Osteology and digital reconstruction of the skull of the early tetrapod Whatcheeria deltae

ABSTRACT

The Early Carboniferous stem tetrapod Whatcheeria deltae is among the earliest-branching limbed tetrapods represented by multiple near-complete specimens, making it an important taxon in understanding the vertebrate water-to-land transition. However, all preserved skulls of Whatcheeria suffer from post-mortem crushing and lateral compression, which has made cranial reconstruction problematic. In this study, computed tomography data of three Whatcheeria specimens were segmented using visualization software to digitally separate each individual skull bone from matrix. Digital methods were used to repair and retrodeform the bones and produce the first complete three-dimensional skull reconstruction of Whatcheeria. We provide a revised description of the cranial and lower jaw anatomy of Whatcheeria based on CT data, focusing on sutural morphology and previously unknown anatomical details. Our findings suggest that Whatcheeria had one of the narrowest skulls of any known early tetrapod, a gap between the nasals, and significant overlap of the lacrimal onto the nasal and prefrontal. Sutural morphology is used to infer loading regime in the skull during feeding and suggests the skull of Whatcheeria was well adapted to resist stresses induced by biting large prey with its enlarged anterior fangs.

ACKNOWLEDGMENTS

Many thanks to E. Lombard (University of Chicago) and J. Bolt (FM) for permission to scan specimens of Whatcheeria, and to W. Simpson (FM) and A. Stroup (FM) for access to and assistance with transporting fossil material. We also thank the Field Museum of Natural History for allowing publication of the µCT data. Many thanks to Z.-X. Luo and A. Neander (University of Chicago) for µCT-scanning fossil material. A. Sánchez-Eróstegui and J.-L. Garnier (Thermo Fisher Scientific) provided assistance with Avizo. Many thanks to all of our colleagues in Bristol, especially T. Davies for assistance with lab accessibility and Avizo. Many thanks to B. Otoo (University of Chicago) for interesting discussion on the anatomy and lifestyle of Whatcheeria. This research was funded by NERC Standard Grant NE/P013090/1 (“Skull evolution and the terrestrialization and radiation of tetrapods”) to E.J.R and L.B.P.