Reappraisal of the morphology and phylogenetic relationships of the middle Eocene alligatoroid Diplocynodon deponiae (Frey, Laemmert, and Riess, 1987) based on a three-dimensional specimen

ABSTRACT

We describe a three-dimensionally prepared specimen of Baryphracta deponiae from the middle Eocene of Messel (Darmstadt, Germany). Based on a phylogenetic analysis that included the addition of 20 novel scorings for characters previously unavailable for this taxon and the recoding of four additional characters, we found B. deponiae to be nested within Diplocynodon. We propose the new combination Diplocynodon deponiae. The name Baryphracta is thus a junior synonym of Diplocynodon. The small species D. deponiae (∼1 m in total length) shares several features with other species of Diplocynodon, including the presence of two subequal alveoli in the maxilla and dentary, exclusion of the splenial from the symphysis, and the shape of the iliac blade. However, it also differs in a few characters, including the presence of molariform teeth and the extension of osteoderms along the limbs and tail. Such osteodermal cover, which developed very early in ontogeny, easily distinguishes even small-sized specimens of D. deponiae from the co-occurring Diplocynodon darwini. The crocodylian fauna of Messel shows an astonishing diversity including at least seven taxa, with two belonging to the same genus. The two congeners exhibit differences in dentition and size that likely allowed for niche partitioning that minimized competition, thereby allowing them to be syntopic.

ACKNOWLEDGMENTS

A. Folie, in charge of the palaeontology collection management at IRSNB, solved technical problems and assisted M. Delfino while studying the collection under her care. Paul Sartenaer and Huggs De Potter (IRSNB) led the Belgian team during the German-Belgian excavations at Messel in 1985. E. Brahm and S. Schaal (SMF) and M. Blume and N. Micklich (HLMD) granted access to specimens of Diplocynodon deponiae hosted in their institutions. G. Lenglet (IRSNB) kindly provided extant comparative materials. T. Scheyer (Zürich) discussed the origin of osteoderms and provided pertinent literature. P. G. Chiadò-Fiorio (Torino) helped with literature research. The Editor, P. O’Connor, Senior Editor, P. Barrett, and the reviewers, C. A. Brochu and J. E. Martin, significantly improved the quality of the manuscript. Brochu and O’Connor kindly offered their time and energy for linguistic revision and for streamlining the manuscript. M. Delfino acknowledges the support of the Dipartimento di Scienze della Terra dell’Università di Firenze where the early stage of the research was developed. Financial support for this research was received from the Spanish Ministerio de Economia y Competitividad (CGL2011-28681), the Italian MIUR PRIN 2009MSSS9L_002 (to G. Pavia, Torino), and the SYNTHESYS Project (http://www.synthesys.info/), the latter of which is funded by European Community Research Infrastructure Action under the FP6 ‘Structuring the European Research Area’ Programme (FR-TAF 967, BE-TAF 4907 to M. Delfino). This paper is a contribution to Research Project MO/36/020, which is financially supported by the Belgian Science Policy Office.

Handling editor: Patrick O’Connor