Evolutionary Patterns in the History of Permo-Triassic and Cenozoic Synapsid Predators

Abstract

Synapsids include modern mammals and their fossil ancestors, the non-mammalian synapsids, or ‘mammal-like reptiles' of old classifications. The synapsid fossil record extends from the Late Carboniferous to the present, a span of nearly 300 million years. However, it can be broken into two distinct phases of diversification, separated by about 150 million years. The first phase extends from the Late Carboniferous to the mid-Triassic, includes the first large land predators on Earth, and is almost entirely non-mammalian. The second phase begins about 65 million years ago after the demise of the dinosaurs, includes only mammals, and extends to the present. In this overview of synapsid predators, we emphasize terrestrial species of large size, and their adaptations for killing and feeding, rather than locomotion. Despite fundamental differences in jaw mechanics and tooth morphology, there are significant parallels in the non-mammalian and mammalian radiations of synapsid predators. Both groups evolve sabertooth forms more than once, and both evolve short-snouted, powerful biting forms. In addition, both the Late Carboniferous—Triassic and Cenozoic phases are characterized by repeated patterns of clade replacement, in which one or a few clades evolve large size and seem to dominate the carnivore guild for several million years, but then decline and are replaced by new taxa. Moreover, within both ancient and Cenozoic predator clades, there are parallel trends over time toward increased body size and hypercarnivory that likely result from a combination of interspecific competition and energetic constraints.