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

Origin of adaptations to open environments and social behaviour in sabretoothed cats from the northeastern border of the Tibetan Plateau

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.

Parallelism and lineage replacement of the late Miocene scimitar-toothed cats from the old and New World

In contrast to large-scale convergence/parallelism, the small-scale convergence/parallelism of sabertooth adaptation within closely related genera and species has been seldom investigated. Here, we describe and analyze the rich material of Nimravides catocopis, and provide evidence using a new phylogenetic analysis that Nimravides was endemic to North America. The late Miocene (10.5-6.5 Ma) Nimravides represents a lineage that shows clearly parallelism with the contemporary Old World lineage of Machairodus-Amphimachairodus.The Old World lineage experienced a higher evolutionary rate of cranial trait than the New World one did. The low density of Amphimachairodus at its first appearance in North America suggests that the derived traits did not provide a direct competitive advantage over Nimravides, but allowed Amphimachairodus to survive the significant faunal change in the early-late Hemphillian (∼6.5 Ma) in North America, a process that probably can be applied to most replacement of closely related lineages.