Plionarctos

Described by Childs Frick in 1926,{{cite journal |last1=Frick |first1=Childs |date=1926 |title=The Hemicyoninæ and an American Tertiary Bear |journal=Bulletin of the American Museum of Natural History |volume=56 |issue=1 |pages=111–119 |hdl=2246/1321}} 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. 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.

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.

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.

Plionarctos existed between the middle Hemphillian faunal stage to the early Blancan faunal stage (7Mya - 2.9Mya). 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){{Cite journal |last1=Morgan |first1=James K. |last2=Morgan |first2=Neil H. |date=1995 |title=A New Species of Capromeryx (Mammalia: Artiodactyla) from the Taunton Local Fauna of Washington, and the Correlation with Other Blancan Faunas of Washington and Idaho |url=https://www.jstor.org/stable/4523614 |journal=Journal of Vertebrate Paleontology |volume=15 |issue=1 |pages=160–170 |doi=10.1080/02724634.1995.10011215 |jstor=4523614 |bibcode=1995JVPal..15..160M |issn=0272-4634}} appears evolutionarily intermediate between P. harroldum and T. floridanus, such as being the size of T. floridanus.{{Cite journal |last1=Tedford |first1=Richard H. |last2=and Martin |first2=James |date=2001-07-20 |title=Plionarctos, a tremarctine bear (Ursidae: Carnivora) from western North America |url=https://doi.org/10.1671/0272-4634(2001)021[0311:PATBUC]2.0.CO;2 |journal=Journal of Vertebrate Paleontology |volume=21 |issue=2 |pages=311–321 |doi=10.1671/0272-4634(2001)021[0311:PATBUC]2.0.CO;2 |issn=0272-4634|url-access=subscription }}

Sites and specimen ages - Middle Pleistocene remains from France were reported but have since been reassigned to Ursus.{{Cite journal |last=Cregut-Bonnoure |first=Evelyne |date=15 August 1996 |title=A review of small Middle Pleistocene bears from France |url=http://www.isez.pan.krakow.pl/journals/azc/pdf/azc_v/39(1)/39(1)_08.pdf |journal=Acta Zoologica Cracoviensia |volume=39 |issue=1 |pages=89–101}} Both Plionarctos edensis and Plionarctos harroldorum coexisted in the Late Hemphillian of the Late Miocene (and possibly the Blancan faunal stage of the Pliocene).

• Rattlesnake Formation, Grant County, Oregon (Plionarctos sp., Hemphillian, ca. 7Mya)
• Montbrook Site, Levy County, Florida (Plionarctos sp., 5.5 - 5Mya){{Cite web |title=About Montbrook |url=https://www.floridamuseum.ufl.edu/montbrook/about-montbrook/ |access-date=2025-06-18 |website=Montbrook Fossil Dig |language=en-US}}
• Eden Formation, Riverside County, California (type P. edensis, Hemphillian)
• Gray Fossil Site, Washington County, Tennessee (Plionarctos sp., ca. 4.9-4.5 Mya, Hemphillian){{Cite journal |last1=Samuels |first1=Joshua X. |last2=Bredehoeft |first2=Keila E. |last3=Wallace |first3=Steven C. |date=2018 |title=A new species of Gulo from the Early Pliocene Gray Fossil Site (Eastern United States); rethinking the evolution of wolverines |journal=PeerJ |volume=6 |pages=e4648 |doi=10.7717/peerj.4648 |doi-access=free |issn=2167-8359 |pmc=5910791 |pmid=29682423}}
• Pipe Creek Sinkhole, Grant County, Indiana (5-4Mya, latest Hemphillian or earliest Blancan){{Cite journal |last=Farlow |first=James O. |last2=Sunderman |first2=Jack A. |last3=Havens |first3=Jonathan J. |last4=Swinehart |first4=Anthony L. |last5=Holman |first5=J. Alan |last6=Richards |first6=Ronald L. |last7=Miller |first7=Norton G. |last8=Martin |first8=Robert A. |last9=Hunt |first9=Robert M. |last10=Storrs |first10=Glenn W. |last11=Curry |first11=B. Brandon |last12=Fluegeman |first12=Richard H. |last13=Dawson |first13=Mary R. |last14=Flint |first14=Mary E.T. |date=2001 |title=The Pipe Creek Sinkhole Biota, a Diverse Late Tertiary Continental Fossil Assemblage from Grant County, Indiana |url=https://www.jstor.org/stable/3083113 |journal=The American Midland Naturalist |volume=145 |issue=2 |pages=367–378 |issn=0003-0031}}
• Aguanga Horizon, Riverside County, California (Blancan)

