An anatomical reappraisal of the dwarf crocodylian Arambourgia gaudryi from the Eocene of Quercy (France) using CT data and its implications for the phylogeny and paleoecology of basally branching alligatoroids

ABSTRACT

After about a century, the skull of the holotype and sole specimen of Arambourgia gaudryi from the Eocene of Quercy (France) is here redescribed. Its interest resides in its extreme morphology, such as its very small skull and altirostral condition, that raises questions about its maturity, taxonomic validity, and paleoecology. In order to reveal previously hidden anatomical details, we used computed microtomography (µCT-scan) data to digitally extract the bones of the cranium and mandible, as well as the endocranial structures. Here, we update the osteological description of A. gaudryi, outlining new characters and re-evaluating anatomical details. Comparing it with ontogenetic series of extant crocodylians, we confirm that the specimen is a mature individual of a dwarf species with pedomorphic features. Our phylogenetic analyses indicate that A. gaudryi is an alligatorine. Comparisons with other small alligatoroid taxa show that it is indeed a valid species. The depositional environment of the finding site suggests a continental context with a karstic network. Taking this into account, together with its peculiar anatomy, we propose that A. gaudryi was a semi-terrestrial animal occupying an ecological niche similar to that of extant juvenile or dwarf crocodylians. Finally, we discuss the current knowledge of the paleobiogeography of Alligatoroidea in Europe during the Paleogene. Arambourgia gaudryi was likely part of a group of small early globidontans that arrived in the Old World, probably from North America, around the end of the Paleocene and that dispersed to at least four localities of the ancient European archipelago.

ACKNOWLEDGMENTS

The authors thank G. Escarguel and G. Perrichon (UCBL) and T. Pellisié for advice and discussion, R. Allain and L. Cazes (MNHN) for access to the specimen and photographs, and also C. Salaviale (UCBL) for technical support. We warmly thank V. Perrier (UCBL) for creating the drawings of Figure 1. The authors also thank R. Allain, F. Goussard, and N.-E. Jalil (MNHN), T. van de Kamp and M. Zuber (SMNK), M. Mouana and A.-L. Charruault (UM), D. Berthet (MHNL), B. Bartschi and E. Robert (UCBL), P. Campbell and S. Maidment (NHMUK), and M. Maillart (ENS Lyon) for access to extant specimens. A special thank-you is also given to F. Quillévéré and G. Cuny (UCBL) for their precise and useful revision of an initial version of this manuscript, written as DC’s final report for his Master’s thesis. This work was supported by the Agence Nationale de la Recherche (SEBEK project no. ANR-19-CE31-0006-01). We thank the phylogenetic editor P. Godoy, one anonymous reviewer, and P. Burke for their constructive comments that helped improve the last version of this work.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author(s).

AUTHOR CONTRIBUTIONS

JEM and YPC designed the project, JA performed the CT data acquisition, DC and NR segmented the CT data and exported 3D models, DC performed the descriptive work and performed the phylogenetic analyses under the supervision of JEM and YPC, DC drafted the manuscript, all authors edited the manuscript and agreed on its final version.

SUPPLEMENTARY FILES

Supplementary File 1: 3D model of the skull of Arambourgia gaudryi.

Supplementary File 2: 3D model of the cranial pneumaticity of Arambourgia gaudryi.

Supplementary File 3: 3D model of the endocast and associated structures of Arambourgia gaudryi.

Supplementary File 4: 3D model of the inner ears of Arambourgia gaudryi.

Supplementary File 5: List of the specimens studied along with their morphologic measurements and ratios for the ontogenetic assessments.

Supplementary File 6: Supplementary anatomical figures and table.

Supplementary File 7: Additional data about the phylogenetic analyses.

Supplementary File 8: Modified version of the Rio & Mannion (2021) matrix used in NEXUS format.

Supplementary File 9: Modified version of the Rio & Mannion (2021) matrix used in TNT format.

Supplementary File 10: Godoy et al. (2021) matrix used in NEXUS format.

Supplementary File 11: Godoy et al. (2021) matrix used in TNT format.

Supplementary File 12: All trees found using the Heuristic Search protocol and the modified matrix of Rio & Mannion (2021).

Supplementary File 13: Consensus tree found using the Heuristic Search protocol and the modified matrix of Rio & Mannion (2021).

Supplementary File 14: All trees found using the New Technology Search protocol and the modified matrix of Rio & Mannion (2021).

Supplementary File 15: Consensus tree found using the New Technology Search protocol and the modified matrix of Rio & Mannion (2021).

Supplementary File 16: All trees found using the New Technology Search protocol and the matrix of Godoy et al. (2021).

Supplementary File 17: Consensus tree found using the New Technology Search protocol and the matrix of Godoy et al. (2021).