DEB 9801342 Hopson Based on a rich fossil record, the reptile-to-mammal transition has long been considered the best-supported example of the origin of a major vertebrate group. This study will examine several prominent hypotheses concerning the tempo and directionality of pre-mammalian synapsid evolution. Using phylogenetic estimates based on recent cladistic analyses and a battery of morphometric techniques, my research will address analytically previous assertions of: (1) the prominent role of morphological convergence, including (2) the prevalence of independent progressive trends towards the mammalian grade of structural organization. The evolution of synapsid morphological disparity (i.e., the range of anatomical variation) from "pelycosaurs" to basal mammaliaforms will be examined using qualitative character descriptions, quantitative measurements, and morphometric outline methods. Results derived from the three methods will be contrasted, and then (1) compared to the pattern of diversification observed among other higher taxa, and (2) examined for significant (i.e., non-random) directionality. The research outlined above integrates aspects of cladistic analysis and biostratigraphy with morphometrics and the quantitative analysis of morphological disparity. It will shed light on the dynamics of synapsid morphological diversification, as well as on important issues such as the recognition of evolutionary trends and the distribution of convergent evolution.
