New discoveries on the middle ear anatomy of Ignacius graybullianus (Paromomyidae, Primates) from ultra high resolution X-ray computed tomography.

A skull of Ignacius graybullianus (USNM 421608) was studied using ultra high resolution X-ray computed tomography (uhrCT). The anatomy of the middle ear in this specimen was previously studied through partial removal of the auditory bulla on one side. The data now available allow for examination of the others unprepared ear, which is more completely preserved, as well as adding to the information available about the previously studied ear. Analysis of the relationships between the bones making up the auditory bulla confirms previous assertions that it is formed from the entotympanic, and not from the petrosal, basioccipital, or basisphenoid. Contrary to previous reconstructions of the middle ear anatomy in all known plesiadapiforms, this specimen exhibits a bony canal for the promontorial artery and/or internal carotid nerves running across the lateral extreme of the promontorium. The identification of this structure is confirmed by the clear presence of a lumen, and its origination at a posterior carotid foramen (pcf) in a position that corresponds to that identified in previous studies of the paromomyid basicranium (Am. J. Phys. Anthropol., 36 (1972) 59, Am. J. Phys. Anthropol., 89 (1992) 477). Remnants of this canal are present bilaterally in USNM 421608, which additionally supports its identification. The presence of bony canals for branches of the internal carotid artery and the internal carotid nerves is a feature seen in scandentians and euprimates that is missing in dermopterans. The unusual lateral route followed by the internal carotid nerves is a primitive euprimate feature missing in all other archontans. As such, this evidence is consistent with a close euprimate-paromomyid relationship, and the inclusion of the latter in the order Primates. The discovery of this feature in paromomyids after almost 30 years of study of the ear region of this family acts as a cautionary note to the interpretation of the middle ear in damaged specimens.