''Plionarctos'' is an extinct genus of short-faced bear endemic to

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

from the

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

to the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Childs Frick Childs Frick (March 12, 1883 - May 8, 1965) was an American vertebrate paleontologist. He was a trustee of the American Museum of Natural History and a major benefactor of its Department of Paleontology, which in 1916 began a long partnership wi ...

in 1926, ''Plionarctos'' is the oldest known genus within the subfamily of the short-faced bears (

Tremarctinae The Tremarctinae or short-faced bears is a subfamily of Ursidae that contains one living representative, the spectacled bear (''Tremarctos ornatus'') of South America, and several extinct species from four genera: the Florida spectacled bear ( ...

), and is believed to be ancestral to ''Arctodus'', ''Arctotherium'' and ''Tremarctos''. A new species, ''P. harroldum'', was described in 2001 from the White Bluffs Fauna in Washington from remains previously attributed to '' Ursus cf. abstrusus''. ''P. edensis'' is thought to be ancestral to ''P. harroldorum''.

Description

''Plionarctos'' is thought to have weighed around the size of a smaller spectacled bear (60-150kg), and a skull length of 20cm. Postcranial skeletons of ''Plionarctos'' are unknown.

Diagnostics

Unlike other tremarctine bears, ''Plionarctos'' did not posses a premasseteric fossa. Although tooth sizes are similar, they also be differentiated from ''Tremarctos'' by the slightly shorter M2 molar.

Paleobiology

''Plionarctos'' existed between the middle Hemphillian faunal stage to the early Blancan faunal stage (7Mya - 2.9 Mya). Present on the coasts but rare in the Great Plains, ''Plionarctos'' is thought to have preferred more humid forested habitats. Like other tremarctine bears, ''Plionarctos'' is believed to have been sexually dimorphic. A ''Plionarctos harroldum'' specimen from Taunton (Washington, 2.9Ma) appears evolutionarily intermediate between ''P. harroldum'' and ''T. floridanus'', such as being the size of ''T. floridanus''.

Fossil distribution

Sites and specimen ages - Middle Pleistocene remains from France were reported but have since been reassigned to ''Ursus''.

''P. edensis''

* Rattlesnake Formation, Grant County, Oregon (''Plionarctos sp.'', Hemphillian, ca. 7Mya) *Montbrook Site,

Levy County Levy County ( ) is a county located in the North central Florida part of the U.S. state of Florida. As of the 2020 census, the population was 42,915. Its county seat is Bronson. It has been included in the Gainesville, Florida Metropolitan ...

, Florida (''Plionarctos sp.'', 5.5 - 5Mya) *Palmetto Mine,

Polk County, Florida Polk County () is a County (United States), county located in the Central Florida, central portion of the U.S. state of Florida. The county population was 725,046, as of the 2020 United States census, 2020 census, and estimated to be 818,330, as ...

(''Plionarctos sp.'', Upper Bone Valley Formation, 5 to 4.5Mya, Hemphillian) *Fort Green Mine, Polk County, Florida paleontological sites (Upper Bone Valley Formation, Hemphillian) *Ogalla Formation, Bear Tooth Slide, Brown County, Nebraska (Hemphillian) *Eden Formation,

Riverside County Riverside County is a county located in the southern portion of the U.S. state of California. As of the 2020 census, its population was 2,418,185, making it the fourth-most populous county in California and the 10th-most populous in the Unit ...

, California (type ''P. edensis'', Hemphillian) * Gray Fossil Site, Washington County, Tennessee (''Plionarctos sp.'', ca. 4.9-4.5 Mya, Hemphillian) * Pipe Creek Sinkhole,

Grant County Grant County may refer to: Places ;Australia * County of Grant, Victoria ;United States * Grant County, Arkansas * Grant County, Indiana * Grant County, Kansas *Grant County, Kentucky Grant County is a county located in the northern pa ...

, Indiana (5-4Mya, latest Hemphillian or earliest Blancan) *Aguanga Horizon, Riverside County, California (Blancan)

''P. harroldorum''

* Bear Springs, Arizona (Early Blancan Red Knolls Fauna, ca. 3.1Mya) * White Bluffs fauna, Franklin County, Washington (

Ringold Formation The Ringold Formation is a Formation (geology), geologic formation in Eastern Washington (state), Washington, United States. The formation consists of sediment laid down by the Columbia River following the flood basalt eruptions of the Columbia R ...

, type ''P. harroldorum'') * Taunton site, Adams County, Washington (ca. 2.9Mya)

References

{{Taxonbar, from=Q3445718 Pliocene bears Miocene bears Pleistocene bears Pleistocene genus extinctions Pliocene mammals of North America Pliocene mammals of Europe Prehistoric carnivoran genera Tremarctinae