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

• Published in: Journal of mammalian evolution 2021-09, Vol.28 (3), p.707-728
• Main Authors: Jiangzuo, Qigao, Hulbert, Richard C.
• Format: Article
• Language: English
• Subjects: Amphimachairodus; Animal Genetics and Genomics; Anthropology; Biomedical and Life Sciences; Carnivora; Coexistence; Evolutionary Biology; Fossil mammals; Human Genetics; Indarctos; Life Sciences; Morphology; Original Paper; Rivers; Skull
• ISSN: 1064-7554; 1573-7055
• DOI: 10.1007/s10914-021-09546-9

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.