During the Late Ordovician, a transgression flooded much of Laurentia. The fauna of these intracratonic basins became differentiated from the fauna of the pericratonic shelves and platforms, typically displaying gigantism and coarser shell ornamentation. In this study, 509 specimens from 11 species of the Plaesiomyidae brachiopod family from the Katian and Hirnantian were measured, of which 198 included in principal component analysis to quantify morphological changes over this interval. Three trends were revealed: 1) increasing globosity and dorsal convexity from the early to late Katian, 2) coarser, but fewer ribs on species from the paleoequatorial intracratonic seas compared to species from the mid-to high-tropical pericratonic shelves and platforms, and 3) enlargement of the cardinal process from early to late Katian and predominance of a trilobed cardinal process in the intracratonic species versus bilobed cardinal process in pericratonic species. Two new species were assigned, and two existing species were reassigned.

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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 intraspecific 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