Coexistence of Indarctos and Amphimachairodus (Carnivora) in the Late Early Hemphillian of Florida, North America

Coexistence of Indarctos and Amphimachairodus (Carnivora) in the Late Early Hemphillian of Florida, North America


Fossils of the ursid Indarctos from Withlacoochee River 4A of Florida (late early Hemphillian North American Land Mammal

Age, Hh2, ~ 7.5–6.5 Ma) represent the best sample of this genus in North America, including both craniodental and

postcranial specimens, yet only the skull has been described. In this study, we describe the other material of this bear from

the same locality and review the records of Indarctos in North America. Indarctos from Withlacoochee River 4A has dental

characters in accordance with those of typical Indarctos oregonensis, but has distinctly more slender postcranial bones.

Indarctos from other localities of North America contains two morphs with regards to postcranial robustness, but lacks a

clear geographic pattern. Indarctos from Withlacoochee River 4A shares traits with the Old World Indarctos zdanskyi and

may be its descendant in the New World. The machairodont from Withlacoochee River 4A shows typical characters (e.g.,

presence of distinct P4 preparastyle and mandibular flange) that allow referral to Amphimachairodus rather than to Nimravides.

Its morphology also shows a stronger affinity of this population to the Old World Amphimachairodus horribilis rather

than to Hh3 Amphimachairodus coloradensis. Previously, most North American records of Amphimachairodus were late

Hemphillian (Hh3–4, ~ 6.5–4.5 Ma), when it co-occurred with a different ursid, “Agriotherium.” The Withlacoochee River

4A specimens provide evidence that Amphimachairodus dispersed from Asia during the Hh2. The special morphology of

Indarctos and the presence of Amphimachairodus in a Hh2 fauna suggest that the environment had begun to change before

the significant fauna turnover between the early and late Hemphillian.


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Qigao, J. (2022). Coexistence of Indarctos and Amphimachairodus (Carnivora) in the Late Early Hemphillian of Florida, North America. National Tibetan Plateau Data Center, DOI: 10.1007/s10914-021-09546-9. (Download the reference: RIS | Bibtex )

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References literature

1.Jiangzuo, Q., Hulbert, R. (2021). Coexistence of Indarctos and Amphimachairodus (Carnivora) in the Late Early Hemphillian of Florida, North America. Journal of Mammalian Evolution(5). (View Details )


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