Neontological evaluation of pelvic skeletal attributes purported to reflect sex in extinct non-avian archosaurs

Sex in non-avian archosaurs has been inferred using a variety of osteological attributes. However, little quantitative data have been presented showing that these phenotypes truly exist. In this study, testing for the presence of pelvic osteological correlates of sex in extant archosaurs was conducted, using skeletons of wild-caught A. mississippiensis as a neontological model. For outgroup comparison, the squamate Iguana iguana is included. A sample of 16 females and 19 males of A. mississippiensis, and 18 females and 10 males of I. iguana were examined. Measurements included pelvic canal area, dorsoventral depth, and mediolateral width of the pelvic canal, mediolateral width between the dorsal edge of each ilium, and ischium orientation. These data were analyzed using analyses of covariance, a t-test, and a recently developed geodesic distance shape analysis. Results indicate that there is sexual dimorphism in the proportions of the pelvic canal in A. mississippiensis, with females typically having deeper pelvic canals than males. This dimorphism might be synapomorphic for Archosauria. No dimorphism was found in I. iguana. The detection of dimorphism in A. mississipiensis required large sample sizes owing to substantial overlap between sexes. Thus, sexing isolated specimens using this metric is tenuous at best. Assuming similar variance in the relative pelvic depth versus width in other non-avian archosaurs, this criterion would also produce imprecise determinations of sex for these taxa.