Evolution of Function-Related Traits in Squamates

The evolution of snakes (Squamata: Serpentes) represents a major transition in squamate reptiles and involves extensive modifications in the body plan. Functional morphology and phylogeny are integrated to discuss adaptive morphological traits in the origin of snakes and in the evolution of the venom-injecting apparatus among lizards, which is convergent with that in snakes. The focus of these analyses is to determine how to best optimize morphological traits on the phylogeny and to use character distribution in terminal taxa to estimate ecological adaptation in ancestral nodes. To study the locomotion transition from lizards to snakes, 45 virtual models of the squamate bony labyrinth in the inner ear were reconstructed. The results show morphological diversifications of the vestibular region among burrowers, generalists and marine swimmers. The vestibule is enlarged in fossorial species, and reduced in marine snakes and the marine lizard Platecarpus coryphaeus. To quantify the morphological differences and provide tractable methods to reconstruct locomotion in ancestral snakes, I performed three-dimensional geometric morphometric analysis of the 45 samples, using six landmarks and 22 semilandmarks. ANOVA tests on the Procrustes coordinates supported differences among fossorial, generalist, and aquatic locomotion categories. Phylogenetic signal was insignificant in the Procrustes coordinates. Using a phylogeny with the 44 extant samples as terminal taxa, I reconstructed shape coordinates of all internal nodes. Reconstructed shape of the vestibular region of key ancestral nodes in snakes preferred a terrestrial origin for crown-group snakes. Morphological proxies used in this study can be applied to stem snakes to estimate locomotion. Methodologically, this study provides a novel approach to resolve the question whether snakes become limbless on land or in the oceans. Evolution of the venom-injection apparatus in lizards is discussed, based on new material of Estesia mongoliensis. A total-evidence phylogenetic analysis of anguimorph lizards is performed with 86 anguimorph taxa coded to 435 morphological characters and four genes. The matrix includes eight new morphological characters. The total-evidence phylogeny suggests that Estesia Mongoliensis is a basal monstersaur whose crown groups are Heloderma, the only living venomous lizards. Presence of tooth flanges with venom grooves is recognized as a new monstersaurian synapomorphy in this study. Estesia mongoliensis has venom grooves comparable to extant Heloderma, but has a deeper root-to-tip groove in the caudal tooth carina, revealing morphological variations of lizard venom delivery apparatus that are not recorded in extant species.