Relationships between the expression of the stapedial artery and the size of the obturator foramen in euarchontans: Functional and phylogenetic implications.

Cranial arterial patterns are commonly used for determining phylogenetic patterns in extant taxa and have often been used in studies investigating the relationships among fossil taxa. In primitive eutherians, the stapedial artery provided blood to the meninges, orbits, and certain regions of the face. In many modern mammals, however, blood supply to most of these areas has been taken over by branches of the external carotid, although some groups (e.g., treeshrews, some families of primates) still retain aspects of the ancestral pattern. Here, we show that the relative size of the obturator foramen of the stapes is a reliable indicator of the presence or absence of a "functional" stapedial artery in Euarchonta. We also describe newly discovered stapedes for extinct euarchontans, Ignacius graybullianus, and Plesiadapis tricuspidens, and use the approach described here to show that these taxa likely did not have a functional stapedial artery. The implications of these findings for auditory function and phylogenetic studies are discussed.