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VAN VALKENBURGH AND JENKINS—HISTORY OF SYNAPSID PREDATORSselection favors the evolution of larger body size.However, large size brings with it a cost—the needto feed on prey as large as or larger than oneself.As shown by Carbone et al. (1999), almost all livingcarnivoran species larger than about 21 kg take preyas large as or larger than themselves, whereassmaller species feed mostly on prey that is 45% orless of their body weight. Using an energetic model,they argued that because of limitations on intakerate and foraging time, it becomes increasinglydifficult to subsist on small prey items as predatorbody mass grows. Consequently, the evolution oflarge body size (>20 kg) in a carnivore must beassociated with craniodental adaptations for killingand feeding on large prey. These include shortenedsnouts to enhance bite force at the canines, as wellas robust canines and large carnassials. Nonmammaliansynapsids seem to have been subjectto these same evolutionary forces, and modifiedtheir skulls to improve the KI bite system as wellas to add the SP bite system.Van Valkenburgh (1999) argued that thistendency toward specialization within carnivoreclades might explain their inevitable declines.Large, hypercarnivorous species may do well inthe short run, but their specialization results in aloss of evolutionary versatility that inhibits theirability to adapt rapidly to changing environmentalconditions. Moreover, families or subfamilies oflarge-sized carnivores tend to be less species-richthan those of smaller carnivores, and consequentlymore vulnerable to extinction. Thus, as clades ofcarnivores evolve toward large size andhypercarnivory, they dwindle in species richness—leading to their decline and ultimate extinction.Thus, the cycle of rising and falling dynasties thatso typifies the fossil record of carnivores seemsbetter explained by the dynamics of competitionamong predators themselves than by changes inprey diversity or adaptations.REFERENCESANDERSON, J. M., AND A. R. I. CRUICKSHANK. 1978. The biostratigraphy of the Permian and the Triassic, Part 5: Areview of the classification and distribution of Permo-Triassic tetrapods. Palaeontologia Africana, 21:15–44.BAKKER, R. T. 1983. The deer flees, the wolf pursues: Incongruencies in predator-prey coevolution, p. 350–382.In D. J. Futuyma and M. Slatkin (eds.), Coevolution. Sinauer Press, Sunderland, Massachusetts.BATTAIL, B., AND M. V. SURKOV. 2000. Mammal-like reptiles from Russia, p. 86–119. In M. J. Benton, M. A.Shishkin, D. M. Unwin, and E. N. Kurochkin (eds.), The Age of Dinosaurs in Russia and Mongolia. CambridgeUniversity Press, Cambridge.BENTON, M. J. 1993. The Fossil Record II. Chapman and Hall, London, 841p.BENTON, M. J. 1998. Vertebrate Palaeontology, 2nd edition. Chapman and Hall, London.BERTA, A. 1981. The Plio-Pleistocene hyaena Chasmoporthetes ossifraga from Florida. Journal of VertebratePaleontology, 1:341–356.BIKNEVICIUS, A. R., AND B.VAN VALKENBURGH. 1996. Design for killing: Craniodental adaptations of predators, p.393–428. In J. L. Gittleman (ed.), Carnivore Behavior, Ecology and Evolution, Vol. 2. Cornell UniversityPress, Ithaca, NY.BIKNEVICIUS, A. R., B. VAN VALKENBURGH, AND J. WALKER. 1996. Incisor size and shape: implications for feedingbehaviors in saber-toothed “cats.” Journal of Vertebrate Paleontology, 16:510–521.BOONSTRA, L. D. 1954. The cranial structure of the titanosuchian Anteosaurus. Annals of the South African Museum,42:108–148.BRAMBLE, D. M. 1978. Origin of the mammalian feeding complex: models and mechanisms. Paleobiology, 4:271–301.BROOM, R. 1932. The mammal-like reptiles of South Africa and the origin of mammals. H. F. and G. Witherby,London, 376 p.BRYANT, H. N. 1991. Phylogenetic relationships and systematics of the Nimravidae (Carnivora). Journal ofMammalogy, 72:56–78.285