Sexual dimorphism in archosaurs: A phylogenetic, morphological, and morphometric approach

Recognizing sexual dimorphism in dinosaurs is problematic. A review of previous research shows that evidence for dinosaur sexual dimorphism is controversial and confined to relatively few species. This study used extant archosaur sister- and derived-taxa, and an outgroup-taxon to identify shared characters and discern the ancestral state for Dinosauria. Based on generally accepted phylogenies, crocodylians (Alligator mississippiensis) and birds ( Rhea americana and Meleagris gallopavo) served as sister and derived taxa respectively, and lepidosaurs (Typinambis teguixin) served as outgroup.; Pelvic and cloacal morphology were examined through detailed dissections. These revealed, across study taxa, consistent reproductive and support structures with osteological correlates, which can be used to distinguish gender. Morphological characters were mapped to currently accepted phylogenetic relationships. This yielded hypothesized ancestral character states for Dinosauria. Based on these, the cloacal structure for Compsognathus longipes, a small Jurassic theropod, was reconstructed.; Osteological correlates in Alligator and Rhea pelvic bones were examined with three-dimensional morphometric techniques and tested with a combined principal components and factor analysis. This analysis established major components of variation among morphometric dimensions. Analytical results support the dimorphic nature of the osteological correlates and suggest their application in future studies of non-avian dinosaurs. The osteological elements giving the most consistent morphological and morphometric determination of sexual dimorphism were the caudal margin of the ischium and haemal arches. Several muscles which insert on these elements directly interact with the cloaca.