Ontogeny and phylogeny of the Archosauriform skeleton

The evolution and development of morphology has formed the foundation of comparative anatomy. Pre-Darwinian workers attempted to use patterns of organismal development to justify patterns of organismal classification. The work presented here has examined patterns of associations between ontogeny and phylogeny. To begin, a conceptual definition of a character was outlined using generalizations of developmental biology. The definition demonstrated a method that may elucidate character states that are maximally independent within a single individual and how the states may be combined across individuals to construct a character that is maximally independent across those taxa.; The associations of ontogeny and phylogeny were based on discrete morphological characters. A phylogenetic trajectory was constructed in a comprehensive study of 115 fossil and extant species of Archosauriformes. The broad sampling of taxa and characters yielded orthodox and unique phylogenetic hypotheses. In particular, the analysis yielded strong support for a paraphyletic distribution of sphenosuchian and protosuchian taxa. Longirostrine crocodyliforms were found to be polyphyletic with thalattosuchians near the base of Mesoeucrocodylia and Dyrosaurus, Sarcosuchus, and Pholidosaurus more closely related to the modern Gavialis than to other extant crocodiles.; The phylogenetic trajectory from Archosauriformes to Alligator mississippiensis yielded 39 clade ranks. All characters used in the phylogenetic analysis were scored for a series of embryonic and juvenile specimens of A. mississippiensis. These data were used to construct an ontogenetic trajectory with 38 ontogenetic ranks. Each ontogenetic rank was diagnosed by a suite of character states using a maximum parsimony criterion. Character states that diagnosed clade and ontogenetic ranks were associated and compared using a Spearman rank correlation. The data yielded a significant rho value of 0.323. The positive value suggests a general trend of von Baerian recapitulation. Further data analyses were performed to examine character state homoplasy in the plot space of ontogeny and phylogeny. These data revealed a general peak of character state homoplasy at high clade ranks and high ontogenetic ranks. The greatest degree of homoplasy occurred at the upper limits of both ranks.