P. G. Baker

The first thecideide brachiopod from the Jurassic of North America

Restudy of a hardground fauna from the Middle Jurassic of south-western Utah revealed the presence of thecideoid brachiopods. The absence of separated dorsal valves and the effects of abrasion and diagenesis hampered the description of the specimens. However, serial sectioning of complete shells revealed a thecidellinid which in its morphology and shell microstructure resembled Rioultina Pajaud from the Middle Jurassic of Europe. Important morphological differences clearly separate the new specimens from Rioultina and they are assigned to Stentorina sagittata gen. et sp. nov. However, the close approximation of certain shell microstructural features does suggest a phylogenetic link between the two genera. S. sagittata is also compared with the only other known American Jurassic species, Ancorellina ageri Mancenido and Damborenea from the Lower Jurassic of Argentina. Differences in morphological characters currently of family rank preclude the possibility of Stentorina being descended from Ancorellina stock. It is considered, therefore, that dispersal from Tethyan thecideoid populations to America occurred several times.