 They have a relatively small and slender skull , with a large external auditory meatus , narrow squamosal and mandibular processes , a minuscule stylomastoid foramen , and usually lack foramina for the external carotid artery and anterodorsal ( meaning in front and toward the back ) nasopharynx . The dental formula of three @-@ toed sloths is : 54 – 5 Two of the teeth in each jaw are incisor @-@ like , although those in the upper jaw are small or may be absent . Many of the features found in pygmy sloths are thought to be indicative of a relatively rapid evolution of a new species in an isolated , island habitat . 
