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| Amphimachairodus hezhengensis Jiangzuo, Werdelin, Sanisidro, Yang, Fu, Li, Wang & Deng, 2023 defending their prey (Hezhengia bohlini) from two Dinocrocuta gigantea. Artwork by Oscar Sanisidro. |
Highlights:
• The earliest Amphimachairodus discovered exhibits craniodental adaptation to open environment and social behaviour.
• Adaptations to change in habitat and killing behaviour evolved prior to other traits.
• Adaptatons to open environments and social behaviour first occurred near the Tibetan Plateau, probably due to aridification as the plateau was formed.
Abstract
The iconic sabretooth Homotherium is thought to have hunted cooperatively, but the origin of this behaviour and correlated morphological adaptations are largely unexplored. Here we report the most primitive species of Amphimachairodus (Amphimachairodus hezhengensis sp. nov.), a member of Machairodontini basal to Homotherium, from the Linxia Basin, northeastern border of the Tibetan Plateau (9.8–8.7 Ma). The long snout, laterally oriented and posteriorly located orbit of Amphimachairodus suggest a better ability to observe the surrounding environment, rather than targeting single prey, pointing to an adaptation to the open environment or social behaviour. A pathological forepaw of Amphimachairodus provides direct evidence of partner care. Our analyses of trait evolutionary rates support that traits correlated with killing behaviour and open environment adaptation evolved prior to other traits, suggesting that changes in hunting behaviour may be the major evolutionary driver in the early evolution of the lineage. A. hezhengensis represents one of the most important transitions in the evolution of Machairodontini, leading to adaptation in open environments and contributing to their further dispersal and radiation worldwide. This rapid morphological change is likely to be correlated with increasingly arid environments caused by the rise of the Tibetan Plateau, and competition from abundant large carnivores in this area.
Keywords: Machairodus, Amphimachairodus, Linxia basin, Eastern Asia, competition
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| (a) Cranium of Amphimachairodus hezhengensis sp. nov. HMV2041. a1, dorsal view; a2, anterior view; a3, ventral view; a4, postero-ventral view; a5, lateral view. (b) Geography of fossil locality. (c) Pathology of the MC2 and MC3 of Amphimachairodus sp. HMV2047 forepaw. (d) Large predator contemporary with A. hezhengensis in the Linxia Basin. d1, A. hezhengensis, HMV2041; d2, Dinocrocuta gigantea, HMV2044; d3, Agriotheriini ursid, HMV2046. |
Systematics
Order Carnivora Bowdich, 1821
Family Felidae Batsch, 1788
Subfamily Machairodontinae Gill, 1872
Tribe Machairodontini Gill, 1872
Amphimachairodus Kretzoi, 1929
Diagnosis: machairotont of large size. Rostrum long, and forehead wide. Orbit anterior border located at P4. Glenoid fossa overhung above basicranium. Mastoid process large, and paroccipital moderate to highly reduced. Mandibular flange weak or moderate, cornoid process small. Incisors large with serration, and upper I1 and I2 with laterally posited accessory cusps. P2 variably present. P3 with distinct anterior accessory cusp. P4 with distinct preparastyle and moderate to very small protocone. m1 with metaconid-talonid complex mostly absent.
Included species: Amphimachairodus giganteus (Wagner, 1848), Amphimachairodus horribilis (Schlosser, 1903), Amphimachairodus palanderi (Zdansky, 1924), Amphimachairodus coloradensis (Cook, 1922), Amphimachairodus alvarezi Ruiz-Ramoni et al. 2019 and Amphimachairodus hezhengensis sp. nov.
Amphimachairodus hezhengensis sp. nov.
Machairodus palanderi p.257, Deng et al. 2013
Amphimachairodus sp. p.11 Jiangzuo et al. 2023
Holotype: HMV2041, a nearly complete cranium
Etymology: After the place (Hezheng Paleozoological Museum, Hezheng, China) where the specimen was found and is currently stored.
Type locality: Houshan, Linxia Basin, Gansu province of northern China.
Chronology and distribution: Thus far only known from the early Late Miocene of northern China.
Diagnosis: medium-sized Amphimachairodus with small incisors and I1 with lingually posited accessory cusps; long C-P3 diastema; presence of P2; relatively small cheek teeth; small P4 preparastyle and moderate protocone.
Differential diagnosis: differs from Machairodus and Nimravides in having different cranial morphology, e.g. lower angle between facial and neurocranial part, wide forehead, retracted orbit and long rostrum, shorter and dorsally arched zygomatic arch, slightly overhanging glenoid fossa, more arched incisor row and more separated lingual accessory cusps in I2, presence of P2, more distinct P4 preparastyle and smaller protocone; differs from Lokotunjailurus in having larger size, longer C-P3 diastema, stronger P3 anterior accessory cusp and more robust P4; differs from other species of Amphimachairodus in having smaller incisors, I1 with two closely located lingual accessory cusps, longer C-P3 diastema, smaller cheek teeth, smaller P4 preparastyle and slightly larger protocone.
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| Reconstruction of two Amphimachairodus hezhengensis defending their prey (Hezhengia bohlini) from two Dinocrocuta gigantea. Artwork by Oscar Sanisidro. |
Qigao Jiangzuo, Lars Werdelin, Oscar Sanisidro, Rong Yang, Jiao Fu, Shijie Li, Shiqi Wang and Tao Deng. 2023. Origin of Adaptations to Open Environments and Social Behaviour in Sabretoothed Cats from the northeastern Border of the Tibetan Plateau. Proc. R. Soc. B. DOI: 10.1098/rspb.2023.0019
https://phys.org/news/2023-04-trait-tibetan-saber-toothed-cat.html
