The entotympanic of Equus caballus (Perissodactyla,Mammalia)

In the majority of extant placental mammals the bulla tympanica is composed of two skeletal elements, the entotympanic and the ectotympanic. Former studies revealed that the presence of an entotympanic in the bulla tympanica of extant Perissodactyla is restricted to Rhinocerotidae. The existence of the entotympanic in Tapiridae and Equidae remained speculative. Here we present the first evidence of an entotympanic, strictly speaking rostral entotympanic, in the domestic horse, Equus caballus. The enchondrally ossified entotympanic can be easily separated from the desmal ectotympanic by its greater thickness and by its cancellous bone texture in a late fetal stage. Both elements are separated by a suture that is in the process of coalescence. The complete fusion of the two elements and the unification of bone texture are almost accomplished at birth but the entotympanic and ectotympanic assume the same thickness obviously not until early postnatal development. Based on modern phylogenetic hypotheses we can conclude that the common ancestor of Perissodactyla must have possessed a well-developed entotympanic, probably only evident in their fetal life. This must be considered as a plesiomorphic character state of this order, because the entotympanic is a neomorphic apomorphy of placental mammals. However, the prenatal fusion of the entotympanic and the ectotympanic is an apomorphy of Equus caballus and possibly of the Equidae as a whole.