INNER EAR AND OLDEST REPORTED CARNIVORAN AUDITORY OSSICLES FROM AN OLIGOCENE CANID, MESOCYON, OF THE BRULE FORMATION, ND

All mammals possess three auditory ossicles, or ear bones, contained within the middle ear: the stapes, incus, and malleus. Differences in the size and morphology of these ossicles can affect the range of hearing across mammalian groups, so investigations of these elements can reveal insights into the biology of extinct animals. However, ear ossicles are extremely small and delicate and often do not survive the fossilization and collection process. Here, we report an unusually well-preserved specimen of the early-diverging canid Mesocyon temnodon, recovered from the Oligocene Brule Formation of Stark County, North Dakota, providing the opportunity to study the ear anatomy of early canids. Computed tomographic (CT) scanning was used to reveal details of the teeth (to confirm taxonomic identity), middle ear, inner ear (bony labyrinth), and brain cavity. Sectioning of these scans revealed some of the oldest ear ossicles among carnivorans, in addition to well-preserved semi-circular canals and cochlea. M. temnodon has been suggested to be one of the earliest dominantly carnivorous, cursorial canids as opposed to its semi-arboreal relatives. The ear ossicles and bony labyrinth exhibit broad similarities to those of modern canids (e.g. wolves) and recently extinct canids, suggesting a near-modern range of hearing and perhaps a more cursorial lifestyle. Measurements of the semi-circular canals roughly align with those of other cursorial genera (Canis, Felis); the anterior semi-circular length 10.41mm, posterior semi-circular length is 10.11mm, and lateral semi-circular length is 7.59mm. Larger relative semi-circular canal radii are generally interpreted to be associated with more agile species, contrasted with smaller radii of slower-moving species. Additionally, the arc radii of M. temnodon are similar to those of Canis and other cursorial genera; the anterior semi-circular arc radius is 2.25mm, posterior semi-circular arc radius is 1.97mm, and lateral semi-circular arc radius is 1.76mm. These findings suggest that early canids possessed hearing and locomotor capabilities on par with modern canids as far back as the early Whitneyan North American Land Mammal Age (~32 mya).