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

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.