EVIDENCE FOR SEXUAL DIMORPHISM, IN BOTH SIZE AND VARIANCE, IN PLACERIAS HESTERNUS REVEALED THROUGH MEASUREMENT OF THE MAXILLARY CANINIFORM PROCESS

Placerias hesternus is a Late Triassic dicynodont, and one of the last megafaunal synapsids of the Mesozoic. The species has tusk-like projections of maxillary bone that have been speculated to be sexually dimorphic since the 1950s, but this claim has never been thoroughly investigated quantitatively. This study examined 36 maxillae, 16 of which are complete, from a single population from Placerias Quarry in the Blue Mesa Member of the Chinle Formation, near St. Johns, AZ. There is no known histological evidence of juveniles, and there was insufficient deposition time for anagenesis to have occurred in the sample. Specimens were measured in three dimensions, analyzed for the development of these caniniform process skull projections. Measurements from complete specimens reveal proportional differences forming two distinct morphs: one with a shorter process and one with a longer process. This was first observed in the quarry sample by Camp and Welles (1956) and this study finds strong statistical support for the sample’s observed bimodal distribution. Interestingly, the variance in length and shape is more complex than initially suggested by Camp and Welles. Firstly, the group with larger maxillae has a substantially greater variance in length than the smaller group, with the larger morph’s coefficient of variation about twice that of the smaller morph, despite the relatively small sample size. Further, the shape of the anterior edge of the caniniform process transitions from flatter to more rounded as maxilla length increases, leading to a positive correlation between width and length that is more pronounced in the larger morph. These results support the long-standing hypothesis that Placerias was cranially dimorphic, possibly based on sex. The greater variation in the length of the caniniform of the larger morph may have been the result of intraspecific competition or courtship, further supporting the supposition of sexual dimorphism. Although dimorphism has been observed in other dicynodonts such as Diictodon and suggested in Kannemeyeriiformes such as Wadiasaurus, this is the first quantitatively recorded instance in a Stahleckeriid, and the only known instance of dimorphism in the caniniform process, in contrast to the dimorphism seen in the tusks or other aspects of the skull in other species, in dicynodonts.