Temporal range: Late Eocene–Early Oligocene
|Skeleton of Hesperocyon gregarius|
Hesperocyon is an extinct genus of canids, family of Canidae and subfamily Hesperocyoninae which was endemic to North America from southern Canada to appearing during the Uintan age-Bridgerian age (NALMA) of the Mid-Eocene 42.5 mya—31.0 Ma. (AEO). Hesperocyon existed for approximately .
Hesperocyon was assigned to Borophagini by Wang et al. in 1999 the and 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 familiaris). At least twenty eight known species of Hesperocyoninae evolved out of Hesperocyon, including Ectopocynus (32-19 mya), Osbornodon (32-18 mya), Paraenhydrocyon (20-25 mya), and Mesocyon-Enhydrocyon (31-15 mya).
This early, 80 cm (2 ft 8 in) long canine looked more like a civet or a small raccoon than a canine. Its body and tail were long and flexible, while its limbs were weak and short. Still, the build of its ossicles and distribution of its teeth showed it was a canid. Although it was definitely a carnivore, it may also have been an omnivore - unlike the hypercarnivore Borophaginae that later split from this canid lineage.
Body mass 
Fossil record 
The oldest fossil evidence was recovered from Saskatchewan dating from 42.5 mya—31.0 Ma. The youngest fossil was recovered from the Dog Jaw Butte site, Goshen County, Wyoming dating to the Arikareean age (NALMA) of the Oligocene and Miocene 42.5 mya—31.0 Ma. (AEO).
- Paleobiology Database Collection 16626, Swift Current Creek, Saskatchewan, Canada. Authorized by Dr. John Alroy, entered by J. Alroy on February 18, 1993
- Wang, Xiaoming, Tedford, Richard H. & Taylor B. E. (1999). "Phylogenetic systematics of the Borophaginae (Carnivora: Canidae)". Bulletin of the American Museum of Natural History 243:1-391.
- Benton, Michael J. & Philip C.J. Donoghue (2007). "Paleontological Evidence to Date the Tree of Life". Molecular Biology and evolution 24 (1): 26–53. doi:10.1093/molbev/msl150. PMID 17047029.
- Wang, X. (1994). "Phylogenetic systematics of the Hesperocyoninae (Carnivora, Canidae)". Bulletin of the American Museum of Natural History 221:1-207.
- Legendre S. & Roth C. (1988). "Correlation of carnassial tooth size and body weight in recent carnivores (Mammalia)". Historical Biology 1(1): 85-98.
- Paleobiology Database, Collection 17492, Dog Jaw Butte site, Goshen County, Wyoming. Authorized and entered by Dr. John Alroy, March 26, 1995.