The petrosal and bony labyrinth of extinct horses (Perissodactyla, Equidae) and their implications for perissodactyl evolution

Perissodactyla, or odd-toed ungulates, are represented today by 16 species of rhinoceroses, tapirs, and horses. Perissodactyls were much more diverse in the past, having a rich fossil record spanning from the earliest Eocene (~56 Ma) to recent, including a myriad of extinct lineages. Despite over a century of study, the inter-relationships of some extinct perissodactyl families remain poorly resolved. New morphological characters are needed to help solve this issue. Recent studies suggest that the ear region, i.e ., the petrosal and the bony labyrinth of the inner ear, is a valuable source of morphological characters for mammalian phylogenetic analyses. The petrosal is the bony structure protecting the inner ear, the organs of hearing and balance in mammals. However, perissodactyl petrosals are poorly documented and have not been used in such a phylogenetic framework. In this study, we describe the petrosals and inner ears of five European fossil equid taxa and perform a preliminary phylogenetic analysis. Despite its small sample size, our phylogenetic analysis recovers important groupings, which suggests the petrosal is phylogenetically informative in equids. This study supports the relevance of the ear region for phylogenetic inference and its potential to better resolve long-contentious relationships within Perissodactyla.