Evolution of anuran assemblages in the Late Cretaceous of Utah, USA

Abstract

Several hundred isolated anuran bones recovered from 37 localities in southern Utah, USA, provide a relatively continuous record of the evolution of anuran assemblages in the central part of the North American Western Interior that spans almost 25 million years, from the early Cenomanian to the late Campanian. Although it is difficult to associate isolated anuran bones from different parts of the skeleton with each other, it is possible to identify distinctive morphs for certain bones (e.g., ilia, maxillae) that can be used to make inferences about the taxonomic diversity of fossil assemblages. Because the samples document a relatively long interval of time, they can also be used to recognize trends in the anatomical evolution of anurans and in the evolution of anuran assemblages. Small-bodied anurans prevailed until the early Campanian, then beginning in the late Campanian, larger-bodied anurans began dominating assemblages. Using iliac morphs as a proxy for taxonomic diversity, it is apparent that some local assemblages were surprisingly diverse. When coupled with previously reported fossils, the new specimens from Utah help document when certain anatomical features appeared and radiated among anurans. Ilia in the majority of early anurans (including the earliest anuran Prosalirus) had an oblique groove on the dorsal margin but lacked a dorsal tubercle. Through the Late Cretaceous, there is a trend towards an increasing majority of ilia having a well-developed dorsal tubercle; this osteological change could be associated with changes in locomotor behavior. Procoelous vertebrae are already present in the Cenomanian samples, which indicates that this derived anuran vertebral condition must have appeared before the Late Cretaceous.