A Review of the Skull Anatomy and Phylogenetic Affinities of Marine Pachyophiid Snakes

Pachyophiidae fossils are among the most complete known for snakes, and include the earliest snakes with fully developed hindlimbs. Pachyophiids have been historically seen as suitable morphological intermediates between lizards and extant snakes, supporting the hypothesis that snakes originated in a marine setting from a macrophagous common ancestor with mosasaurian lizards. Pachyophiids have been subject to conflicting interpretations of their anatomy, fuelling renewed debate on snake origins and early diversification. We revisit pachyophiid cranial anatomy, providing additional evidence from new preparations, high resolution CT scans, and Synchrotron images. We address challenges posed by fossil (in)completeness to the study and interpretation of these specimens, and reassess phylogenetic affinities. We critically reassess morphological evidence supporting the Marine Hypothesis, concluding that (i) snakes are not especially closely related to mosasaurians, and (ii) pachyophiids are relatively deeply nested within the snake crown, so that they are of greater importance for understanding early crown-snake evolutionary history than they are for understanding snake origins.