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Amphicyonidae is an extinct family of terrestrial belonging to the suborder Caniformia. They first appeared in North America in the middle Eocene (around 45 mya), spread to Europe by the late Eocene (35 mya), and further spread to Asia and Africa by the early Miocene (23 mya). They had largely disappeared worldwide by the late Miocene (5 mya), with the latest recorded species at the end of the Miocene in Africa. They were among the first carnivorans to evolve large body size. Amphicyonids are colloquially referred to as " bear-dogs".Xiaoming and Richard H. Tedford Wang (2025). 9780231135283, Columbia University Press. ISBN 9780231135283
Amphicyonids should not be confused with the similar looking (and similarly nicknamed) "dog-bears", a more derived group of caniforms that is sometimes classified as a family (Hemicyonidae), but is more often considered a primitive subfamily of ursids (Hemicyoninae). They should also not be confused with Amphicynodontidae (another family of extinct caniforms which were related to bears or Pinniped) or Arctocyonidae (a family of "Condylarthra" which literally translates to "bear-dogs").
Amphicyonids had a relatively rudimentary form of auditory bulla, a bony sheath which encases the Middle ear. The bulla is small, mostly formed by the crescent-shaped ectotympanic bone below the middle ear. The Entotympanic only make a minor contribution whenever they are ossified, which only becomes commonplace in Miocene amphicyonids. In these regards, amphicyonids are similar to living bears, Otter, Walrus, Eared seal, and the red panda. The bulla also helps to distinguish the evolutionary trajectory of amphicyonids: early bears such as Cephalogale have large bullae which are reduced through the course of their evolution, while dogs start out with large bullae which persist through their entire existence. Amphicyonids differ from both dogs and bears in that they start with a small bulla which gradually becomes more strongly developed later in their evolution.
At the start of their evolution, amphicyonids retained the typical placental dental formula of , but each subfamily follows their own trend in modifying their teeth. Daphoeninae, for example, have dog-like teeth, with substantial Premolar and reduced second and third molars. Temnocyoninae and Haplocyoninae take this approach even further, with massive crushing premolars akin to Hyena. Amphicyoninae follow the opposite path, reducing most premolars and greatly enlarging and strengthening the carnassials and second molar. Bears also have large molars, but their teeth are modified into wide rectangular forms for grinding plant material. Amphicyonids did not pursue the same adaptations; their upper molars always maintain a roughly triangular profile for shearing and crushing meat. Thaumastocyonines were the most specialized for hypercarnivory, emphasizing massive blade-like carnassials at the expense of the rest of their postcanine teeth.
Fossils of juvenile Agnotherium, Ischyrocyon, and Magericyon all show an unusual type of tooth eruption in which there is a vulnerable stage at about two or three years of age where the subadult animal has no functional molar or carnassial teeth, the only functional cheek teeth being several milk premolars. This period is suggested to be short and would have left the animal somewhat vulnerable.
Many amphicyonid lineages instead adopted a digitigrade posture and locomotion (walking on their toes) and long legs specialized for running with a primarily front-to-back arc of movement. These cursorial wolf- or hyena-like forms included temnocyonines, haplocyonines, and some species of the large daphoenine Daphoenodon.
Other New World amphicyonids include the oldest known amphicyonid, Daphoenus (37–16 Mya).
Amphicyonids began to decline in the late Miocene, and disappeared by the end of the epoch. The exact reasons for this are unclear. The most recent known amphicyonid remains are teeth known from the Dhok Pathan horizon, northern Pakistan, dating to 7.4-5.3 mya. The species is classically named Arctamphicyon lydekkeri, which may actually be synonymous with a species of Amphicyon.
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