HOMOLOGIES OF THE PROOTIC CANAL IN MAMMALS AND NONMAMMALIAN CYNODONTS

The term prootic canal has been used to describe vascular canals in the adult petrosal bones of extant monotremes and marsupials and presumed vascular canals in the petrosals of extinct ''non-tribosphenic'' mammals (i.e., morganucodontids, docodontids, triconodontids, multituberculates, Vincelestes). Study of the ontogeny of these canals in extant mammals and of their morphology in extinct mammals reveals that the term prootic canal has actually been applied to four different arrangements of canals. The prootic canal variously encloses the prootic sinus (most extinct ''nontribosphenic'' mammals), the prootic sinus in company with the ramus superior of the stapedial artery (the platypus Ornithorhynchus and most multituberculates), the prootic sinus and the lateral head vein (the echidna Tachyglossus), and the lateral head vein alone (didelphid, caenolestid, and some dasyurid marsupials). We propose restricting the term prootic canal to the canal enclosing the prootic sinus (middle cerebral vein) at its exit from the cranial cavity. As so defined, the prootic canal is a canal that: (1) in extinct ''non-tribosphenic'' mammals passes through the lateral trough of the petrosal; (2) in the platypus forms between the petrosal and lamina obturans and also transmits a meningeal branch of the ramus superior; (3) in the echidna forms within the petrosal bone; and (4) in marsupials forms between the petrosal and squamosal. Several presumed vascular canals in non-mammalian cynodonts have been proposed as precursors of the canal enclosing the prootic sinus in mammals. We review the vascular structures of the ear region and lateral braincase in non-mammalian cynodonts and conclude that the most likely precursor is a canal through the lateral flange of the prootic similar to those interpreted as enclosing the prootic sinus in Probainognathus jenseni and Massetognathus pascuali. However, until a prootic canal is found in other non-mammalian cynodonts, those in Probainognathus and Massetagnathus are most reasonably viewed, given current phylogenies, as acquisitions independent of the prootic canal in mammals.