Hesperocyon was assigned to Borophagini by Wang et al. in 1999[2] and was the earliest of the canids to evolve after the Caniformia-Feliformia split some 42 million year ago. Fossil evidence dates Hesperocyon gregarius to be at least 37 million years old, but the oldest Hesperocyon has been dated at 39.74 mya from the Duchesnean North American Land Mammal Age.[3]