It was comparable in build and size to a fisher or small raccoon, with an estimated body mass of 4.3 kg, and possessed a variety of features that indicate a mix of terrestrial and scansorial locomotion.
Its most notable feature is its unique dentition, with its massive premolars and hypocarnivorous molars, as well as its robust mandible, indicating that it consumed hard-shelled prey, possibly making it the oldest molluscivorous carnivoran known.
In 1982, a crew led by paleontologist Robert Emry an almost complete, beautifully preserved fossil of a male arctoid, which was later designated as holotype (name-bearing specimen) USNM 637259.
So far remains of this taxon, nicknamed "kitten-otter-bear" by its describers, only been discovered in the Fitterer Ranch locality, which dates to the late Orellan or early Whitneyan, and an outcrop of the Chadronian Chadron Formation near Roundtop, Nebraska.
[1][2] The genus name is a combination of Eos, the Greek goddess of the dawn, and arctos, “bear”, while the species name vorax is Latin for “voracious eater”.
The relatively smooth supraoccipital shield also creates a large and wide concavity for attachments of deep neck muscles, markedly the rectus capitis dorsalis major.
Eoarctos possessed a moderately inflated bulla, across the ventral surface of which a distinct ridge runs across half of its diagonal length.
The anterior border is mediolaterally thick, indicating a large masseteric fossa muscle, which is quite deep and can be divided into ventral and dorsal portions.
These rims are asymmetrical in the posterior half of the groove, and the buccal ones are elevated, with the overall muscle attachment areas being slightly tilted dorsally and lingually.
Such a distinct groove is not found in other carnivorans, although the extinct, durophagous otter Siamogale shares the unusually thickened horizontal ramus with a rough ventral surface.
These teeth are broadened relative to their length, and, typically for arctoids, possess a surrounding cingulum while lacking the anterior and posterior accessory cusps seen in canids.
[1] The overall build of Eoarctos was comparable to that of a raccoon or fisher, with an unspecialized neck and predominantly terrestrial locomotion, and it possessed a mix of plesiomorphic and advanced features.
[1] Its scapula had a roughly trapezoid outline, which is diagonally divided by a gently curved spine and possesses nearly parallel cranial and caudal borders.
The robust humerus possessed a rounded head and a greatly expanded lateral supracondylar crest and a large medial epicondyle, and did not share the slender proportions of cursorial carnivorans.
The distinct deltoid tuberosity and a crista for pectoralis muscles extending from the greater tubercle is weakly developed, unlike in fossorial carnivorans, and the reduced deltopectoral crest suggests it was terrestrial and not arboreal.
The ulna's relatively long olecranon leans anteriorly and possesses a strong tilt toward the medial side in cranial view.
The fourth metacarpal has a noticeably enlarged facet for articulation with the uniform, with a maximum mediolateral width that is about equal to that of the dorsoventral length, unlike in most other carnivorans.
The greater trochanter is of the same height as the femoral head, as in Canis, but unlike the condition seen in modern ursids, but its intertrochanteric crest, which is well-developed in canids, is almost indistinguishable, with a trochanteric fossa that isn't particular steep or excavated.
The orientation of the trochlea indicates that it was may have been capable of performing hindfoot reversal, although Wang et al. consider this unlikely due to the rarity of this feature in modern carnivorans.
The distribution of this family, as well as that of a variety of other early arctoids, indicates that North America played a major role in the diversification of these carnivorans.
This diversification, alongside that of the related canids, occurred around the Eocene-Oligocene boundary, during a time in which open environments became more and more common, and terrestrials and even cursorial adaptions evolved in response.
Ursus arctos A large variety of features, including the presence of an astragalar foramen, metatarsals without a keel on the distal face of their head, and widespread toes and fingers, showcases that Eoarctos was a plantigrade animal.
Overall, Eoarctos was likely similar to raccoons and fishers in its lifestyle, spending most of its time foraging on the ground, but being capable of escaping into trees when faced with danger.
However, another specimen with similar injuries survived into old age, despite the loss of its premolars, as it shifted the main focus of crushing hard objects to the first lower molars, which showcase extensive wear.
All this suggests that Eoarctos fed on hard-shelled food, most likely mollusks, which would make it the oldest known arctoid with a durophagous diet, although its teeth were not yet strong enough to resist breakage.
However, its Encephalization Quotient of 0.427 is much lower than that of Hesperocyon (0.71) and the similarly progressive carnivoran Paragale (0.59-0.70), and more comparable to those of the hyaenodont Thinocyon (0.41–0.49) and the miacid Procynodictis (0.44–0.52).
[1] Eoarctos, and other members of its family, lived alongside, and possibly competed with, early canids such as Hesperocyon and Leptocyon, although they were far less common and seem to have been more geographically restricted.
It shared its habitat with a variety of other mammals, including the marsupials Herpetotherium and Nanodelphys, the oreodont Miniochoerus, the ruminant Leptomeryx, the canid Osbornodon and the bat Quinetia.