Composition of the canid auditory bulla and a new look at the evolution of carnivoran entotympanics

The higher carnivoran taxa significantly differ in the morphology of the auditory bulla, but little is known about its non-ectotympanic elements and their contribution to phylogenetically informative bullar characters. The ventral entotympanic sinus, a principal hypotympanic compartment unique to Canidae, expands in post-ossification ontogeny from a distinct portion, rather than the whole, of what is considered the ‘caudal entotympanic’. To trace the earlier development of this sinus and to clarify the potential roles of individual entotympanics in formation of the canid auditory bulla, osteological observations were made on younger skulls of Canis lupus and four additional species. The ventral entotympanic sinus was found to invariably originate at a separate bone provisionally designated the ventral entotympanic. The rest of the caudal entotympanic is a fusion of the posterior (or the proper) caudal entotympanic ossifying near the tympanohyal, and the anterior caudal entotympanic ossifying between the ectotympanic and rostral entotympanic. Examination of the rostral entotympanic also revealed previously unknown details. In sum, the canid auditory bulla includes at least four rather than earlier recognised two (or suspected three) entotympanics. Based on these findings, the composition of the canid intrabullar septum and the homologies of the carnivoran entotympanics are discussed. Within the established phylogenetic framework, the rostral entotympanic and posterior caudal entotympanic appear as plesiomorphic for crown-group Carnivora, while the anterior caudal entotympanic is synapomorphic for Caniformia, and the ventral entotympanic is autapomorphic for Cynoidea. This hypothesis implies that the carnivoran entotympanic patterns may have emerged before complete fusions of bullar bones observed in the fossil record.