A comprehensive review of the morphological diversity of the quadrate bone in mosasauroids (Squamata: Mosasauroidea), with comments on the homology of the infrastapedial process

ABSTRACT

We examined a selection of three-dimensionally preserved quadrate bones from representatives of all major clades of mosasauroid reptiles, an extinct group of marine lizards inclusive of aigialosaurs and mosasaurs (Squamata, Mosasauroidea). The quadrate bones appear to be very diverse within and across mosasauroid clades, and show variable combinations of ridges, crests, and processes. Because of its complex features and diversity, taxonomists have often relied quite heavily on quadrate morphology both in alpha-level classification of mosasauroids and for use as characters in phylogenetic analyses. However, the terminology applied to some quadrate features has been inconsistent, in particular regarding the ‘infrastapedial process.’ Such a lack of clarity of even anatomical terms hampers our understanding of the evolution of the morphology of this complex bone. Here we provide a comprehensive comparison of mosasauroid quadrates contextualized against a recent phylogeny of the group, and demonstrate that some features of the posteroventral region of these bones are likely homoplastic and require the establishment of a new set of anatomical terms.

ACKNOWLEDGMENTS

We are grateful to H. Street (T. rex Discovery Centre, Eastend, Saskatchewan, Canada) for providing pictures of the quadrate of G. alabamaensis; we also thank former and current students of the University of Alberta (Edmonton, Canada) A. Dutchak, T. Bullard, P. Jiménez-Huidobro, I. Paparella, T. Simões, M. Campbell-Mekarski, and A. LeBlanc, for discussions and contributions to data collection on quadrate morphology from the diverse clades of mosasauroids they studied during their dissertation research. We thank M.N. Hutchinson and M.S.Y Lee (Flinders University, Adelaide, South Australia, Australia) for discussion. We are grateful to D. Madzia and an anonymous reviewer for their useful comments and suggestions. M.W.C. thanks the Natural Sciences and Engineering Research Council of Canada (grant no. 23458) and a Chairs Research Allowance for long-term financial support. The authors have no conflict of interest to declare.