The Euro-American genus Eopelobates, and a re-definition of the family Pelobatidae (Amphibia, Anura)

Abstract

The extinct Eopelobates (Eocene of western North America; Eocene–Pliocene of Europe) and Pelobates (Oligocene–Recent of Europe; Recent of northern Africa and the Middle East) are superficially toad-like anurans that are united within the family Pelobatidae mainly on the basis of a unique, tripartite frontoparietal complex. Both genera have a relatively good fossil record consisting of isolated bones, skeletons, and developmental series of tadpoles through adults, all of which are potentially informative for tracing the evolutionary history of the family. Eopelobates is of interest for several reasons. Of the two pelobatid genera, Eopelobates appears earlier in the fossil record (early Eocene vs. late Oligocene) and it is more primitive in lacking many of the features associated with fossoriality in extant Pelobates. The taxonomic composition of Eopelobates has been contentious and at least one putative new species has long been recognised, but never formally named. Here, we provide updated taxonomic accounts for Pelobatoidea, Pelobatidae, Pelobates, and Eopelobates and document development within a series of tadpoles and juveniles of E. bayeri from Bechlejovice (late Oligocene in age), Czech Republic. We also provide updated accounts for the five previously named and currently accepted species of Eopelobates. For the European congeners, E. anthracinus (late Oligocene) and E. bayeri (early Oligocene–middle Miocene) can confidently be regarded as separate species; although the distinction between E. hinschei and E. wagneri (both middle Eocene) is less certain, we provisionally maintain them as separate species. Micro-CT scans for the holotype skeleton of E. grandis (latest Eocene, USA) help resolve some problematic features, most notably showing that the cranial sculpture is of the pit-and-ridge style that is typical for Eopelobates. A sixth congener is named and described based on two skeletons from the middle Eocene portion of the Green River Formation, in Wyoming, USA. We caution that reports of Eopelobates-like anurans from the pre-Eocene of western North America and the early Eocene of India are based on isolated bones that cannot be assigned with confidence to that genus. The presence of Eopelobates in both North America and Europe may be explained by dispersal via the high latitude land bridge that connected those two continents during the late Paleocene through Eocene. The pelobatid fossil record is informative for documenting the nature and timing of changes in cranial features (e.g. ornament patterns, shape of nasals, pattern of frontoparietal–squamosal contact) from the inferred primitive condition seen in most Eopelobates to the more derived condition seen in extant Pelobates, but it is less informative for tracing the evolution of fossoriality, which is a key attribute of extant Pelobates.