The chorda tympani and its significance for rodent phylogeny

The topographical relationship of the chorda tympani to the tensor tympani muscle in the middle ear region of rodents was recorded, finding that this character provides new phylogenetic information. For this purpose we examined histological serial sections of 63 rodent species representing most families. Two lagomorph species were used as an outgroup. Computed 3D models of middle ear structures of several rodent taxa were prepared for the first time. We observed two different character states concerning the topography of the chorda tympani and the insertion tendon of the tensor tympani muscle: hypotensoric and epitensoric; the latter displays three subtypes. We assume that the shift from a plesiomorphic hypotensoric chorda tympani to an apomorphic epitensoric condition occurred at least six times independently in rodent phylogeny; however, this derived state is bound to higher taxonomic levels. The anomalurids under study show an epitensoric course of the chorda tympani through a mallear channel above the insertion of the tendon – just as in Pedetes capensis. Therefore, we suggest a new synapomorphy for these two taxa, confirming other new evidence as to their monophyly. The principal quality of the character used in this study is discussed.