Postcrania and locomotor function of Mesocyon coryphaeus (Canidae, Carnivora) from the Arikareean of North America

Canids increased in cursoriality through the Cenozoic, as environments transitioned from closed-canopy forest to open grassland and steppe. Canids have evolved through a series of radiations since their origin in the Eocene, but it is unclear if cursorial adaptations appeared in the earliest of these radiations. In the middle Oligocene, the basal hesperocyonines ecologically diversified, and the coyote-sized Mesocyon coryphaeus exemplified the transition from smaller, omnivorous canids to larger, hypercarnivorous forms. M. coryphaeus is exclusively known from the John Day Formation of North America. Although M. coryphaeus is a relatively common fossil in this formation, first recognized in the late 19 th century, no postcranial material from this species has ever been formally described. Here, we present a near-complete skeleton of M. coryphaeus , JODA 3366, which includes a complete cranium, near-complete presacral spine, all long bones, elements of both the manus and pes, and a baculum. The short, robust limbs, mobile elbow joint, and tarsal morphology of M. coryphaeus indicate that this species retained a plantigrade to semidigitigrade posture, similar to the earliest canid Hesperocyon , and lacked the cursorial adaptations found in more derived canids. Based on this morphology, we interpret M. coryphaeus as a terrestrial ambush predator, more similar to large mustelids than extant canids, likely hunting small prey like hypertragulids. Although the habitat of M. coryphaeus would have been cooler and more open than the dense closed-canopy forests of the Eocene, enough vegetation cover was still present in the Oligocene for ambush hunting to remain a successful strategy.