Plionarctos

Described by Childs Frick in 1926, Plionarctos is the oldest known genus within the subfamily of the short-faced bears (Tremarctinae),'''' and is believed to be ancestral to Arctodus, Arctotherium and Tremarctos. Once suggested to have emerged from the Ailuropodinae bears (Agriotherium and Indarctos), Plionarctos probably evolved from Ursavus which emigrated into North America from Eurasia during the Miocene, or possibly Protarctos. A new species, Plionarctos harroldum, was described in 2001 from the White Bluffs Fauna in Washington from remains previously attributed to Protarctos abstrusus''. Diagnostics Tremarctinae Various dental characteristics link Plionarctos with other tremarctine bears, such as a deep mandible underneath the molars, a notable groove underneath the premolars, a masseteric fossa at its deepest in line with the teeth, an M1 molar with a labial enamel bulge and high talonid with a very shallow basin, and a rectangular M2 molar. Archaic characteristics which separate Plionarctos from other tremarctines such as Arctodus and Tremarctos include a lack of a premasseteric fossa (and the associated extension of the strongly ridged masseteric crest) and a tri-rooted (rather than birooted) P4 premolar, with its protocone anterior (instead of behind) the carnassial notch. Tremarctos In addition to the lack of premasseteric fossa in Plionarctos, the M2 molar of Plionarctos is slightly shorter than that of Tremarctos (though tooth sizes are similar). Plionarctos Plionarctos harroldorum shares dental characteristics with Tremarctos, with both lacking a well-defined lingual cinguli, and a longer and wider talon on the M2, which is not as dorsal as Plionarctos edensis. The M2 of P. harroldorum is broad compared to the width of M1 with a longer talonid than in P. edensis. == Evolution ==

Likely part of the Hemphillian 2 migration wave (Hh2) across the Beringinian land bridge in the late early Miocene, Plionarctos first appeared in North America (during the late Miocene epoch) being recorded from the Rattlesnake Formation of Oregon circa 7Ma. The Hh2 migration was the largest migration wave of Carnivorans between Asia into North America, and was also one of the largest mammalian dispersal events of the Neogene. The ascendancy of Asian fauna in this exchange may have been due to the intensification of the monsoon season in northeastern Asia expanding humid forests into the Arctic (in contrast to the global trend of aridification and expansion of C4 grasslands), with the more static & arid ecologies of North America potentially acting as refugia for Asian fauna. Plionarctos was the sole representative of the Tremarctinae subfamily of bears during its existence, with P. edensis being thought to be ancestral to P. harroldorum. Plionarctos was followed by three new genera of short-faced bear around the Plio-Pleistocene boundary. These were Arctodus (pristinus), Tremarctos (floridanus) and Arctotherium sp., which emerged in the Late Blancan age of North America circa 2.6Ma. An investigation into the mitochondrial DNA of bear species indicates that the short-faced bears diverged from the Ursinae subfamily approximately 5.7 million years ago. Around the Miocene-Pliocene boundary (~5.3 Ma) short-faced bears, along with other ursids, experienced an explosive radiation in diversity, as C4 vegetation (grasses) and open habitats dominated, the world experienced a major temperature drop and increased seasonality, and a faunal turnover which extinguished 60–70% of all Eurasian faunal genera, and 70–80% of North American genera.'''' Correspondingly, the three succeeding genera appear to genetically diverge around this time, with Arctodus splitting from Arctotherium and Tremarctos at between 5.5Ma and 4.8Ma,'' and Arctotherium and Tremarctos'' branching off at 4.1Ma. == Description ==

Plionarctos is thought to have weighed around the size of a smaller spectacled bear (60–150 kg), and a skull length of 20 cm. while a P. harroldorum individual from Washington was calculated to 165.5 kg. == Map of fossil localities ==

Plionarctos existed between the middle Hemphillian faunal stage to the Blancan faunal stage (7Mya - 2.9Mya). Present on the coasts but rare in the Interior Plains, Plionarctos is thought to have preferred more humid forested habitats. however specimens of these index fossils younger than 4.5Mya put this temporal bracketing in doubt. Another individual (originally described as P. edensis) was also recovered from Riverside County (Aguanga Horizon, Blancan faunal stage). A sagebrush steppe with semi-arid wooded shrubland and tall grasslands, the Rattlesnake Formation fauna is considered a reference fauna for the Hemphillian faunal age. These fauna were recovered alongside Megalonyx, Amebelodon, Hemiauchenia, Megatylopus, Pliauchenia, Ilingoceras, Pediomeryx, Parablastomeryx, Mylohyus, Platygonus, Teleoceras, Hippotherium, Neohipparion, Pliohippus, Borophagus secundus, Borophagus pugnator, Eucyon, Metalopex, Vulpes, Lynx, Rhizosmilodon, and Nimravides. The presence of Castor californicus, Dipoides, Pekania, and an unidentified tapir suggest riparian forests also existed in this ecosystem. Conversely, the youngest representatives of Plionarctos have also been recovered from the Ringold Formation of Washington (White Bluffs & Taunton, Early & Late Blancan respectively). The White Bluffs specimen is the type specimen of P. harroldorum, Recovered from a freshwater fluvial environment, the Taunton P. harroldorum was preserved alongside numerous bony fish, turtles, waterfowl, Equus simplicidens and Capromeryx, along with Megalonyx, Platygonus, Camelops, Hemiauchenia, Dinofelis cf. paleonca, Borophagus, Canis lepophagus, Puma lacustris and cervids. Interior Plains The Pipe Creek Sinkhole of the Interior Plains (Indiana, 4-5Mya, latest Hemphillian or earliest Blancan) preserves a wetland environ which was surrounded by a dry, open herbaceous savanna with some denser vegetation (such as Fagus, Platanus, Populus and Salix), representing an extension of the Early Pliocene Great Plains. The site records abundant leopard frogs and pond turtles, but also Plionarctos edensis, Borophagus sp., Vulpes, Canis sp., Castor, Spermophilus, deer, Hemiauchenia, Aepycamelus, Titanotylopus and Teleoceras. The Hemphillian-age Ash Hollow Formation (Ogalla / Bear Tooth Slide, Nebraska) preserves Plionarctos sp. close to but distinguished from Plionarctos edensis. The Pipe Creek Sinkhole and Palmetto Fauna are the most biologically similar sites to the Gray Fossil Site. Dating to the early Pliocene of the Atlantic Plain, the Palmetto fauna resembled an archaic Clarendonian faunal assemblage from the Interior Plains (ca. 10Mya) due to its breadth & diversity of browsing, mixed-feeding, and grazing herbivores. 19 specimens of Plionarctos sp. have been recovered from the Fort Green, Palmetto, and Gardinier mines. Together with Agriotherium, bears make up 22% of carnivoran teeth & jaws in the Palmetto Fauna. Although half the species recovered also occurred in late Hemphillian faunas of the western United States and Mexico (Mount Eden, California; Santee, Nebraska; Buis Ranch, Oklahoma; Yepómera, Chihuahua; El Ocoté, Guanajuato), the more humid & forested site also preserves many endemic southeastern species (particularly ungulates), along with marine fauna such as sharks, rays, and cetaceans. ==References==