Eoarctos is an extinct genus of arctoid carnivorans, known from the latest Eocene to early Oligocene of North Dakota and Nebraska. It is known from several remains, the most notable of which is the almost perfectly preserved skeleton of a large male. 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.
Discovery and etymology Fossils belonging to Eoarctos were first discovered in the 1940s, when its unique jaws were found at the Fitterer Ranch fossil site in North Dakota, but not described until decades afterward, although they were occasionally referred to the species Subparictis dakotensis. 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. Emry and his colleague Richard Tedford, who recognized that the fossil represented a new species, planned to describe it, but the already slow moving project remained unfinished after the death of Tedford in 2011. However, several other paleontologists were invited by Emry to continue the project, and the description of Eoarctos vorax was finally published in 2023. 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.
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”.
Description
Skull and dentition The general skull proportions of Eoarctos are similar to those of early canids, such as Hesperocyon, although the rostrum and frontal shield are broader, the orbital area between the postorbital process of frontal and the postorbital constriction more elongated. Their snout appears to be relatively short, however this is the result of its broadening and not any shortening. Atypically, the sutures between individual skull bones are very strongly fused even among young adult individuals, which may be an adaption for its durophagous diet. Whereas the premaxilla and maxilla are completely fused, there is an unfused suture between the premaxilla and the nasal, indicating a rather wide nasal process and likely resulting in a gap between the posterior tips of premaxilla and anterior tips of the frontal not seen in modern ursids. The flat surface of the wide frontal suggests that it lacked the domed forehead typical for early canids. The crests located just behind the postorbital process form a sharp ridge, with a thin grove located just in front of it and slight bulges along the central segment. The sagittal crest is comparatively high, especially at its posterior end. The postorbital constriction, which has a minimum width of 13.13 mm, is located 13.5 mm caudal to the postorbital process, which is more than twice the distance seen in early canids. This is because many ursoids, including Eoarctos, possessed an elongated postorbital area, with a possible explanation being that their brains are less inflated than those of canids. The laterally expanding wings of the interparietal form a large attachment area for the temporalis muscle. The relatively smooth supraoccipital shield also creates a large and wide concavity for attachments of deep neck muscles, markedly the rectus capitis dorsalis major. It possesses a strong zygomatic arch, with a dorsal rim that curves smoothly on the right side and has a small postorbital on the left side. The distinct medial rim of the rim of the squamosal process creates a distinct, flat, dorsolaterally facing area. This feature seems to be unique to Eoarctos, although many other taxa lack similarly well-preserved zygomatic arches, making comparison difficult. This area of the arch didn't possess any muscular attachments, but was likely related to the processing of hard food, possibly being related to the attachment of the masseter musculature between the ascending ramus and zygomatic arch. The lateral scar for the attachment of the masseter muscle is prominent and runs the entire length of the jugal. Another unique feature of the genus is a distinct, large muscle scar area located between bullae, which crosses over to the basisphenoid surface. Among modern wolves and pandas, this scar serves as attachment area for the longus capitis and rectus capitis ventralis muscles, however it is located farther anteriorly in Eoarctos than in these extant carnivorans. Eoarctos possessed a moderately inflated bulla, across the ventral surface of which a distinct ridge runs across half of its diagonal length.
The coronoid process of the mandible is proportionally quite large, tall, and wide, with a rectangular outline when viewed laterally. 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. It also possessed a well-developed superficial temporalis muscle. A unique, longitudinal groove is located in the ventral border of the horizontal ramus. The bone is distinctly swollen on the lingual and buccal sides of the groove, which is therefore flanked by two longitudinal rims. 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. As the bone textures suggest muscle attachment, the likeliest explanation for this groove is the insertion of the digastric muscle. However, the lateral facet of the paroccipital process, where the digastric muscle originates, is not enlarged. 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. Another possible explanation for this unusual feature is the attachment of the genioglossus muscle. The horizontal ramus is greatly thickened, measuring 6.9 mm in mediolateral thickness and only 12.7 mm deep, resulting in a ratio of thickness to depth of 0.54, comparable to Siamogale and the bone-crushing canid Borophagus. However, considering the massive proportions of the jaw, the symphysis is rather weak. Notably, the two hemimandibles are not fused, possibly because of unilateral bites during maximum exertions.
Among the upper incisors, I1 and I2 have similar dimensions, except for the crown height, whereas the I3 is almost twice as large. The same dimensions can be observed in the lower incisors, with the third incisor being twice as large as the first two. A cutting blade is formed by a chisel-like cutting edge at the top of their crowns, while the lingual surface possesses a shovel-like concave surface. The robust canine is proportionally shorter and less curved than in Hesperocyon, and its enamel surface lacks the distinct groove seen in some other arctoids. The lower canine protrudes forwards, and possesses a bulbous base. Among many arctoids, the canines showcase signs of sexual dimorphism, however, this does not seem to be the case in Eoarctos. As in other subparictids, the premolars are robust and enlarged, however this development is more advanced in Eoarctos than in other members of the family. 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. Another features it shares with other derived subparictids is are the extensive crenulated textures found on the enamel surfaces of their upper premolars. The P1 is the smallest upper premolar, and there is almost no diastema separating it and the slightly larger P2. As in Nothocyon the P3 possesses a prominent lingual bulge supported by an extra root, although this feature is more extremely developed in Eoarctos. A cingulum, which is thickest mesially and lingually, surrounds the P4. The protocone is lingually dispalaced, and the paracone buccolingually thickened, resulting in the occlusal outline of the tooth being overall widened. Among the lower premolar, the single rooted p1 is reduced in size relatively to the other teeth. The other premolars are double rooted, and p1-p3 possess ridges along the occlusal surface from the anterior margin, which almost appear as a single ridge if seen from occlusal view. The p2 and p3 are of similar dimensions and shape as one another, but the p4 is notably larger. As in other ursoids, the upper molars appear to be mesiodistally lengthened and furthermore possess a somewhat rectangular outline. Both M1 and M2 have protocones with mesiodistally greatly expanded preprotocrista and postprotocrista as well as enlarged paraconule and metaconule, whereas the lingual cingulum is shortened. The first lower molar (m1) is low crowned, with a trigonid that is not even half the height of the talonid. A distinct but low carnassial blade is formed by the paraconid and protoconid. The wear facet of the tooth indicates contact with the lingual surface of the P4 carnassial blade and a paraconule as well as the paracone of M1. A notable feature of the m2 is the extreme shift of the strongly reduced trigonid to the mesiolingual corner of the tooth. Relative to m1, m2 is reduced in anteroposterior length, and m3 is completely absent. The molars show adaptions towards hypocarnivory.
Postcranial skeleton 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. Its baculum was overall similar to that of Hesperocyon, although with a deeper urethral groove at its proximal end, and not as derived as that of later ursids.