Adriosaurus and the affinities of mosasaurs, dolichosaurs, and snakes

The poorly-known, long bodied, limb-reduced marine lizard Adriosaurus suess i Seeley, 1881, is reassessed. Adriosaurus and a number of other marine liz ards are known from Upper Cretaceous (Upper Cenomanian-Lower Turonian) mari ne carbonate rocks exposed along the Dalmatian coast of the Adriatic Sea, f rom Komen, Slovenia, to Hvar Island, Croatia. A revised vertebral count rev eals 10 cervical, 29 dorsal, and at least 65 caudal vertebrae. The projecti ons previously interpreted as hypapophyses are instead transverse processes . Openings on the anterior part of the skull, previously described as exter nal nares, are probably internal nares. Important features not noted previo usly include accessory articulations on all presacral vertebrae, pachyostos is of dorsal vertebrae and ribs, and the presence of two pygal vertebrae. P hylogenetic analysis of 258 osteological characters and all the major squam ate lineages suggests that Adriosaurus and dolichosaurs are successive sist er-taxa to snakes. This is consistent with their long-bodied, limb-reduced morphology bring intermediate between typical marine squamates (e.g., mosas aurs) and primitive marine snakes (pachyophiids). The analysis further reve als that up to five successive outgroups to living snakes (pachyophiids, Ad riosaurus, dolichosaurs, Aphanizocnemus, and mosasauroids) are all marine, suggesting a marine (or at least, semi-aquatic) phase in snake origins. The se phylogenetic results are robust whether multistate characters are ordere d or unordered, thus refuting recent suggestions that snakes cluster with a mphisbaenians and dibamids (rather than aquatic lizards) if multistate char acters are left unordered. Also, the recent suggestion that Pachyrhachis sh ares synapomorphies with advanced snakes (macrostomatans) is shown to be po orly supported, because the reinterpretations of the relevant skull element s are unlikely and, even if accepted, the character states proposed to unit e Pachyrhachis and advanced snakes are also present in more basal snakes an d/or the nearest lizard outgroups, and are consequently primitive for snake s.