Sproat and Zhan—A new paleogeographically important rhynchonellide from the Tarim plate 92(6):1005–1017 1013


Figure 7. Internal structures of Altaethyrella tarimensis n. sp. from ATX-402. Distance from the ventral umbo indicated in mm for each section. Dorsal valve uppermost. In this specimen, the dental plates are apparently missing, but dental cavities have likely been infilled by secondary shell growth. Due to the preservational state of the shell, this is difficult to determine.


Some specimens of Altaethyrella tarimensis n. sp. are


similar to an atrypide from the study area to be described in a future study. The atrypides are much more abundant than Altaethyrella, and can usually be differentiated externally from Altaethyrella by their smaller size, lower dorsal convexity, and slightly more rounded outline. However, differentiating between small Altaethyrella and


atrypide shells can be difficult. Small shells of Altaethyrella are also less convex and do not have a clearly subpentagonal outline as in larger shells. The only reliable external feature to differentiate the genera at this stage is the number of ribs in the sulcus and fold —Altaethyrella generally has at least three ribs in the ventral sulcus and four ribs on the dorsal fold with a sharp boundary between the fold or sulcus and shell flanks on each side, while the


atrypides often have a much weaker fold and sulcus, with only a single rib in the sulcus and two ribs on the fold. Altaethyrella also tends to have sharp, angular ribs while the atrypides feature less-prominent rounded ribs. In small and/or poorly preserved material, these characters can be difficult to determine reliably. The differences in internal shell structures are immediately obvious, because the atrypides possess larger crura that diverge toward the anterior and two mediodorsally directed spiralia— rhychonellides do not have skeletal lophophore supports amen- able to preservation in the fossil record. One of the species previously classified as Rhynchotrema


in China mentioned in Zhan and Li (1998), R. pentagonia Fang (in Liu and Fang, 1990) also from the Hadabulaktag Formation in Xinjiang, is likely one of these atrypide forms. Figures show


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