LATE ORDOVICIAN AND EARLY SILURIAN PENTAMERIDE BRACHIOPOD HOLORHYNCHUS KIAER, 1902 FROM NORTH CHINA, THE

Journal of Paleontology, Mar 2004 by Rong, Jia-Yu, Zhan, Ren-Bin, Jin, Jisuo

ABSTRACT-Holorhynchus giganleits Kiacr, 1902, a common Late Ordovician (mid-Ashgill) pentameride brachiopod in lhe 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 pscudodeltidium 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. Holorhynchiis has rodlike crura ( = brachial processes) that do not form flanges at their junctions with the inner hinge plates (=outer plates = 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

HOLOKHYNCUUS KIAHR, 1902 is a large and primarily smooth-shelled 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 stratigraphie 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 stratigraphie utility is hampered only to a minor degree by its limited recurrence in the Early Silurian (Elandovery) 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 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 Holorhynclms 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, pi. 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.