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Journal of Paleontology

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

¹ Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China 210008, <jyrong{at}nigpas.ac.cn; rbzhan{at}nigpas.ac.cn>
² Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada, <jjin{at}uwo.ca>

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 (=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.

This article has been cited by other articles:

				J.-Y. RONG, J. JIN, and R.-B. ZHAN TWO NEW GENERA OF EARLY SILURIAN STRICKLANDIOID BRACHIOPODS FROM SOUTH CHINA AND THEIR BEARING ON STRICKLANDIOID CLASSIFICATION AND PALEOBIOGEOGRAPHY Journal of Paleontology, November 1, 2005; 79(6): 1143 - 1156. [Abstract] [Full Text] [PDF]