Bear dog


Amphicyonidae is an extinct family of terrestrial carnivorans belonging to the suborder Caniformia. They first appeared in North America in the middle Eocene, spread to Europe by the late Eocene, and appear in Asia, and Africa by the early Miocene. They had largely disappeared worldwide by the late Miocene, with the latest recorded species at the end of the Miocene in Pakistan. They were among the first carnivorans to evolve large body size. Later in their history, they came into competition with hesperocyonine and borophagine canids. As dogs evolved similar body sizes and cranial and dental adaptations, the rise of these groups may have led to their extinction. Amphicyonids are often colloquially referred to as "bear-dogs".

Taxonomy

The family was erected by Haeckel . Their exact position has long been disputed. Some early paleontologists defined them as members of the family Canidae, but the modern consensus is that they form their own family. Some researchers have defined it as the sister clade to ursids, based on morphological analysis of the ear region. However, cladistic analysis and reclassification of several species of early carnivore as amphicyonids has strongly suggested that they may be basal caniforms, from lineages older than the origin of both bears and dogs.

Description

Amphicyonids ranged in size from as small as and as large as and evolved from wolf-like to bear-like body forms.
Early amphicyonids, such as Daphoenodon, possessed a digitigrade posture and locomotion, 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.
There is often some confusion with the similar looking "dog-bears", a more derived group of caniforms that is sometimes classified as a family, but is more often considered a primitive subfamily of ursids.

Evolution

It has long been uncertain where amphicyonids 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 miacids Miacis cognitus and M. australis. As these are of North American origin, but appear to be early amphicyonids, it may be that the Amphicyonidae actually originates in North America.
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 of faunal exchange between Asia and North America in the early Miocene, using the trans-Beringian route.
New World daphoenines and temnocyonines coexisted with Old World amphicyonines 23.7 to 17.5 million years ago. With estimated weights of, these were the largest terrestrial carnivorans to have evolved on the North American continent up to this time. Other New World amphicyonids include the oldest known amphicyonid, Daphoenus.
Amphicyonids began to decline in the late Miocene, and disappeared by the end of the epoch. The reasons for this are unclear; possibly it was due to competition with other carnivorans, as large canids had better adaptations to pursue swift prey in open country, a habitat which spread globally as the new grassland biome replaced more closed environments. However, no direct evidence for this idea has been found. 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.

Classification