The petrosal bone and inner ear of Micromeryx flourensianus (Artiodactyla, Moschidae) and inferred potential for ruminant phylogenetics

While petrosal bones have a long research history in artiodactyl phylogenetics, the inner ear embedded in this bone has rarely been investigated. I describe here a set of petrosals and the associated inner ears of the Middle Miocene moschid Micromeryx flourensianus from the German locality Steinheim and compare them to the extant musk deer Moschus moschiferus (Moschidae), the four-horned antelope Tetracerus quadricornis (Bovidae) and the white-tailed deer Odocoileus virginianus (Cervidae). Inner ears were reconstructed using high resolution x-ray computed tomography scans. In phylogenenetic reconstructions built on morphological and molecular data, Moschidae has variously been shown to be a sister taxon to Bovidae or Cervidae. Its position hasn’t reached a consensus yet. Studying the inner ear morphology adds new morphological characters that will help resolving this question. Micromeryx flourensianus is an abundant fossil moschid and I show indeed that its petrosal bone and inner ear share several similarities with that of the extant musk deer such as a ventral basicapsular groove, a well-developed anterior process of the tegmen tympani, or a fossa for the tensor tympani muscle in the musk-deer that may well have evolved from a Micromeryx-like condition. Inner ears share a thick basal cochlear whorl, a bulky vestibule, or a short and thick cochlear aqueduct. This shows that inner ears have a high potential for taxonomy and phylogenetics. Including the inner ear of a fossil skull of Micromeryx flourensianus also from Steinheim, four inner ears are described here and give insights into the morphological variability of this structure at an intraspecific level as well as into the post-natal ontogenetic changes that occur. This contribution is a first step towards a comprehensive understanding of the evolution of the ruminant inner ear.