Amphicyonidae

Amphicyonidae is an extinct family of terrestrial carnivorans 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".

The family was erected by Haeckel in 1866 (also attributed to Trouessart 1885). Their exact position has long been disputed. Early paleontologists usually defined them as members of Canidae (the dog family) or Ursidae (the bear family), 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, a lineage older than the origin of both bears and dogs.

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 pinnipeds) or Arctocyonidae (a family of "condylarths" which literally translates to "bear-dogs").

Amphicyonids ranged in size from as small as 5 kg (11 lb) and as large as 100 to 773 kg (220 to 1,704 lb) and evolved from wolf-like to bear-like body forms.

Amphicyonids tended to have relatively large skulls, with the snout shorter than the rear portion of the cranium. In some large members of the family, such as Amphicyon, the back of the skull develops a sharp sagittal crest which defines attachment points for large jaw muscles.

Amphicyonids had a relatively rudimentary form of auditory bulla, a bony sheath which encases the middle ear cavity. The bulla is small, mostly formed by the crescent-shaped ectotympanic bone below the middle ear. The entotympanics 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, otters, walruses, eared seals, 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.

Like most carnivorans, amphicyonid teeth were adapted for carnivory, with large canines near the front and shearing carnassials at the back of the jaw. Amphicyonids were typically mesocarnivorous (majority meat-eating, like dogs) or hypercarnivorous (entirely meat-eating, like cats), and some were adapted for tough abrasive food. Only two small Miocene amphicyonines, Pseudarctos and Ictiocyon, show any evidence for a hypocarnivorous (majority plant-eating) diet.

At the start of their evolution, amphicyonids retained the typical placental dental formula of 3.1.4.33.1.4.3, but each subfamily follows their own trend in modifying their teeth. Daphoenines, for example, have dog-like teeth, with substantial premolars and reduced second and third molars. Temnocyonines and haplocyonines take this approach even further, with massive crushing premolars akin to hyenas. Amphicyonines 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.