A Comprehensive Review of the Morphological Diversity of the Quadrate Bone in Mosasauroids (Squamata: Mosasauroidea), with Comments on the Homology of the Infrastapedial Process

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.