Amphicyonidae is an extinct family of large terrestrial carnivorans belonging to the suborder Caniformia which inhabited North America, Europe, Asia, and Africa from the Middle Eocene subepoch to the Pleistocene epoch 46.2—1.8 Mya, existing for about 44.4 million years. Amphicyonids are often colloquially referred to as "bear-dogs", but are more closely related to true dogs (Canidae) than to bears (Ursidae).
The Forgotten Story of the Beardogs
The family was erected by Haeckel (1886) [also attributed to Trouessart 1885]. It was assigned to Carnivora by Sach and Heizmann (2001); to Arctoidea by Hunt (2001), Hunt (2002), and Hunt (2002); affirmed as Arctoidea by Zhai et al. (2003); affirmed to Arctoidea by Carroll (1988), Hunt (1998), and Wang et al. (2005); and to Caniformia by Morlo et al. (2007).
It is uncertain where Amphicyonids, often referred to as "bear dogs", originated. It was thought that they may have crossed from Europe to North America during the Miocene epoch, but recent research suggests a possible North American origin from the renamed Miacidae, Miacis cognitus and Miacis australis (now renamed as the genera Gustafsonia and Angelarctocyon respectively). As these are of North American origin, but appear to be early amphicyonids, it may be that amphicyonidae actually originates in North America.
The New World amphicyonids include the oldest known amphicyonid, Daphoenus (37-16 Mya) and the subfamily Temnocyoninae (33-20 Mya). These coexisted with their Old World counterparts, including Ysengrinia (30—20 Mya), Cynelos (24—7 Mya) and Amphicyon (23—5 Mya). There is often some confusion with the similar looking (and similarly named) "dog-bears", which are members of the family Hemicyonidae.
Amphicyonids ranged in size from as small as 5 kg (11 lb) and as large as 100 to 600 kg (220 to 1,320 lb) and evolved from wolf-like to bear-like body forms. Early amphicyonids, such as Daphoenodon, possessed a digitigrade posture and locomotion (walking on their toes), while many of the later and larger species were plantigrade or semiplantigrade. The amphicyonids were obligate carnivores, unlike the Canidae, which are hypercarnivores or mesocarnivores.
While amphicyonids were previously thought to be closely related to ursids (bears), there is increasing evidence that they may be basal caniforms.
During the early Miocene, a number of large amphicyonids are thought to have migrated from Eurasia into North America. These taxa belong to the Old World amphicyonid subfamily Amphicyoninae. The earliest to appear is the large bear dog Ysengrinia Ginsburg, followed by Cynelos Jourdan, and then by Amphicyon. This influx of amphicyonines, accompanied by Old World ungulates and small mammals, indicates a prolonged interval (from 23 to 16.5 Mya) of faunal exchange between Asia and North America in the early Miocene, using the trans-Beringian route. New World daphoenines (Daphoenodon, Borocyon) and temnocyonines coexisted with Old World amphicyonines (Ysengrinia, Amphicyon, Cynelos) 23.7-17.5 million years ago. These are the largest terrestrial carnivorans 50 to 200 kg (110 to 440 lb) to have evolved on the North American continent up to this time. The amphicyonines Ysengrinia, Cynelos and Amphicyon appear at 23, 19.2, and 18.8 Mya, respectively, and herald the beginning of a Eurasian amphicyonine migration into North America that continued into the mid-Miocene.
Amphicyonids began to decline in the late Miocene, and largely disappeared in the Pliocene. The reasons for this are unclear: possibly it was due to competition with other carnivorans, but no direct evidence for this has been found. The most recent known amphicyonid remains are teeth known from the Dhok Pathan horizon, northern Pakistan, dating to the early Pleistocene, classically named Arctamphicyon lydekkeri, which may actually be synonymous with/a species of Amphicyon.