of H. giganteus will be important for determining the validity of other Holorhynchus species. 4. Median furrow. A narrow and shallow median furrow of variable relief can be seen on both valve surfaces in some of the Inner Mongolian material (Fig. 5.16, 5.17) and on a dorsal valve from Norway (Boucot et al., 1971, pl. 5, fig. 5). Such a median furrow is regarded as one of the diagnostic characters of H. giganteus latisulcifer from mid-Ashgill rocks of

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295

FIGURE 6—Serial sections of conjoined shell of Holorhynchus giganteus, NIGP135546, Badanjilin Formation, mid-Ashgill, Inner Monglolia. Note clearly defined median septum in apical area (0.2–0.3 mm from apex).

southern Kazakhstan (Rukavishnikova and Sapelnikov, 1973), although the furrow may vary from prominent to inconspicuous in the Kazakhstan species. Thus, the wide range of variation in the median furrow development implies that the structure is not reliable for species-level classification. 5. External ribbing. Although Holorhynchus usually has a smooth shell, irregular, faint to moderately developed radial ribbing is not uncommon in H. giganteus (St. Joseph, 1938, pl. 4, fig. 1; Nikiforova and Sapelnikov, 1973), H. latisulcifer (Rukavishnikova and Sapelnikov, 1973, pl. 1, fig. 1), and H. sinicus (Fig. 5.26). In summary, a detailed restudy of the type material of H. giganteus and its comparison with conspecific material from other regions with respect to the five variable characters discussed above are important not only for clarifying the confusions over Holorhynchus taxonomy, but also for determining whether or not H. giganteus is a Lazarus taxon at the species level. HOLORHYNCHUS SINICUS Fu, 1982 Figure 5.19–5.21, 5.26, 5.27 Holorhynchus sinicus FU, 1982, p. 135, pl. 38, figs. 1, 2; FU, 1985, p. 93, pl. 1, figs. 4, 5.

Types.Syntypes, XIGM B910 and XIGM B911. Zhaohuajing Formation (upper Rhuddanian-lower Aeronian), Tongxin County, Ningxia, North China. Fu (1982) selected two syntypes but did

not designate a holotype. Specimen B910 is a large ventral valve (for measurements see Table 2) and is selected herein as the lectotype (Fig. 5.26, 5.27) and B911 as a paralectotype (Fig. 5.19– 5.21). Other material examined.Approximately 15 specimens collected by Rong and Zhan from the type locality in 1995; most specimens are incomplete ventral valves. Figured hypotypes: NIGP135533, NIGP135534, NIGP135535, NIGP135536, and NIGP135537. Discussion.Fu (1982) described Holorhynchus sinicus from the upper Zhaohuajing Formation (upper Rhuddanian-lower Aeronian) of Ningxia, North China. Reinvestigation of the type and topotype material of Fu’s species suggests its close affinity to H. giganteus in shell size, outline, and weak costae in large forms. Fu (1982) separated H. sinicus from H. giganteus on the basis that the spondylium is less strongly curved to the dorsal direction in H. giganteus than in H. sinicus, although this is most likely a result of different orientations of the transverse sections. In the present paper, the spondylial curvature is not considered to be of any taxonomic significance. Some large shells of H. sinicus have pronounced trilobation, which is common in the Early Silurian Pentamerus but usually absent or poorly developed in the Late Ordovician Holorhynchus of the Baltic region. On the other hand, the degree of consistency of such structures as the ventral median septum, pseudodeltidium, and the median ridge inside

← FIGURE 5—1–18, 22–25, Holorhynchus giganteus Kiaer, 1902 from the Badanjilin Formation, mid-Ashgill, Danmianshan, Inner Mongolia; 1–5, NIGP135525, dorsal, ventral lateral, posterior, and anterior views of immature shell, 32; 6–10, NIGP135526, dorsal, ventral, lateral, posterior, and anterior views of small shell; note open delthyrium, 32; 11–15, NIGP135527, ventral, lateral, dorsal, posterior, and anterior views of relatively small shell, 32.5; 16–18, NIGP135528, dorsal, ventral, and lateral view of medium-sized shell with damaged posterior, 32; 22, NIGP135529, lateral view of posterior fragment, 32.4; 23, NIGP135530, exterior of ventral valve with cracked surface, 32; 24, NIGP135531, exterior of ventral valve, 31.9; 25, NIGP135532, lateral view of weathered shell, 31.8. 19–21, 26, 27, Holorhynchus sinicus, Zhaohuajing Formation, lower Llandovery, Tongxin, Ningxia; 19–21, paralectotype, XIGM-B911, dorsal, lateral, and posterior views of relatively small shell, 31.5; 26, 27, lectotype, XIGM-B910, ventral and lateral views of large ventral valve, 31.

