Linear morphometrics fail to support strong sexual dimorphism in Uintatherium anceps.

Uintatheres, mammals belonging to the extinct order Dinocerata, are among the most recognizable of all Paleogene (~66-23 Ma) organisms. Unmistakable for their bizarre skulls with multiple pairs of horns and saber-like upper canines, uintatheres have captivated paleontologists since the late nineteenth century. Since their initial discovery, uintatheres have been regarded as a classic example of dramatic sexual dimorphism in the fossil record, with males purported to be larger and possess more prominent horns and canines than females. However, the hypothesis that uintatheres were highly sexually dimorphic has never been formally tested. Here, traditional, linear morphometrics are implemented with a collection including most known skulls of Uintatherium anceps to quantify patterns of cranial variation within this taxon. Use of univariate and multivariate statistical approaches provides some evidence of dimorphism in Uintatherium, particularly in the length of the canines, but the expected signals of strong sexual dimorphism are generally lacking. To verify the approaches used, the same suite of analyses was performed on a similarly sized dataset from Bison bison. Statistical signals of sexual dimorphism are clearer in this extant taxon. Comparison with other extant mammalian herbivores reveals that the estimated level of sexual dimorphism in skull length for Uintatherium is not especially high. In light of these findings, as well as the current understanding of uintathere systematics and paleoecology, it is argued that extreme sexual dimorphism should not be treated as the null hypothesis for this clade. These results may point to a broader pattern in mammalian evolution in which traits like horns and large canines emerge in both males and females of a species despite being primarily selected for in one sex.