The postcranial skeleton of Cerrejonisuchus improcerus (Crocodyliformes: Dyrosauridae) and the unusual anatomy of dyrosaurids.

Isaure Scavezzoni,Valentin Fischer

doi:10.7717/peerj.11222

Abstract

Dyrosauridae is a clade of neosuchian crocodyliforms that diversified in terrestrial and aquatic environments across the Cretaceous-Paleogene transition. The postcranial anatomy of dyrosaurids has long been overlooked, obscuring both their disparity and their locomotive adaptations. Here we thoroughly describe of the postcranial remains of an unusually small dyrosaurid, Cerrejonisuchus improcerus, from the middle-late Paleocene Cerrejón Formation of Colombia, and we provide a wealth of new data concerning the postcranial anatomy of the key dyrosaurids: Congosaurus bequaerti and Hyposaurus rogersii. We identify a series of postcranial autapomorphies in Cerrejonisuchus improcerus (an elliptic-shaped odontoid laterally wide, a ulna possessing a double concavity, a fibula bearing a widely flattened proximal end, a pubis showing a large non-triangular distal surface) as well as functionally-important traits such as a relatively long ulna (85% of the humerus’ length), short forelimb (83% of hindlimb’s length), or thoracic vertebra bearing comparatively large lateral process (with widened parapophysis and diapophysis) along with strongly arched thoracic ribs allowing a more sturdy and cylindrical rib cage. These indicate a more terrestrial lifestyle for Cerrejonisuchus compared to the derived members of the clade. We also built a dataset of 187 traits on 27 taxa, that extensively samples the cranial and postcranial architectures of exemplar crocodyliforms. We analyze these data in via Principal Coordinate Analysis (PCoA) to visualize the postcranial morphospace occupation of Dyrosauridae, Thalattosuchia, and Crocodylia. Our data reveal the existence of a distinctive postcranial anatomy for Dyrosauridae that is markedly distinct from that of crocodylians. As a result, modern crocodylians are probably not good functional analog for extinct crocodyliformes. Postcranial data should also be more widely used in phylogenetic and disparity analyses of Crocodyliformes.

Highlights

Dyrosauridae is an extinct family of neosuchian crocodyliforms that is first recorded in the Campanian–Maastrichtian Shendi Formation of Sudan (Salih et al, 2015)
We chose to follow the terminologies from de Souza (2018) because we found the definition of transverse processes of dorsals too ambiguous for this work on a more basal dyrosaurid; we decided to use the general term of ‘lateral process’ for all bony structures of the vertebrae laterally emerging instead of sporadically using ‘transverse process’ which has a restricted meaning among Crocodylia
Even though some taxa share similar lifestyles, the three clades are clearly separated along the first axis of the Principal Coordinate Analysis (PCoA) meaning that this axis appears strongly influenced by the phylogenetic signal. Whereas this was expected for thalattosuchians, our results indicate that dyrosaurids have a distinctive postcranial anatomy

Summary

Introduction

Dyrosauridae is an extinct family of neosuchian crocodyliforms that is first recorded in the Campanian–Maastrichtian Shendi Formation of Sudan (Salih et al, 2015). Dyrosauridae showed several dispersal episodes during the Late Cretaceous, with at least three exchanges with America (Hastings, Bloch & Jaramillo, 2014; Jouve et al, 2020)

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