Shape variation and ontogeny of the ruminant bony labyrinth, an example in Tragulidae

Despite its growing use in anatomical and ecological studies, the morphological variability and ontogenetic development of the bony labyrinth have very rarely been investigated in ruminants. Here we study its morphology in 15 adult and 10 juvenile specimens in the three extant tragulid ruminant genera. Intraspecific and interspecific variability is quantified using morphometric and 3D geometric morphometrics analyses. The bony labyrinth of Tragulus, Hyemoschus, and Moschiola is strikingly different, clustering in clearly different morphospaces despite similar ecological adaptations. Although the bony labyrinths within two species of the same genus cannot be distinguished from each other based on the chosen semi-landmarks, discrete interspecific differences exist. We were able to show for the first time that an artiodactyl mammal in a late fetal stage possesses an almost fully formed bony labyrinth similar to that of adults. No significant change either occurs in size or morphology after ossification of the petrosal bone. Some intraspecific variation is observed on the shape of the lateral semi-circular canal, the size and shape of the common crus, the coil of the cochlea or the stapedial ratio. Variable structures are expected to be highly informative characters for a large cladistic analysis. They can be used for phylogenetic studies in ruminants. Incorporating juvenile specimens in studies is not problematic, as they fall within the morphological range of adults.