THE PHYLOGENY OF VARANOID LIZARDS AND THE AFFINITIES OF SNAKES

Evidence that platynotan squamates (living varanoid lizards, snakes an d their fossil relatives) are monophyletic is presented. Evolutionary relationships within this group are then ascertained through a cladist ic analysis of 144 osteological characters. Mosasauroids (aigialosaurs and mosasaurs), a group of large marine lizards, are identified as th e nearest relative of snakes, thus resolving the long-standing problem of snake affinities. The mosasauroid-snake clade (Pythonomorpha) is c orroborated by 40 derived characters, including recumbent replacement teeth, thecodonty, four or fewer premaxillary teeth, supratemporal-pro otic contact, free mandibular tips, crista circumfenestralis, straight vertical splenio-angular joint, loss of posterior ramus of the corono id, reduced basipterygoid processes, reduced interpterygoid vacuity, z ygosphene-zygantal articulations, and absence of epiphyses on the axia l skeleton and skull. After mosasauroids, the next closest relatives o f snakes are varanids (Varanus, Saniwa and Saniwides) and lanthanotids (Lanthanotus and Cherminotus). Derived features uniting varanids and lanthanotids include nine cervical vertebrae and three or fewer pairs of sternal ribs. The varanid-lanthanotid-pythonomorph clade, here term ed Thecoglossa, is supported by features such as the anteriorly positi oned basal tubera, and the loss of the second epibranchial. Successive outgroups to thecoglossans are Telmasaurus, an unresolved polytomy (E stesia, Gobidermatidae and Helodermatidae), Paravaranus and Proplatyno ta. The 'necrosaurs' are demonstrated to be an artificial (polyphyleti c) assemblage of primitive platynotans that are not particularly close ly related to each other. Snakes are presumed to have evolved from sma ll, limbless, burrowing lizards. The inability of previous analyses to resolve the affinities of snakes has been attributed to extensive con vergence among the numerous lineages of such lizards. The present stud y contradicts this claim, demonstrating that the problem is due instea d to omission of critical fossil taxa. No modern phylogenetic analysis of squamate relationships has simultaneously included both mosasauroi ds and snakes: previous studies have therefore failed to identify the mosasauroid-snake association and the suite of derived characters supp orting it. Mosasauroids are large aquatic animals with well-developed appendages, and non of the derived characters uniting mosasauroids and snakes is obviously correlated with miniaturization, limb reduction o r fossoriality. Recognition that mosasauroids, followed by varanids an d lanthanotids, are the nearest relatives of snakes well also facilita te studies of relationships within snakes, which until now have been h ampered by uncertainty over the most appropriate (closely related) liz ard outgroups.