The auditory bulla in carnivora: An anatomical basis for reappraisal of carnivore evolution

Abstract

Basicranial anatomy of mammalian carnivores is significant in the determination of carnivore evolution. One of the least understood yet most often studied features of the basicranium is the auditory bulla, a bony capsule enclosing the middle ear. Although previously believed to be formed by a tympanic bone alone, or by tympanic and entotympanic, it is shown here that the carnivore bulla is formed by three and in some cases four ontogenetic elements: tympanic, rostral entotympanic, caudal entotympanic(s). These elements in Carnivora appear to correspond to bulla elements discovered over 50 years ago by Van der Klaauw in representative species of several other orders of mammals. Increased auditory sensitivity appears to have been attained in various carnivore lineages by increase in the volume of the middle ear cavity, principally by hypertrophy of the caudal entotympanic, but also by different yet less common anatomical strategies such as invasion of the mastoid bone by the middle ear space. Five basic types of bulla among living Carnivora can be recognized, based largely on the relationship of the caudal entotympanic to the other bulla elements. Closely associated with the bulla in the auditory region are the median and promontory branches of the internal carotid artery. These branches exhibit a precise relationship to the bulla elements, particularly the rostral entotympanic. The promontory branch is much reduced or completely lacking in all living carnivores. Reduction or loss of the median branch is explained as the result of the development of arterial retia on the anastomotic artery in the orbital region. Reduction of the median branch is particularly characteristic of the aeluroid Carnivora, in which a greater proportion of the cerebral blood supply passes to the brain in the external carotid artery via the anastomotic branch, and is cooled in the orbital region by a countercurrent heat exchange mechanism formed by arterial retia closely associated with venous blood in surrounding sinuses. The arctoid and cynoid Carnivora are characterized by only rudimentary development of retia along the anastomotic artery, and the median branch is much better developed than in the aeluroids. Among arctoids, ursids parallel the aeluroids in the development of a countercurrent mechanism but employ the median branch of the internal carotid rather than the anastomotic branch of the external carotid artery. Bulla structure and nature of the carotid circulation in the auditory region can be determined from fossil Carnivora as well as for living forms. This study attempts to define basicranial anatomical patterns characteristic of living Carnivora which then can be used to trace the evolution of lineages in the fossil record.