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.

Mosasaurian phylogenetics has been one of the most controversial topics in squamate systematics, with various studies and authors arguing in favor of a varanoid affinity (the Varanoid Hypothesis), a snake affinity (the Pythonomorph and Ophidiomorph Hypotheses) or only distant affinities to these lineages (the Stem-scleroglossan Hypothesis). We review the classification history of mosasaurians over the past two centuries, focusing on non-mosasaurid mosasaurians (dolichosaurs and aigialosaurs). A reappraisal is provided based on a new phylogenetic analysis. Our results clearly support the Varanoid Hypothesis. The Pythonomorph and Ophidiomorph Hypotheses are reviewed, and characters traditionally inferred to support these hypotheses are discussed and reinterpreted. Taxonomic sampling and fossil completeness likely play a major role—our (hopefully improved) phylogenetic hypothesis being based on denser taxon sampling and more complete character scoring resulting from additional studies, including the application of modern imaging techniques. Based on current data, our interpretation is that a particularly close phylogenetic relationship between mosasaurians and snakes can be rejected.