Published online by Cambridge University Press: 30 July 2022
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.