Journal of Paleontology, 92(6), 2018, p. 1005–1017 Copyright © 2018, The Paleontological Society 0022-3360/15/0088-0906 doi: 10.1017/jpa.2018.31


Altaethyrella (Brachiopoda) from the Late Ordovician of the Tarim Basin, Northwest China, and its significance


Colin D. Sproat,1 and Renbin Zhan1,2


1Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China 210008; ⟨csproat@nigpas.ac.cn⟩, ⟨rbzhan@nigpas.ac.cn⟩ 2University of the Chinese Academy of Sciences, Beijing, China 100049


Abstract.—Altaethyrella tarimensis, a new species of rhynchonellide brachiopod, is described from the late Katian (Late Ordovician) Hadabulaktag Formation in the Kuruktag region of Xinjiang, Northwest China on the northeastern edge of the Tarim Basin. Serial sections of the shell clearly show no dorsal median septum or septalium in the dorsal valve, and no spiralia or atrypide-style crura. Like other species of the genus, A. tarimensis n. sp. exhibits a high degree of intraspe- cific variation, including variations in shell shape and size, number of ribs in the sulcus at the anterior, and degree of asymmetry. The discovery of Altaethyrella in Tarim has important paleogeographic implications, indicating a close relationship between the Late Ordovician brachiopod faunas of Tarim and those of the Kazakh terranes and North and South China paleoplates, supporting a recently published paleogeographic projection that places Tarim near the Chu-Ili


terrane during the Late Ordovician. The abundant large biconvex shells of A. tarimensis n. sp. would have provided a firm substrate for encrusting filter feeders like bryozoans to establish on the Kuruktag Platform. UUID: http://zoobank.org/df8843cd-4db0-48e7-ba03-bf0ce81c4f01


Introduction


Altaethyrella is a late Katian (Late Ordovician) rhynchonellide brachiopod found in Central Asia and China. The genus has a typical rhynchonellide form, featuring a biconvex lateral profile, strong dorsal fold and ventral sulcus with a distinct uniplicate anterior, and strong, simple, angular ribs. Shells vary in size and are generally subcircular to subpentagonal in outline (Fig. 1). The genus was established by Severgina (1978) as a lissa-


trypide when she studied the Katian (Ashgill) brachiopods of northwestern Altai, and was systematically revised by Kulkov and Severgina (1989). Subsequently, it has been found elsewhere in Central Asia and China, often first identified as the home- omorphic genus Rhynchotrema. Several key internal shell fea- tures can be used to differentiate these two genera, however. This is the first record of Altaethyrella on theKuruktag Platform on the edge of the Tarim Basin in the Xinjiang region of northwestern China—part of the Tarim paleoplate during the Late Ordovician. During the Late Ordovician, the Tarim paleoplate was


located approximately in tropical latitudes (e.g., Cocks and Torsvik, 2013, fig. 7; Popov and Cocks, 2017, fig. 13), although its precise location is still uncertain. Given the limited paleo- magnetic data available from Tarim, fossils are useful in deter- mining its position in relation to other plates. For example, recently the position of microbial reefs on the carbonate plat- form has been used to suggest a position either slightly north or south of the equator, rotated 90˚ or in a similar orientation to the present (Zhang et al., 2017). The limited paleogeographic dis- tribution of Altaethyrella, along with other elements of the


brachiopod fauna, makes it a valuable paleogeographic tool in determining the relative location of the Tarim plate to the nearby North and South China plates and Kazakh terranes during the Late Ordovician. The primary objective of this study is to systematically


aspects of the significance of Altaethyrella and this new species below. The differentiating features between Altaethyrella and the homeomorph Rhynchotrema as well as atrypides associated with the new species in Tarimare reviewed, focusing on internal shell structures. A high degree of intraspecific variation is noted in the Tarimspecies, but this is also characteristic of other Altaethyrella species (e.g., Li and Zhan, 1998). Shells encrusted by bryozoans


describe a new species of Altaethyrella from the Hadabulaktag Formation (upper Katian) in the Tarim Basin, including basic measurements of shells and serial sectioning to examine key diagnostic structure of the interior of the shell, for comparison with previously described species on other paleoplates. The study provides key material for future taxonomic revisions, given that Altaethyrella was treated as nomia dubium in the latest revision of the Treatise on Invertebrate Paleontology (Savage, 2002a). Data from this investigation are used to discuss several


indicate the important role that Altaethyrella shells played as a firm substrate for filter feeders on the carbonate platform.


Previous studies of Altaethyrella Altaethyrella was first described as an atrypide by Severgina (1978), but was re-classified to the Rhynchonellida by Kulkov


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