[2][1] 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.

[8][9] Amphicyonids had a relatively rudimentary form of auditory bulla, a bony sheath which encases the middle ear cavity.

[10][9] Like most carnivorans, amphicyonid teeth were adapted for carnivory, with large canines near the front and shearing carnassials at the back of the jaw.

Only two small Miocene amphicyonines, Pseudarctos and Ictiocyon, show any evidence for a hypocarnivorous (majority plant-eating) diet.

[9][11] 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.

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.

[8][9] Thaumastocyonines were the most specialized for hypercarnivory, emphasizing massive blade-like carnassials at the expense of the rest of their postcanine teeth.

[14] Many amphicyonids had cat-like bodies, with a long tail and relatively short, strong limbs suitable for stalking and pouncing on their prey.

[9] 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.