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FIGURE 7—Serial sections of dorsal valve of Holorhynchus giganteus, NIGP135547, Badanjilin Formation, Danmianshan, Inner Mongolia. Note that the distal portions of the crura were not preserved in the disarticulated dorsal valve.

each valve is not fully understood in H. giganteus from the Norwegian type area. Pending further detailed investigation, therefore, H. sinicus is retained provisionally as a separate species. Should it be confirmed that the Norwegian forms are completely devoid of a pseudodeltidium but have a consistently developed incipient median septum, H. sinicus would be considered a junior synonym of H. giganteus, but the Tien Shan forms of H. giganteus with a pseudodeltidium (Nikiforova and Sapelnikov, 1973) would require systematic revision. ACKNOWLEDGMENTS

Z.-C. Zheng of the Geological Surveying Team of Ningxia Autonomous Region, Z.-G. Zhou of the Xi’an Institute of Geology and Mineral Resources, X. Chen, H.-K. Xu, and Y. Wang of the Nanjing Institute of Geology and Palaeontology (NIGP) provided invaluable field assistance. L.-P. Fu of the Xi’an Institute of Geology and Mineral Resources generously gave access to the type material of Holorhynchus sinicus. Z.-Y. Zhou (NIGP) kindly identified the trilobites associated with H. sinicus in the Zhaohuajing Formation. Y.-G. Ren and Z.-G. Chen (NIGP) assisted in preparing the line drawing and photographic illustrations. This research project is funded by the Major Basic Research Project (G2000077700) from the Ministry of Science and Technology of China (Rong and Zhan), Senior Visiting Scientist Fellowship from the Chinese Academy of Sciences (Jin), and the Natural Sciences and Engineering Research Council of Canada (Jin).

REFERENCES

AMSDEN, T. W., AND G. BIERNAT. 1965. Pentamerida, p. 523–552. In R. C. Moore (ed.), Treatise on Invertebrate Paleontology, Pt. H. Volume 2. Geological Society of America and University of Kansas Press, Lawrence. AMSDEN, T. W., A. J. BOUCOT, AND J. G. JOHNSON. 1967. Conchidium and its separation from the subfamily Pentamerinae. Journal of Paleontology, 41:861–867. ANGELIN, N. P. 1854. Palaeontologia Scandinavica, I: Crustacea formationis transitionis Fasc. 2. Samson and Wallin, Lund, 21–92. APOLLONOV, M. K., S. M. BANDELETOV, AND I. F. NIKITIN. 1980. Granitsa ordovika i silura v Kazakhstane. Nauka, Alma-Ata, 300 p. BARRANDE, J. 1879. Systeˆme silurien du Centre de la Boheˆme. Primie`re Partie: Recherches Pale´ontologiques, v. 5, Classe des Mollusques, Ordre des Brachiopodes. Author, Paris, 226 p. BEZNOSOVA, T. M. 1994. Biostratigrafiya i brakhiopody silura Evropeiskogo Severo-Vostoka Rossii. Nauka, Sankt-Peterburg, 127 p. BILLINGS, E. 1859. On some new genera and species of Brachiopoda, from the Silurian and Devonian rocks of Canada. Canadian Naturalist and Geologist, 4:131–135. BLODGETT, R. B., D. M. ROHR, A. G. HARRIS, AND J.-Y. RONG. 1988. A major unconformity between the Upper Ordovician and Lower Devonian strata in the Nanook Limestone, Shublik Mountains, northeastern Brooks Range, p. 18–26. In J. P. Galloway and T. D. Hamilton (eds.), Geologic Studies in Alaska by the U.S. Geological Survey During 1987. United States Geological Survey Circular, 1016. BLODGETT, R. B., K. L. WHEELER, D. M. ROHR, A. G. HARRIS, AND F. R. WEBER. 1987. A Late Ordovician age reappraisal for the upper

TABLE 1—Measurements (mm) of Holorhynchus giganteus from the Badanjilin Formation, Inner Mongolia.

