The osteology and myology of the cranio-cervical region in squamate reptiles : a comparative study
Modem squamates (lizards, snakes and amphisbaenians) form a highly successful group of terrestrial reptiles with a range of habitats and lifestyles. Some features of their anatomy (head, limbs) are relatively well-studied, but there are significant gaps in our knowledge. The neck is one such area. It provides the mobile unit between the head and the body, serving for the attachment of the musculature to the head, the forelimb and the pharyngeal region. The mobility of the head is dependent on arrangement of such muscles and on the structure of the craniovertebral and intervertebral joints across which the muscles act. Preliminary studies have shown interesting patterns of variation in both skeletal anatomy and musculature. The aim of the project was to explore, catalogue and analyse this variation, and to find functional and/ or taxonomic explanations for it. Details of the surface morphology of bones, and the origin and insertion of muscles were recorded for Caiman, Sphenodon and squamates (including the amphisbaenian Diplometopon and the snake Trimeresurus). Where possible, comparisons were also made within and between taxa to determine levels of inter- and intraspecific variation, as well as variation between genera. Several limbless or limb-reduced taxa were compared to assess levels of convergence in these forms. The results of the study suggest that some craniocervical characters may be phylogenetically useful although there is a high level of variation within and between species. Craniocervical characters generally support the monophyly of Lepidosauria, Squamata and Sderoglossa (all non-iguanian squamates); and major clades like Iguarda, Gekkota, Sdncomorpha and Angtiimorpha. Also supported are groups like Acrodonta (agamids + chamaeleons) and Scincoidea; and families like Chamaeleonidae and Varanidae. Within Squamata, craniocervical features seem to place Gekkota closer to Anguimorpha than Sdncomorpha. Varanid lizards and snakes (Trimeresurus) share muscular features (origin and extension of longus colli in the craniocervical region), although varanids themselves are distinctive in their neck morphology. Amongst the limbless taxa examined, the amphisbaenian Diplometopon and the snake Trimeresurus had a spedalised neck morphology which differs from that of lizards, but the craniocervical features of Diplometojjon resembled those of gekkotans and limbed anguimorphs (e.g. varanids) more than sdncomorphs.