Oxyaenidae

The name of order Oxyaenodonta comes from Ancient Greek ὀξύς (oxús) 'sharp', name of hyena genus Hyaena and from Ancient Greek ὀδούς (odoús) 'tooth'.

The name of family Oxyaenidae comes from Ancient Greek ὀξύς (oxús) 'sharp', name of hyena genus Hyaena and taxonomic suffix "-idae".^[6]

They were superficially otter- or badger-like placental mammals that walked on flat feet, in contrast to most modern Carnivora, which walk and run on their toes. Though most genera were medium-sized by modern standards, they may have been the earliest group of large carnivorous mammals.^[7][8] The largest known oxyaenid was Sarkastodon mongoliensis, which could have weighed 800 kg (1,800 lb). However, this may have been an overestimate.^[9] All had two molars on each side of both the upper and lower jaw.^[10] While many oxyaenids, such as Patriofelis, were hypercarnivores,^[11] some such as Oxyaena were more omnivorous, with meat-based but varied diets similar to modern brown bears and racoons.^[12] Of the four families:

• The Tytthaeninae were the earliest representatives of the group, generally smaller and with more unspecialized bodies and primitive features.^[13]
• The Paleonictinae included intermediate-sized hypercarnivores and mixed feeders.^[13]
• The Oxyaeninae included the largest genera, such as Sarkaskadon, Patriofelis, and Oxyaena. Some genera showed Hunter-Schrager bands on their molars, typical of bone-cracking. They were adapted for terrestrial locomotion and probably hunted or scavanged on forest floors.^[11]
• The Macheroidinae were a small number of specialized hypercarnivores adapted for arboreal locomotion, with long slender bodies and strong grappling forelimbs^[14] This group was the first saber-toothed mammals in the fossil record. A flange on the lower jaw partially protected their long, slender saber canine teeth. They ranged from civet- to leopard-sized, probably lived in the trees of closed-canopy forests, and ambushed prey larger than their bodies. Since most large herbivores at the time were terrestrial, it is likely they dropped from trees onto their prey.^[15]

Overall, oxyaenids had long, flat heads, long bodies with short legs, large claws, and reinforced, inflexible lower spines. The forelimbs were strong and flexible, and could twist to embrace and grapple, like the forelimbs of cats and unlike those of dogs. These adaptations suggest that hunting Oxyaenids were solitary ambush predators that would capture larger prey.^[7][14] Their extinction in the Eocene may be connected to the reduction of closed-canopy tropical jungles, which dominated the world in the hothouse of the Paleocene-Eocene Thermal Maximum. No modern carnivoran has an inflexible spine, which would reduce maneuverability and running speed in more open environments.^[16]

Oxyaenodonts were believed to have evolved in the middle Paleocene in North America with the oldest known oxyaenodont, Tytthaena, being found there.^[17][18] Oxyaenodonts would disperse into Europe near the Paleocene-Eocene boundary via the De Geer route, however the timing of arrival in Asia is unknown.^[17]

• Mammal classification
• Ferae
• Creodonta