Length Width Thickness Median septum Inner hinge plate length

1

2. NIGP 135546

3

4

5. NIGP 135547

6

7

5.8 6.1 3.4 P 0.8

5.9 7.6 3.4 P 0.8

8.5 8.8* 4.8 P 1.6

13.2* 14.1* 7.5 ? 3.1

14.6 13.8 8.7 ? ?

15.6 18.5* — ? 4

20.5* 25.0* 11.2 ? 5.2

* 5 estimate; P 5 present; ? 5 not visible from shell exterior.

RONG ET AL.—ORDOVICIAN-SILURIAN HOLORHYNCHUS FROM NORTH CHINA TABLE 2—Measurements (mm) of Holorhynchus sinicus from the Zhaohuajing Formation, Ningxia Province. XIGM B910 Length Width Thickness/depth

.68 .77 22 (ventral valve)

XIGM B911 .33 39.4 22.1 (conjoined shell)

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Fossil data for paleobiostratigraphic discussion. Baltica and adjacent blocks.Horizon 5b (Langa˚ra Shale and Limestone), mid-Ashgill, Asker, Norway (H. giganteus Kiaer, 1902; St. Joseph, 1938; Boucot et al., 1971; Brenchley et al., 1997). Mid-Ashgill beds, Sweden (Jaanusson, 1982; Pedersen et al., 1992). Yaptikshor beds, Kyra

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horizon, mid-Ashgill, western slope of Boreal Urals (H. giganteus; Modzalevskaya and Sapelnikov, 1973; Sapelnikov and Beznosova, 1980; Beznosova, 1994). Kabala and Taucionys Formations, Pirgu Stage, mid-Ashgill, Estonia, Latvia, and Lithuania, eastern Baltic region (H. giganteus; H. sp. Paskevicius, 1982; Kovalevskii et al., 1991; Hints, 1993). Kazakhstan and adjacent blocks.Tolen horizon, mid-Ashgill, Chingiz and Tarbagatai ranges, Chu-Illi Mountains, and Dzhungar Alatai (H. giganteus, H. giganteus latisulcifer, Rukavishnikova and Sapelnikov, 1973; Sapelnikov and Rukavishnikova, 1975). Akadombak beds, mid-Ashgill, Chingiz Range, eastern Kazakhstan (H. giganteus; Klenina, 1984). Chashmanklon and Archalyk beds, Ashgill, Zeravshan-Gissar ranges, Tien-Shan (H. giganteus; Nikiforova and Sapelnikov, 1973; Rozman, 1978; Kim et al., 1978). Lower Archalyk beds, mid-Ashgill, Zeravshan Range, Tian-Shan (H. giganteus; Nikiforova and Sapelnikov, 1973). Alpeis horizon, lower Llandovery, Kazakhstan (Chingiz-Tarbagatai and Chu-Ili Mountains) (H. cinghizicus, Borissiak, 1955, 1964; Rukavishnikova and Sapelnikov, 1973; Sapelnikov and Rukavishnikova, 1975). Badanjilin Formation, mid-Ashgill, western Inner Mongolia (H. giganteus, Alxa block; this paper). Zhaohuajing Formation, lower Llandovery, Ningxia (H. sinicus, Alxa block; this paper). Laurentia and adjacent blocks.Nonda Formation, lower Llandovery, northern British Columbia, Canada (Nondia norfordi Boucot and Chiang, 1974). Tirekhtyakh Formation, beds N and O, mid-Ashgill, Omulev Highland, Kolyma (H. giganteus; Oradovskaya, 1983; Kovalevskii et al., 1991). Fossil Creek Volcanic Suite (upper part, Ashgill), east-central Alaska (Holorhynchus n. sp. Blodgett et al., 1987). Siberia and adjacent blocks.Ashgill beds, central Taimyr, Siberia (H. ex gr. giganteus, H. giganteus; Nikiforova, 1989; Cocks and Modzalevskaya, 1997).