Aquatic adaptation, cranial kinesis, and the skull of the mosasaurine mosasaur Plotosaurus bennisoni

ABSTRACT

New anatomical observations of the holotype skull of Plotosaurus bennisoni from the Maastrichtian Moreno Formation of California, U.S.A., are used as a framework to examine cranial kinesis in derived members of the Mosasaurinae. Enlarged posteromedial flanges of the frontal and extensive lateral contacts of the prefrontal and postorbitofrontal contributed to increased rigidity along the frontoparietal suture (the mesokinetic joint). Sutural contacts of the parietal with the supraoccipital posteriorly and the prootic ventrally would have restricted metakinetic movements. Furthermore, the unusual shape of the epipterygoid, and its dorsal contact with the prootic and parietal, shows that the epipterygoid and pterygoid were probably not capable of anteroposterior movements. Most strikingly, Plotosaurus exhibits a tight association of the quadrate with the temporal arcade, suggesting that streptostyly was limited or lost in this derived mosasaurine, the loss of such a feature having never been described in a mosasaur. These charcteristics are placed in a functional context to examine aquatic adaptations in mosasaurs. As one of the most specialized mosasaurs known, the loss of cranial kinesis may have evolved as a result of its piscivorous diet.

ACKNOWLEDGMENTS

We would like to thank T. Konishi for photographs of CIT 2750 and L. Chiappe for assistance to the authors during a visit to CIT. A.L. would also like to thank the members of the University of Alberta Laboratory for Vertebrate Paleontology labs for discussion and help with the manuscript. We thank A. S. Schulp, J. L. Conrad, and F. R. O'Keefe for comments and suggestions that greatly improved the manuscript. Support for this project came from a Natural Sciences and Engineering Research Council of Canada Alexander Graham Bell Canada Graduate Scholarship (CGSM) to A.L., and from a Natural Sciences and Engineering Research Council of Canada Discovery Grant (no. 238458-01) and Chair's Research Allowance to M.C.

Handling editor: Robin O’Keefe