From Toad to Frog, a CT-based Reconsideration of Bufo servatus , an Eocene Anuran mummy from Quercy (France)

Abstract

In the XIXth century, natural mummies of amphibians were discovered in the Quercy Phosphorites. The specific collection site has never been formally reported, which hampered precisely dating these specimens. The name Bufo servatus was erected for one these mummies, based on the external morphology of the specimen.The tomography of similarly preserved specimens, originating from this same unknown site, revealed almost complete preserved skeletons, preserved soft-tissues and gut contents. We analyze here the holotype of Bufo servatus using CT-scanning in order to investigate its potentially preserved internal features. As for the previous specimen, a subcomplete articulated skeleton was identified. Surprisingly, this skeleton is almost identical to that of Thaumastosaurus gezei, an Eocene anuran from Western Europe, to which other specimens from this mummy series were previously assigned. Few differences between the specimen skeletons highlight ontogenetic and intraspecific variations, making T. gezei a junior synonym of B. servatus, creating the new combination Thaumastosaurus servatus. Given its association with previously described Quercy specimens, this redescribed anuran is probably from the same time interval as T. gezei. Previous phylogenetic analyses have assigned T. servatus to Ranoides, with natatanuran affinities. Using data from this newly described specimen, we test here further its taxonomic affinities. Our analyses confirm this position, and formally identify T. servatus as a Natatanuran member of Pyxicephalidae (currently endemic of Equatorial Africa) and more precisely, a stem-Pyxicephalinae. This result confirms the origin of Thaumastosaurus, a member of the African herpetofauna occupying Western Europe before the Grande Coupure at the Eocene/Oligocene transition.