Aplasia of the maxillary sinus in a Tibetan macaque (Macaca thibetana) with implications for its evolutionary loss and reacquisition.

The skull of an adult female Tibetan macaque, Macaca thibetana, was found to completely lack the maxillary sinus (MS). This absence was accompanied by a slight lateral concavity where the ostium should have formed in the MS, a slight drop of the orbital floor, posterior and medial displacement of the zygomaxillary suture, an unusual position of the lacrimal canal, malocclusion with severely worn cheek teeth, and abnormalities in the temporomandibular joints. The facial component was disproportionally large compared with the neurocranium and mandible. This hypertrophic face probably caused the malocclusion and associated anatomical disorders and simultaneously displaced the lacrimal canal posterior to other nasal structures to preclude the possibility of maxillary pneumatization. These modifications in the spatial relationships to nasal structures might help explain the evolutionary loss and reacquisition of the MS in some primate lineages displaying great variations in facial anatomy.