Sproat and Zhan—A new paleogeographically important rhynchonellide from the Tarim plate 92(6):1005–1017 1015


Figure 10. Serial sections of Rhynchotrema increbescens (Hall, 1847) and Altaethyrella tarimensis n. sp. showing differences in internal structures of the shell, despite similar external shell morphology (dorsal valve uppermost). Numbers indicate distance in millimeters from the posterior-most point of the shell. Rhynchotrema from Sproat et al. (2014, Fig. 7); Altaethyrella from Fig. 7. herein.


Most shells have three ribs in the sulcus at the anterior


commissure that correspond to four ribs on the dorsal fold. This pattern is similar to the homeomorphic Rhynchotrema. How- ever, whereas Rhynchotrema almost always has only three ribs in its sulcus, Altaethyrella exhibits greater variation similar to the rhynchonellide genus Rostricellula. Some shells of A. tarimensis n. sp. feature as many as four, and rarely five ribs in the sulcus, with five or six corresponding ribs on the fold. These extra ribs commonly cause a deflection of the tongue formed by the prominent dorsal fold and ventral sulcus (e.g., Fig. 5.11). Similar variability was also noted by Zhan and Li (1998) in A. zhejiangensis. Although these extra ribs apparently contribute to asym-


America before extending across the continent by the late Katian (Sproat et al., 2014; Sproat and Jin, 2017). Rhynchotrema has been described from outside Laurentia.


metry at the anterior, there is significant degree of asymmetry in shells that only have three ribs in the sulcus as well. Such asymmetry is apparent in both small and large shells, indicating that it is independent of ontogeny, beginning early in the life of the brachiopod. Like the variation in rib number, this also sets Altaethyrella apart from Rhynchotrema in North America that are almost always symmetrical. This relatively high degree of intraspecific variation could be


caused by deformation of the shells during deposition and diagenesis. However, this may be a feature of Altaethyrella because a high degree of intraspecific variation has been noted in other species fromdifferent paleoplates. Zhan and Li (1998) noted significant variation in shell sizes and shape, number of ribs in the fold and sulcus, andmorphology of the ventral beak, evenwithin a single collection. Nikitin et al. (2006) noted variation in the number of ribs between populations of Altaethyrella otarica,and their shell measurements show significant variation in shell size and shape within single populations.


Paleogeographical significance.—Until Altaethyrella was dif- ferentiated from Rhynchotrema, it seemed as though Rhyncho- trema had a worldwide distribution during the Late Ordovician. It is now known that Altaethyrella was confined to the plates and terranes that comprise modern day Kazakhstan and China. Conversely, Rhynchotrema was mainly limited to Laurentia during the Ordovician, where by the early Katian, it and the closely related Hiscobeccus had colonized eastern North


Nikitin et al. (2003) described R. seletensis from Selety River Basin of Kazakhstan. Laurie (1991) and Percival (1991) noted several species from Australia (from Tasmania and New South Wales, respectively). Detailed serial sections are lacking in the species from Australia, although figures and descriptions show a septalium in these species (Laurie, 1991, figs. 46.6, 46.26, 47.14; Percival, 1991, figs. 18.40, 18.41 as internal molds), indicating that these species probably cannot be classified as Altaethyrella. The species from Kazakhstan also possess a septalium (Nikitin et al., 2003, fig. 6), but the septalium only completely connects the hinge plates in the extreme posterior of the shell, unlike in species from Laurentia where the septalium extends further toward the anterior (e.g., Sproat et al., 2014, figs. 6, 7). These Central Asian and Australian species may, in fact, represent distant relatives of the species from Laurentia where Rhynchotrema is dominant. This geographic separation can be traced back to the early


evolution of these lineages. Early rhynchonellide evolution is not well known, but by the Sandbian (early Late Ordovician), early rhynchotrematoides (such as Rostricella and Rhyncho- trema) and ancistrorhynchoides (of which Altaethyrella is a member) had become established on the margin of Laurentia. By the middle Katian, the ancistrorhynchoides largely went extinct in Laurentia while rhynchotrematoides dispersed throughout the expanding epicontinental seas. On the plates that now comprise Central and Eastern Asia,


however, the ancistrorhynchoides became established in the shallow carbonate platforms and survived into the Devonian in Siberia. Rhynchotrema was rare on these platforms, although the closely related Rostricellula would become established there in the Silurian. This clear paleogeographic division between these two


brachiopod lineages during the Late Ordovician has some utility in paleogeographic studies. The limited geographic range of Altaethyrella re-affirms that Tarim, North and South China, and the Kazakh terranes were located close to one another. The similarities of the specimens in this study to those of Altaethyrella otarica from the Chu-Ili terrane of Kazakhstan (see discussion


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