Transformations in the extracranial course of the internal carotid artery in mammalian phylogeny

Study of cranial-artery ontogeny reveals that three adult character states of the extra-cranial portion of the single mammalian internal carotid artery should be recognized: (1) transpromontorial—on the ventral surface of the promontorium of the petrosal bone, within the middle-ear cavity; (2) perbullar—within a canal in the medial wall of the auditory bulla; and (3) extrabullar—medial to the auditory bulla and middle-ear cavity. The phylogenetic transformation of these three character states is assessed via five criteria of testing morphocline polarity: (1) ontogenetic transformations; (2) commonality; (3) out group comparisons; (4) relative geochronologic age; and (5) “Form–function complexes.” Each of these “tests” supports the (laterally-placed) transpromontorial character state as primitive for Mammalia and Eutheria and the (medially-placed) perbullar and extrabullar states as derived conditions. This character-state polarity, the reverse of that generally accepted at present, supports one superordinal grouping within Mammalia—a perissodactyl-hyracoid clade—and subordinal and lower taxonomic groupings within Carnivora, Lagomorpha, Chiroptera, and Primates.