The petrosal of Omomys carteri and the evolution of the primate basicranium.

The first omomyine petrosals, those of Omomys carteri, are described. Omomys probably had a tympanic bulla and canals for the intratympanic carotid circulation derived from the petrosal bone. The stapedial and promontory canals were complete, large and subequal. The posterior carotid foramen entered the bulla posteromedially. The intratympanic portion of the facial nerve was fully enclosed in bone, the stapedius fossa is extrabullar and the parotic fissure is patent. The mastoid was pneumatized from the epitympanic recess and a supracochlear cavity may have been present. The Omomys petrosals exhibit a generic omomyiform morphology, exhibiting no features that can be interpreted as autapomorphies and only one feature shared with adapiforms. The monophyly of Omomyiformes is based on other cranial characters, dental and postcranial characters assessed elsewhere. The similarity of the Shanghuang petrosal to the petrosals of omomyiforms, as well as the ambiguous evidence of its association, suggest that an omomyiform affinity for that petrosal cannot be ruled out.