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Hesperocyon is a canid from late Eocene to early Oligocene North America. ranging from southern Canada to Colorado. It appeared during the Uintan age-Bridgerian age (NALMA) of the Mid-Eocene 42.5 mya—31.0 Ma. (AEO). Hesperocyon existed for approximately 11.5 million years. 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. The Canidae subfamily Hesperocyoninae probably arose out of Hesperocyon to become the first of the three great dogs groups: Hesperocyoninae (~40-30 Ma), Borophaginae (~36-2 Ma), and the Caninae lineage that led to present-day Canidae, inclusive of modern-day wolves, foxes, coyotes, jackals and dogs (Canis lupus familiaris). At least 28 known species of Hesperocyoninae evolved out of Hesperocyon, including those in the following five genera: Ectopocynus (32-19 mya), Osbornodon (32-18 mya), Paraenhydrocyon (20-25 mya), Mesocyon (31-15 mya) and Enhydrocyon (31-15 mya).

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