POSSIBLE SEXUAL DIMORPHISM IN THE PLEISTOCENE SHASTA GROUND SLOTH (NOTHROTHERIOPS SHASTENSE)

Potential sexual dimorphism in extinct ground sloths has already been reported for the gigantic Eremotherium laurillardi by Cartelle in 1982, and for the larger La Brea ground sloth Paramylodon harlani by McDonald in 2006. We examined the skulls of a smaller ground sloth, Nothrotheriops shastense, from Rancho La Brea, California, Rampart Cave in the Grand Canyon, San Josecito Cave, Mexico, and several other localities. There is no real size distinction between the two morphs, as evidenced by the total skull length measurements, and also by the sample of post-cranial bones such as astragali and patellae in the La Brea collections. However, like P. harlani, the skulls of N. shastense seem to have much wider frontal region in the presumed male skull (LACM HC 1800-5) with robust supraorbital ridges, versus the few other skulls which are narrower laterally in dorsal view and are presumed to represent females. In addition, in lateral view, the rostrum is dorsoventrally deeper in the presumed male skull than in presumed female skulls. The differences are not as extreme as those reported by McDonald in 2006 and by Cartelle in 1982, but are consistent with the fact that for most ground sloths that have been studied so far, there is at least some apparent sexual dimorphism in the skulls.