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The Ursidae family belongs to the order Carnivora and is one of nine families in the suborder Caniformia, or "doglike" carnivorans. Bears' closest living relatives are the pinnipeds, a clade of three families: Odobenidae (the walrus), Otariidae (fur seals and sea lions), and Phocidae (true or earless seals). Bears comprise eight species in three subfamilies: Ailuropodinae (monotypic with the giant panda), Tremarctinae (monotypic with the Spectacled Bear), and Ursinae (containing six species divided into one to three genera, depending upon authority).
The origins of Ursidae can be traced back to the very small and graceful Parictis that had a skull only 7 cm (3 in) long. Parictis first occur in North America in the Late Eocene (ca. 38 million years ago), but this genus did not appear in Eurasia and Africa until the Miocene.[6] The raccoon-sized, dog-like Cephalogale, however, is widely regarded as the most primitive ursid and is ideally suited as a representative basal taxon for the family. Cephalogale first appeared during the middle Oligocene and early Miocene (approximately 20–30 million years ago) in Europe. Cephalogale gave rise to a lineage of early bears of the genus Ursavus. This genus radiated in Asia and ultimately gave rise to the first true bears (genus Ursus) in Europe, 5 million years ago. Even among its primitive species, such as C. minor, it exhibits typical ursid synapomorphic dentition such as posteriorly oriented M2 postprotocrista molars, elongated m2 molars, and a reduction of the premolars. Living members of the ursids are morphologically well defined by their hypocarnivorous (non-strictly meat-eating) dentitions, but fossil ursids include hypercarnivorous (strictly meat-eating) taxa, although they never achieved the extreme hypercarnivory seen in mustelids. Cephalogale was a mesocarnivore (intermediate meat-eater).[7] Other extinct bear genera include Arctodus, Agriarctos, Plionarctos and Indarctos.
It is uncertain whether ursids were in Asia during the late Eocene, although there is some suggestion that a limited immigration from Asia may have produced Parictis in North America due to the major sea level lowstand at ca. 37 Ma, but no Parictis fossils have yet to be found in East Asia. Ursids did, however, become very diversified in Asia later during the Oligocene. Four genera representing two subfamilies (Amphicynodontinae and Hemicyoninae) have been discovered in the Oligocene of Asia: Amphicticeps, Amphicynodon, Pachycynodon, and Cephalogale. Amphicticeps is endemic from Asia and the other three genera are common to both Asia and Europe. This indicates migration of ursids between Asia and Europe during the Oligocene and migration of several taxa from Asia to North America likely occurred later during the late Oligocene or early Miocene. Although Amphicticeps is morphologically closely related to Allocyon, and also to Kolponomos of North America, no single genus of the Ursidae is known to be common to both Eurasia and North America. Cephalogale, however, do appear in North America in the early Miocene. It is interesting to note that rodents, such as Haplomys and Pseudotheridomys (late Oligocene) and Plesiosminthus and Palaeocastor (early Miocene), are common to both Asia and North America and this indicates that faunal exchange did occur between Asia and North America during the late Oligocene to early Miocene. Ursid migration from Asia to North America would therefore have also been very likely to occur during this time.[8] Three major carnivoran migrations between Eurasia and North America are recognized in the late Neogene that definitely included ursids. The first around 20 Ma (probably 21–18 Ma) were waves of intermittent dispersals that included Amphicynodon, Cephalogale and Ursavus. The second migration occurred at about 7–8 Ma and included Agriotherium. And the last wave took place in the early Pliocene 4 Ma with Ursus.[9]
The giant panda's taxonomy has long been debated. Its original classification by Armand David in 1869 was within the bear genus Ursus, but in 1870 it was reclassified by Alphonse Milne-Edwards to the raccoon family.[10] In recent studies, the majority of DNA analyses suggest that the giant panda has a much closer relationship to other bears and should be considered a member of the family Ursidae.[11] The status of the red panda remains uncertain, but many experts, including Wilson and Reeder, classify it as a member of the bear family. Others place it with the raccoons in Procyonidae or in its own family, the Ailuridae. Multiple similarities between the two pandas, including the presence of false thumbs, are thought to represent convergent evolution for feeding primarily on bamboo.
There is also evidence that, unlike their neighbors elsewhere, the brown bears of Alaska's ABC islands are more closely related to polar bears than they are to other brown bears in the world. Researchers Gerald Shields and Sandra Talbot of the University of Alaska Fairbanks Institute of Arctic Biology studied the DNA of several samples of the species and found that their DNA is different from that of other brown bears. The researchers discovered that their DNA was unique compared to brown bears anywhere else in the world. The discovery has shown that while all other brown bears share a brown bear as their closest relative, those of Alaska's ABC Islands differ and share their closest relation with the polar bear.[12] There is also supposed to be a very rare large bear in China called the blue bear, which presumably is a type of black bear. This animal has never been photographed.
Koalas are often referred to as bears due to their appearance; they are not bears, however, but marsupials.
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