During the mid-Pliocene, the Canadian High Arctic would have been forested, and the latitudinal gradient was much less than modern, so that although global temperatures were 3-4 degrees warmer than modern, the mean annual temperature of the terrestrial High Arctic was ~22 °C warmer . The Beaver Pond site comprises the remains of a Pliocene forest wetland community that was dominated by larch , and also supported alder and birch , spruce , pine and cedar . Multiple proxies consistently suggest a Pliocene mean annual temperature at the Beaver Pond site of slightly above freezing, with plant community composition indicating a warmest summer air-temperatures of ~20°C. Coldest winter temperatures have been recently estimated from vegetation to be ~−12°C, though a prior estimation from beetle fauna suggest −27 °C. Modern Mean Annual precipitation in the area is 104mm/year, whereas in the Pliocene the plant community implies precipitation to have been ~550mm/year . In fossil vertebrates, the Beaver Pond site, in combination with the nearby Fyles Leaf Bed fossil site, has produced four native North American mammals: a castoroidine beaver Dipoides sp., an archaeolagine rabbit Hypolagus cf. H. vetus, a small canine dog Eucyon, and a cameline camel. Of these, Eucyonand Paracamelus had arrived at Eurasia near the Mio-Pliocene boundary,pots with drainage holes and they may be closely related to the ancestral stock that gave rise to the Eurasian forms.
The rest of the faunal components include a frog, a percid fish, Sander teneri, of Eurasian origin, and ten mammal taxa which share considerable similarity to equivalent-aged faunal assemblages in East Asia, including a neomyine shrew Arctisorex polaris, a microtine-like cricetid similar to Microtodon or Promimomys, a large wolverine , a fisher -like carnivore, a marten-like carnivore Martes cf. M. americana, a weasel Mustela sp., a meline badger Arctomeles sotnikovae, a three-toed horse Plesiohipparion, a possible cervoid Boreameryx braskerudi of unknown origin, plus an ursine bear “Ursus abstrusus” described herein. Te third author has also identified a duck closest to the Greater Scaup .Skeletal remains of the fossil bear were collected in different years from the Beaver Pond site . Te skull specimen, with upper teeth appears to be a young adult . Te exoccipital-basioccipital, exoccipital-supraoccipital, and premaxilla-maxilla sutures are largely fused, whereas the internasal and interfrontal elements are unfused. In the modern black bear this degree of fusion of the cranial elements suggests the individual is between five and seven years old. Te upper teeth, particularly the premolar and molar cheek teeth, are essentially pristine and show wear only on the tip of the upper right canine, incisors, and anterior edge of M1, which also suggest a relatively young individual . In contrast, there is extensive wear on the lower teeth , indicating the mandibles are from an individual much older than that of the cranium . Te symphysial sutures of the leftt and right dentaries occlude perfectly, and the wear patterns on the lower teeth on either side are comparable, indicating a single individual for the lower jaws. There are thus a minimum of two individuals. Judging by the lack of fusion between epiphysis and diaphysis, the postcranial elements may belong to the younger individual represented by the skull .A phylogenetic analysis was conducted using 24 taxa and 59 morphological characters and all seven living ursines . A new single shortest tree was found by New Technology search in TnT with a tree length of 145, consistency index of 0.51, and retention index of 0.70 .
The topology of modern taxa was constrained using nuclear DNA evidence of Kutschera et al. and the whole genome analysis of Kumar et al. . Assuming the molecular relationship is correct, six extra morphological steps are required to account for this new relationship. Protarctos abstrusus appears basal to all modern bears, including Tremarctos, the spectacle bear of South America. Moreover, its phylogenetic position suggests aEurasian origin for this lineage. Asia appears to be of vital importance in the early diversifcation of ursines: Not only is Asia home to all basal ursines still alive today but the most advanced stem form, Ursavus tedfordi, leading to the ursines is also found in east Asia, as well as early ursines such as Ursus yinanensis .Judging from dental wear, the partial skull and mandibles from the Beaver Pond site represent two individuals of Protarctos abstrusus. Both show evidence of dental caries, particularly on teeth that have sustained the most wear; a pit usually develops on the exposed dentine surface . We used microCT scanning to investigate features of the left upper second left molar , and the right side lower first and second molars of the mandibular specimen, CMN 52078-A. The M2 has deep occlusal and proximal surface lesions. Scans show that both lesions are characterized by a thin zone of demineralization at the cavity boundary and deeper sclerosis of the dentinal tubules. There is also evidence of mild formation of reparative dentin formation in the adjacent pulp associated with each lesion. The lower carnassial, m1, revealed five structures of interest . Feature m1.1 is a fragment of dentin that is slightly elevated from a worn surface because of cracks in the desiccated dentin. Feature m1.2 represents a series of carious lesions extending apically to the worn surface . Tree small pit-like lesions and one large lesion are identified. MicroCT scans reveal that lesions undercut the worn surface and show slight demineralization of their margins.
