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VAN VALKENBURGH AND JENKINS—HISTORY OF SYNAPSID PREDATORSEVOLUTIONARY PATTERNS IN THE HISTORY OFPERMO-TRIASSIC AND CENOZOIC SYNAPSID PREDATORSBLAIRE VAN VALKENBURGH 1 AND IAN JENKINS 21Department of Organismic Biology, Ecology and Evolution, University of California,Los Angeles, California 90095-1606, USA2Department of Earth Sciences, University of Bristol, Wills Memorial Building,Queen’s Road, Bristol, BS8 1RJ, EnglandABSTRACT—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 tothe 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 beginsabout 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 forkilling and feeding, rather than locomotion. Despite fundamental differences in jaw mechanics and toothmorphology, 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. Inaddition, both the Late Carboniferous–Triassic and Cenozoic phases are characterized by repeated patterns ofclade replacement, in which one or a few clades evolve large size and seem to dominate the carnivore guild forseveral million years, but then decline and are replaced by new taxa. Moreover, within both ancient and Cenozoicpredator clades, there are parallel trends over time toward increased body size and hypercarnivory that likelyresult from a combination of interspecific competition and energetic constraints.INTRODUCTIONTHE SYNAPSIDA IS a huge clade of amniotesthat is of major evolutionary significance andincludes modern mammals and their fossilancestors—the non-mammalian synapsids, or‘mammal-like reptiles’ of old classifications(Carroll, 1988; Benton, 1998). Synapsids possessonly one temporal opening for the jaw muscles,located low on the side of the skull, as opposed tothe two openings in diapsid reptiles and birds. Thesynapsid skull type gave rise to an astonishinglyversatile jaw system, allowing side-to-side as wellas fore-aft chewing movements (Romer, 1966).Synapsids are currently considered to comprise theparaphyletic pelycosaurs (known as pelycosaurgradesynapsids), the Therapsida, non-mammaliancynodonts (derived non-mammalian synapsids), andmammals (Kemp, 1988; Hopson, 1991; Laurin andReisz, 1995). The synapsid fossil record extendsfrom the Late Carboniferous to the present day, aspan of nearly 300 million years. However, it canbe broken into two distinct phases of diversification,separated by about 150 million years. The first phaseextends from the Late Carboniferous to the mid-Triassic, includes the first large land predators onEarth, and is almost entirely non-mammalian. Thesecond phase begins about 65 million years ago afterthe demise of the dinosaurs, includes only mammals,and extends to the present.The Carboniferous to Triassic fossil record ofnon-mammalian synapsids includes a wide varietyof carnivores. Not only are sphenacodont synapsidsthe first mega-carnivores on Earth, but succeedingsynapsid predators—such as dinocephalians andLate Permian gorgonopsids—made up the firstdiverse carnivore guilds on Earth and pre-datethose of Cenozoic mammals by as much as 200267