Topography of the chorda tympani nerve and the tensor tympani muscle in carnivores provides a new synapomorphy for Herpestidae (Carnivora, Mammalia)

Abstract

The topographical relationship of the chorda tympani nerve (chorda tympani) to the tensor tympani muscle in the middle ear of carnivores provides new phylogenetic information. The examination of histological serial sections of 16 carnivore species representing most families revealed two distinct character states concerning the course of the chorda tympani: a hypotensoric state with the nerve running below the insertion tendon of the tensor tympani muscle, and an epitensoric state with the nerve running above the tendon. The shift from the plesiomorphic hypotensoric chorda tympani to the apomorphic epitensoric condition occurred once in carnivore phylogeny: Only in the herpestid species under study does the chorda tympani cross above the tensor tympani muscle. Therefore, we introduce the epitensoric pattern as a new synapomorphy for herpestids. Within the herpestids we find the following structural distinctions: Herpestes javanicus and Galerella sanguinea have a chorda tympani running in a sulcus directly above the insertion of the tensor tympani muscle, whereas in the eusocial herpestid species Suricata suricatta and Mungos mungo the chorda tympani lies far above the insertion of the muscle.