A Re-Interpretation of the Eocene Anuran Thaumastosaurus Based on MicroCT Examination of a ‘Mummified’ Specimen

Fabien Laloy, Jean-Claude Rage, Susan E. Evans, Renaud Boistel, Nicolas Lenoir, Michel Laurin

What originally appeared to be only an external cast of an anuran 'mummy' from the Quercy Phosphorites (southwestern France) was described as Rana plicata during the 19th century. Its geographical provenance is only vaguely known; therefore its precise age within the Paleogene was uncertain. The taxon was erected on the basis of the external morphology of the specimen, which includes few diagnostic characters. As a further complication, the name Rana plicata was recently shown to be unavailable at the time of the description, and the name Rana cadurcorum was proposed as a replacement. In order to see whether internal features were fossilized, the fossil was CT scanned. This showed that a large part of the skeleton is preserved. Unexpectedly, the scans revealed that the skull of the mummy is almost identical to that of Thaumastosaurus gezei, another anuran from the late middle or late Eocene of the Quercy Phosphorites. The few observed differences are attributable to intraspecific and ontogenetic variation, and R. cadurcorum is a junior subjective synonym of T. gezei. The mummy is therefore probably from the same time interval as T. gezei. The latter was previously known only by its skull, but the mummy provides important information on the postcranial skeleton. Earlier assessments, based only on the skull, placed Thaumastosaurus close to South American hyloid anurans, but a new phylogenetic analysis including postcranial characters reveals ranoid affinities. This study exemplifies the usefulness of modern imaging technologies that allow non-destructive study of previously inaccessible internal anatomical features.