File:Three toed (Bradypus variegatus) and Two toed (Choloepus hoffmanni) Sloths (5332567654).jpg

English: The three and two toed sloths belong in different genera, and in fact, different families (Bradypodidae and Megalonychidae) within the sloth suborder of Folivora. The three toed sloths are actually the most basal lineage of the sloth radiation, with all of the known giant ground sloths (represented by multiple families) being more closely related to the two toed sloth. In fact, Megalonychidae harbored many giant ground sloth species itself. Thus, what you see here are two relics of a once highly diverse group of herbivorous mammals, most of which were giant, heavily clawed ground dwelling herbivores that could reach to the size of modern African elephants and lived from the high reaches of snowy North American boreal forests, to the desert shores of South America (where there were actually aquatic forms that probably lived like modern marine iguanas).

These two skulls in particular belong in the collections of the Biology Department at Centenary College of Louisiana and were collected in Costa Rica (likely by or in association with Dr. Brad McPherson).

Note that the jugal is broken off of the Bradypus skull, otherwise you would see a rather flamboyant, incomplete zygmoatic process, just as with the Choloepus skull, only with a higher dorsal process. Also, the Choloepus skull is lacking all of its post-caniniform and lower caniniform teeth (and I mean caniniform, not canine. It is believed that the caniniform teeth of sloths are actually just premolars or molars. The fact that the lower caniniform is posterior to the upper is a rather glaring reason why, as in all mammals with canines, the lower is always anterior to the upper canine).