A Unique Pterioid Bivalve from the Early Triassic of Utah

The new species Bakevellia? silberlingi is described from a single locality in the upper Thaynes Formation (mid-Spathian) near Salt Lake City. The 133 specimens studied are natural casts of siltstone freed from limestone in the laboratory by acid dissolution. In external form, B? silberlingi resembles inequivalved and auriculate taxa of several pterioid families. However, its peculiar combination of hinge characters precludes unqualified assignment to either an established genus or family. The shallow, troughlike ligament scar extending backward from the beak resembles that of some pteriids, whereas the numerous teeth form a pattern typical of multivincular bakevelliids and their putative duplivincular ancestors. The new form is provisionally assigned to Bakevellia in recognition of the numerous characters it has in common with that familiar genus. The single-locality occurrence and unlikely mode of fossilization of this unusual taxon suggest that a significant amount of the diversity of early Triassic marine faunas may consist of short-lived, geographically isolated taxa awaiting discovery.