The poorly known marine squamate Mesoleptos is reassessed based on two previously known specimens and a newly referred specimen. The three specimens of Mesoleptos zendrinii share unique characters such as long, posteriorly tapering centra and distally straight but non-pachyostotic ribs. Mesoleptos had a narrow neck (and presumably small head), long laterally compressed body, and small fore- and hindlimbs. Phylogenetic analysis suggests that Mesoleptos is the nearest relative of snakes; this phylogenetic position is consistent with its morphology being intermediate between typical marine squamates (e.g. mosasauroids) and primitive marine snakes (pachyophiids). However, this interpretation remains tentative because Mesoleptos is very poorly known, and many of the characters uniting it with mosasauroids and primitive snakes are correlates of marine habits and/or limb reduction.
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