Anatomy, ontogeny, and phylogeny of coelophysoid theropods

Coelophysoid theropods (Dinosauria: Saurischia) were viewed as members of Ceratosauria in early cladistic phylogenetic hypotheses, but more recent analyses placed Coelophysoidea and Dilophosaurus as successively closer outgroups to Neotheropoda (Ceratosauria + Tetanurae). Most cladistic studies did not appreciate the importance of relative maturity to the expression of some characters in coelophysoids. Often the role played by ontogenetic variation was not considered, or maturity-dependent characters were deleted from analyses. I used cladistic techniques to derive a hierarchy of relative maturity and map the sequence of ontogenetic transformations among coelophysoid specimens. I then conducted an extensive phylogenetic analysis of basal theropod relationships. Taxa and characters used in the phylogenetic analysis were examined in detail. Separate phylogenetic tests were run to evaluate different approaches to dealing with the relative maturity of specimens in cladistic analyses. The first test coded characters as if all taxa were represented by adults, and resulted in a taxon known only from immature specimens being placed in a basal position. The second test removed maturity-dependent characters from consideration, and resulted in Coelophysoidea being placed outside Ceratosauria, basal to Neotheropoda. The third test incorporated the results of the ontogenetic analysis. Maturity-dependent characters were coded as 'missing data' instead of 'absent' in taxa represented by immature specimens if the ontogenetic analysis showed the derived states of characters were expressed only at stages of maturity more advanced than the representative specimens. The third test resulted in Coelophysoidea being placed as the sister lineage to Ceratosauroidea within Ceratosauria. My study shows that the approach used to deal with ontogenetic variation in fossil taxa can alter the outcome of phylogenetic analyses. The use of a quantitative ontogenetic analysis to determine the relative maturity of fossil specimens, the subsequent approach to coding maturity-dependent characters for taxa represented by immature specimens, and the addition of new anatomical data all contributed to more robust hypotheses of relationships among basal Theropoda.