 Despite the similarities , there are several features that distinguish Mesopropithecus skulls from those of living indriids . The skull , including the zygomatic arch , is more robustly built . The temporal lines join together anteriorly into a sagittal crest and there is a distinct nuchal ridge that joins the rear of the zygomatic arch . The skull has a more rounded braincase , slightly smaller and more convergent orbits , more pronounced postorbital constriction ( narrowing of the skull behind the eye sockets ) , more robust postorbital bar ( bone that encircles the eye socket ) , a steeper facial angle , more robust and <unk> convex zygomatic bone , and a broader , squared snout . The upper incisors and canines are larger . The more robust mandible ( lower jaw ) and mandibular symphysis ( point where the two halves of the lower jaw meet ) suggest a more folivorous diet , which requires extra grinding . The orbits are as large ( in absolute size ) as those in smaller living indriids , which suggests low visual acuity . Mesopropithecus and its closest sloth lemur relative , Babakotia , did share a few ancestral traits with indriids , unlike the largest sloth lemurs , Palaeopropithecus and Archaeoindris . These include the aforementioned four @-@ toothed toothcomb , an inflated auditory bulla ( bony structure that encloses part of the middle and inner ear ) , and an <unk> ectotympanic ring ( bony ring that holds the eardrum ) . 
