The diverse yet not-so-convergent transformations of crocodyliformes to the aquatic realm

Crocodyliformes is a diverse and long-lived clade of archosaurs that thrived in many distinct environments. This ancient diversity is notably embodied by the semi-aquatic to marine clades Dyrosauridae and Thalattosuchia. Dyrosauridae is a clade of neosuchian crocodyliformes that diversified in fluviatile and marine environments across the Cretaceous-Paleogene transition. Thalattosuchia is a clade of marine crocodyliforms which spanned over the Jurassic period and disappeared during the Early Cretaceous. Both clades crossed important extinction events (i.e. Jurassic-Cretaceous boundary for Thalattosuchia; Cretaceous-Paleogene boundary for Dyrosauridae). The postcranial anatomy of both groups has long been overlooked in anatomical descriptions and diagnoses, shrouding the locomotive adaptations that possibly underpinned their aquatic radiations.
We thoroughly surveyed the morphology of the girdle elements of Dyrosauridae, Thalattosuchia, and Crocodylia, and recreated their girdles in three-dimensions using several tens of high-precisions 3D models. We identified osteological correlates for ecology and behaviour a comparative framework. In addition, we performed landmark-based geometric morphometric analyses on girdle elements (193 landmarks on scapulae, 194 on coracoids, 180 on humeri, 262 on ilia, 170 in ischia, 116 on pubes, 200 on femora) within the same three clades.
Dyrosauridae, Thalattosuchia, and Crocodylia appear well separated in our morphospaces. Extinct crocodyliform clades colonizing similar environments appear markedly distinct from one another, indicating the existence of clade-specific features limiting the strength of evolutionary convergence. As such, our work reveals the previously unsuspected potential of postcranial anatomy as an abundant source of phylogenetic characters to assess the relationships within crocodyliformes. Moreover, extant crocodylians do not appear as an optimal functional analogue for every group of extinct crocodyliformes, because they show a restricted range of postcranial morphologies compared to what has been evolved by dyrosaurids and thalattosuchians. Incorporation of postcranial anatomy therefore appears crucial to fully assess the ecology of extinct crocodyliforms and their disparity.