Published online by Cambridge University Press: 30 July 2022
This chapter assesses morphological characters proposed to support the Pythonomorph Hypothesis— a purported close relationship between snakes and mosasaurians. With an emphasis on early diverging (non-mosasaurid) mosasaurians and mosaurids, new morphological data (including from high-resolution CT) for well-preserved dolichosaurid and mosasaurid fossils are presented. Details of the skull and mandibles are interpreted as supporting the monophyly of Mosasauria as the proximal outgroup of Varanoidea, to the exclusion of snakes. However, mosasaurians do deviate from the typical varanoid condition in aspects of their infraorbital foramen, ventral part of the lacrimal and its relationship with the prefrontal, anterior ramus of the ectopterygoid and its contact with the maxilla and jugal, lack of plicidentine, and (at least in early diverging mosasaurians) anguinoidean tooth replacement. We consider most characters previously reported as supporting the Pythonomorph Hypothesis to be problematic, because of incomplete fossil preparation, artefacts of taphonomy, limited comparisons, misinterpretations of anatomy, incomplete taxon sampling, or inadequate character formulation and/or scoring.