Partitions in the carnivoran auditory bulla: their formation and significance for systematics

The common notion that the septa in the carnivoran auditory bulla are formed by the growth of bone edges inwards the bullar cavity is a mistaken assumption based on the data of the late 19th century. Intrabullar partitions are in fact a result of the differential resorption of the bulla internal surface during the growth of the external surface. A septum develops at the boundary between local, relatively independent, ‘inflations’ of the bulla wall. This explanation, given by Van Kampen in 1905 for the case of the Canidae and some Mustelidae, can be applied to the whole order, including the aeluroid families with their ‘bilaminar’ septum bullae. Such an approach seems to solve the problem of homology of the intrabullar septa throughout the Carnivora, a question which has long been confused because of insufficient knowledge of septum morphogenesis. The partitions can really be considered as indicators of independent attempts to increase the size of a middle‐ear cavity among the infraorders. This conclusion follows immediately from the difference between major carnivoran taxa in the arrangement of separate inflations on the bulla wall, which can be considered as additional sinuses enlarging the hypotympanic space. It is precisely this difference that conditions the relative contribution of several bones (mainly of the ectotympanic and caudal entotympanic) to the intrabullar septa. Thus, the initial topographies of the above sinuses – whichever subsequent bone modelling of septa occurs – represent unique patterns useful in the higher‐level systematics of the Carnivora.