• Palmetto Mine, Polk County, Florida (Plionarctos sp., Upper Bone Valley Formation, 5 to 4.5Mya, Hemphillian){{Cite book |last1=Janis |first1=Christine M. |url=https://books.google.com/books?id=I-RgojcDyWYC&q=plionarctos |title=Evolution of Tertiary Mammals of North America: Volume 1, Terrestrial Carnivores, Ungulates, and Ungulate Like Mammals |last2=Scott |first2=Kathleen M. |last3=Jacobs |first3=Louis L. |date=1998-05-28 |publisher=Cambridge University Press |isbn=978-0-521-35519-3 |language=en}}{{Cite journal |last1=Doughty |first1=Evan M. |last2=Wallace |first2=Steven C. |last3=Schubert |first3=Blaine W. |last4=Lyon |first4=Lauren M. |date=2018 |title=First occurrence of the enigmatic peccaries Mylohyus elmorei and Prosthennops serus from the Appalachians: latest Hemphillian to Early Blancan of Gray Fossil Site, Tennessee |journal=PeerJ |volume=6 |pages=e5926 |doi=10.7717/peerj.5926 |doi-access=free |issn=2167-8359 |pmc=6276594 |pmid=30533292}}{{Cite web |title=Palmetto Fauna |url=https://www.floridamuseum.ufl.edu/florida-vertebrate-fossils/sites/palmetto-fauna/ |access-date=2025-06-18 |website=Florida Vertebrate Fossils |language=en-US}}
• Fort Green Mine, Polk County, Florida paleontological sites (Upper Bone Valley Formation, Hemphillian)
• Bear Springs, Arizona (Early Blancan Red Knolls Fauna, ca. 3.1Mya){{Cite journal |last1=White Jr. |first1=Richard S. |last2=Morgan |first2=Gary S. |date=2005 |title=Arizonan Blancan Vertebrate Faunas in Regional Perspective |url=https://www.researchgate.net/publication/267844516 |journal=Vertebrate Paleontology of Arizona, Mesa Southwest Museum Bulletin |volume=11}}{{Cite journal |last1=Morgan |first1=Gary S. |last2=White Jr. |first2=Richard S. |date=2005 |title=Miocene and Pliocene vertebrates from Arizona |url=https://www.researchgate.net/publication/40661398 |journal=Vertebrate Paleontology in Arizona. New Mexico Museum of Natural History and Science Bulletin |volume=29 |via=ResearchGate}}
• Ash Hollow Formation, Ogalla / Bear Tooth Slide, Brown County, Nebraska (Hemphillian)
• White Bluffs fauna, Franklin County, Washington (Early Blancan Ringold Formation, type P. harroldorum)
• Taunton site, Adams County, Washington (Blancan, ca. 2.9Mya){{Cite journal |last1=Tedford |first1=Richard H. |last2=and Martin |first2=James |date=2001-07-20 |title=Plionarctos, a tremarctine bear (Ursidae: Carnivora) from western North America |url=https://doi.org/10.1671/0272-4634(2001)021[0311:PATBUC]2.0.CO;2 |journal=Journal of Vertebrate Paleontology |volume=21 |issue=2 |pages=311–321 |doi=10.1671/0272-4634(2001)021[0311:PATBUC]2.0.CO;2 |issn=0272-4634|url-access=subscription }}{{Cite journal |last1=Morgan |first1=James K. |last2=Morgan |first2=Neil H. |date=1995 |title=A New Species of Capromeryx (Mammalia: Artiodactyla) from the Taunton Local Fauna of Washington, and the Correlation with Other Blancan Faunas of Washington and Idaho |url=https://www.jstor.org/stable/4523614 |journal=Journal of Vertebrate Paleontology |volume=15 |issue=1 |pages=160–170 |doi=10.1080/02724634.1995.10011215 |jstor=4523614 |bibcode=1995JVPal..15..160M |issn=0272-4634}}

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Category:Pliocene bears

Category:Miocene bears

Category:Pleistocene bears

Category:Pleistocene genus extinctions

Category:Pliocene mammals of North America

Category:Pliocene mammals of Europe

Category:Prehistoric carnivoran genera

Category:Tremarctinae