The smallest-known species of the Late Ordovician rhynchonellide brachiopod Altaethyrella from a reef-stromatoporoid bioherm in Xinjiang, Northwest China
The rhynchonellide brachiopod Altaethyrella Severgina, 1978 is a key component of the shallow-water Altaethyrella brachiopod fauna that flourished in the tropical Proto-Tethys during the Katian (Late Ordovician). This study describes an unusually small species of the genus, Altaethyrella xuejiaoi new species, from the lower Katian (Ka-1) Yingan Formation in northwestern Tarim, Xinjiang, Northwest China. With an average length of 8.60 mm, width of 9.73 mm, and depth of 5.69 mm, it is approximately two-fifths the size of typical Altaethyrella species, although the presence of a central myophragm in the dorsal cavity and well-developed teeth confirms that the specimens of the new species are mature and not simply juvenile forms. This species inhabited the shallow-water reef-stromatoporoid bioherm, providing new insights into the origination and early evolution of the rhynchonellides. Furthermore, this study contributes new generic occurrence data that will improve our understanding of the paleogeographic distribution of the Altaethyrella fauna prior to the Late Ordovician Mass Extinction (LOME).
