Squamate phylogeny and the relationships of snakes and mosasauroids

Cladistic analysis of extant and fossil squamates (95 characters, 26 taxa) finds the fossil squamate, Coniasaurus Owen, 1850, to be the sister-group o f the Mosasauroidea (mosasaurs and aigialosaurs). This clade is supported i n all 18 shortest cladograms (464 steps; CI 0.677; HI 0.772) by nine charac ters of the dermatocranium, maxilla, and mandible. A Strict Consensus Tree of the 18 shortest trees collapses to a basal polytomy for most major squam ate clades including the clade (Coniasaurus, Mosasauroidea). A Majority Rul e Consensus Tree shows that, in 12 of 18 shortest cladograms, the clade Con iasaurus-Mosasauroidea is the sister-group to snakes (Scolecophidia (Alethi nophidia, Dinilysia); this entire clade, referred to as the Pythonomorpha ( [[Scolecophidia [Alethinophidia, Dinilysia]], [Coniasaurus, Mosasauroidea]] is the sister-group to all other scleroglossans. Pythonomorpha is supporte d in these 12 cladograms by nine characters related to the Lower jaw and cr anial kinesis. In 6 of 18 shortest cladograms, snakes are the sister-group to the clade (Amphisbaenia (Dibamidae (Gekkonoidea, Eublepharidae))). None of the cladograms support the hypothesis that coniasaurs and mosasauroids a re derived varanoid anguimorphs. Two additional analyses were conducted: (I ) manipulation and movement of problematic squamate clades while constraini ng 'accepted' relationships; (2) additional cladistic analyses beginning wi th extant taxa, and sequentially adding fossil taxa. From Test I: at 467 st eps, Pythonomorpha can be the sister-group to the Anguimorpha, Scincomorpha , 'scinco-gekkonomorpha' [scincomorphs, gekkotans, and amphibaenids-dibamid s]. At 471 steps Pythonomorpha can be placed within Varanoidea. Treating on ly mosasauroids and coniasaurs as a monophyletic group: 469 steps, mosasaur oids and coniasaurs as sister-group to Anguimorpha; 479 steps, mosasauroids and coniasaurs nested within Varanoidea. Test II finds snakes to nest with in Anguimorpha in a data set of only Mosasauroidea + Extant Squamates; the sistergroup to snakes + anugimorphs is (Amphisbaenia (Dibamidae (Gekkonoide a, Eublepharidae))). No one particular taxon is identified as a keystone ta xon in this analysis, though it appears true that fossil taxa significantly alter the structure of squamate phylogenetic trees. (C) 1999 The Linnean S ociety of London.