A phylogenetic and functional approach to the study of the evolution of body shape in lizards (Squamata)
Measures of species, morphological, functional, and ecological diversity are interrelated although not always highly correlated. Morphology tends to influence how organisms function, which in turn influences what they do in their environment. Here I study how morphology, function and ecology relate to one another and to diversity in the Squamata. Squamates have high variability in vertebral number, body shape, and functional and ecological diversity. First, I construct a supertree of the Squamata and use it to study the evolution of presacral vertebral number and its variability in squamates, relating these traits to morphological and species diversity. Squamate lineages vary in the modal number and degree of variability in number of vertebrae. Most lineages with increased variability, or relaxed constraint, on vertebral number have diversified more in terms of both species richness and morphological disparity relative to more constrained lineages. Second, I use high speed video data to show that more elongate species tend to bend their bodies more, and species with relatively longer hind limbs tend to take longer, quicker strides, resulting in faster sprinting in the elongate genus Lerista and the stocky phrynosomatines. However, differently shaped species in these two clades do not select different substrate characteristics or perform differentially on sand versus sandpaper substrates. Third, conducting a morphometric analysis of the Phrynosomatidae, I show that the horned lizards (genus Phrynosoma ) have undergone continued directional evolution towards increasingly stocky body shapes. In addition, species that are stockier also run more slowly, have relatively longer horns for predator defense, and are increasingly specialized on an ant diet. Phrynosomatid morphology, locomotor function and ecology have co-evolved extensively. When all of my findings are considered, there is evidence that a relaxation of constraint on presacral vertebral number is a key innovation in squamates. Relaxation of constraint facilitates the evolution of more diverse body shapes through changes in vertebral number, and this increased diversity of body shapes allows for the invasion of new niches through increased functional diversity. Ultimately, these factors enhance diversification in lineages with high variability in vertebral number.