Demineralization of dentinal tubules is a reaction to actively spreading caries while dentinal sclerosis and formation of reparative dentin are evidence of protective responses. Feature m1.3 demonstrates subsurface demineralization extending about 0.1mm from the margins of the cavity. Features m1.4 and m1.5 are early carious lesions that clearly extend below the worn surface. Seven features of interest were identified in the occlusal surface of m2 . Five early carious lesions are identified under the scale bars of m2.1, m2.3, m2.4, m2.5 and m2.6. Feature m2.2 shows demineralization of the pulpal surface of the lesion. There is also evidence of demineralization of the dentinal tracts between the depth of the lesion and the pulp as well as a mildly sclerotic peripheral zone. Further, it is most likely that some reparative dentin has formed in the region of the pulp adjacent to the demineralized dentinal tracts. Feature 2.7 also shows slight demineralization of the lesion surface as well as slight demineralization of dentinal tracks just pulpal to the lesion as well as deeper sclerotic changes. For comparative purposes, we assessed the prevalence of caries in modern American black bear populations using museum collections, as well as published data from museum collections and a living population. Dental caries in extant black bears are seen in both museum specimens and in vivo bears , in contrast to their general absence in other carnivores. Moreover, examination of northern boreal forest black bears from Canadian Museum of Nature collections revealed prevalence of caries increasing with age .Analysis of new fossil material of Protarctos abstrusus from the North America High Arctic shows that, although ecomorphologically similar to the modern North American black bear , P. abstrusus represents a basal ursine. The most prominent cranial features of P. abstrusus are its relatively short rostrum,drainage pot fat forehead above the orbit, and high sagittal crest that extends posteriorly and overhangs the occipital condyles , characters that generally signal primitive status within Ursinae. P. abstrusus appears to have been an isolated immigration event from Eurasia to North America, separate from Ursus, representing a time of Asian-North American high latitude foral and faunal interchange, when the high-latitude forests of Asia and North America were connected across the Beringian isthmus.Te American black bear, by contrast, appears in the North America fossil record in the Early Pleistocene as a result of an independent dispersal event from Eurasia. Fossil records of true American black bear, Ursus americanus Pallas, range from Irvingtonian to late Rancholabrean. From the Irvingtonian age, Brown described abundant materials from the Conard Fissure, Arkansas, which he referred to U. americanus. Gidley named Ursus vitabilis from the Cumberland Cave, Maryland, which was later referred to Euarctos vitabilis. By late Rancholabrean black bears appear widespread throughout North America. Several species or subspecies of late Pleistocene black bears have been named, which were sometimes confused with brown bears because of overlap in size and pronounced sexual dimorphism, such as Ursus optimus from late Pleistocene McKittrick brea deposits of southern California, which was determined by Graham to be a brown bear. Grahamconcluded that only one species, Ursus americanus, is valid throughout the Pleistocene with late Pleistocene fossil forms being larger than their living descendants, suggesting continuity of the black bear linage in North America, as was also pointed out earlier by Kurtén.
Within Ursinae, P. abstrusus represents a stage of dental evolution that is intermediate in its specialization for ingesting plants, and significantly less than modern bears – polar bears being an exception, showing evolutionary reversal toward increased carnivory. Te evolutionary history of ursines is generally characterized by a shift in dental specialization from carnivory to increased omnivory, with the posteriormost molars of more recent forms being more elongate, and wrinkled, allowing for more crushing surface . Although, morphologically, P. abstrusus is less specialized than modern bears, the presence of dental caries suggests the diet of this 3.5 million-year-old transitional form already included a significant carbohydrate component. Dental evidence from the beaver pond site P. abstrusus appears to be from two individuals, including an apparent young adult, and both show dental caries, suggesting their diets included high amounts of fermentable carbohydrates early in their lives. Simple sugars, such as glucose and fructose, are readily metabolized by many bacteria found in the oral bio-film into various acids. These acids demineralize enamel and dentin and may lead to dental caries. Cariogenicity is highly correlated to the amount and frequency of sugar intakes. Te type of sugar consumed and associated dental caries are also found to differ. Despite their high sugar content, raw fruits by themselves are not always implicated for cariogenicity, although high frequency may induce caries. In humans, there is convincing evidence that free-sugar consumption of more than four times a day or more than 6–10% energy intake will increase incidents of dental caries. Historically in humans, increase in prevalence of dental caries has generally been associated with dietary shifts, linked with a reduction of nomadic lifestyles, the development of agriculture in Neolithic populations, and even more so with industrialization. In bears, carbohydrate intake may account for the appearance of dental caries , and may also be related to sedentary behavior, particularly for northern bears which hibernate. Northern black bears hibernate five to seven months, and survive better if they have high fat reserves. In bears, the optimal diet for production of fat reserves appears to be one of high-energy carbohydrates and low in protein. High-latitude berries often have a wide, circumpolar distribution and can be found in a variety of northern habitats including forest, woodland, wetland and tundra habitats. Black bears and grizzly bears in boreal forest eat berry fruits in the autumn, but some fruits, such as cranberry and bearberry, frequently remain on the vine over winter and are important to bears coming out of hibernation in the early spring. Bearberry fruits are relished and highly important to black bear in Pelly River Valley of Yukon Territory. Berries are found in nearly 80% of bear scats collected during the fall period and consistently represent a large component of black bear diet in Alaska, with blueberries being the most common. However, fruit intake may be mitigated by factors such as fruit abundance and body size. For example, larger bodied bears appear to tend toward carnivory, as they are less efficient than smaller bears at exploiting small fruits. These factors may underlie the high variation observed in caries prevalence seen among populations of modern black bears . Floral macrofossils from the Beaver Pond shows a diversity of berries would have been available to U. abstrusus, including Empetrum nigrum , Vaccinium sp. , Rubus idaeus , and their abundance may have been enhanced following forest fires, which is evident at this site. Therefore berries may have constituted a component of the Beaver Pond bear’s diet, particularly during the peak seasons, and their high sugar and acid contents could have resulted in the observed pronounced dental caries.