From toad to frog, a CT-based reconsideration of Bufo servatus, an Eocene anuran mummy from Quercy (France)

ABSTRACT

In the 19th century, natural mummies of amphibians were discovered in the Quercy Phosphorites. The specific collection site was never formally reported, which hampers precise dating of these specimens. Still, the name Bufo servatus was erected based on the external morphology of one of the mummified specimens. A tomography of a similarly preserved specimen revealed a preserved skeleton, 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. Like the previous specimen, a subcomplete articulated skeleton was identified in the B. servatus holotype. 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. The few differences between the specimen skeletons highlight ontogenetic and intraspecific variations, making T. gezei a junior synonym of B. servatus and 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 assigned T. servatus to Ranoides, with natatanuran affinities. Using data from this newly described specimen, we tested 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.

ACKNOWLEDGMENTS

We thank D. Germain (CR2P, MNHN) for providing the access to historical specimens and Y. Laurent (MHNT) for the loan and information about the specimens from MNHT. We are grateful to T. Pélissié (Global Geopark UNESCO and Parc Naturel Régional des Causses du Quercy, France) and E. Cassan (granddaughter of a miner of Escamps quarries) for sharing their knowledge on the Quercy region and sites on the search for the putative site of origin of the mummies. We thank A. Ohler (MNHN) for bringing her expertise in nomenclature in the study of the nomenclatural history of Bufo servatus and other taxa named on the basis of the Quercy anuran mummies. We thank D. Dosso (IBPC) and P. Prevost Marcihacy (IRHIS) for fruitful discussion and exchange on the history surrounding the mummies, bringing another angle into this mystery. We warmly thank A. Mazurier (IC2MP, Poitiers University) for supporting the whole tomographic acquisition process and advising on it. We are also grateful to F. Goussard and N. Poulet (CR2P, MNHN) for their help and advice on 3D reconstructions. We are grateful to Z. Roček, J. Jia and a third anonymous reviewer for their useful comments and reviews on the manuscript. The fossil tomography was supported by the Irish grant irc4c5ffcf3b92ac2f8dda1f349764b177e (“The evolution of the integument in terrestrial vertebrates: insights from taphonomy and exceptionally preserved Eocene fossils”) awarded to N. Robin. This study was funded by a grant from the Fondation pour la Recherche sur la Biodiversité (FRB, Paris) constituted by the patronage of A., C. Lemierre; I. Baszanger; J. Tillit and P. Bernard.