View - Kowalewski, M. - Virginia Tech

VAN VALKENBURGH AND JENKINS—HISTORY OF SYNAPSID PREDATORSFIGURE 1—Kinetic-Inertial (KI) vs. Static-Pressure(SP) bite systems as illustrated by a gorgonopsid(Arctognathus) and a modern cat (puma, Pumaconcolor), respectively. 1, In the KI system, thelargest jaw adductor (P) is the anterior pterygoideusmusculature. Note that it has its maximum leveragefor jaw closing when the jaw is wide open, and caninitiate a rapid closure; A = external adductors. 2, Inthe SP system, the largest jaw adductor is thetemporalis musculature (T) followed by the masseter(M); the pterygoideus musculature is reduced. Inthe SP system the major jaw adductors have theirmaximum advantage when the jaws are nearlyclosed. (1) modified from Kemp, 1982, fig. 40.1996). The anterior pterygoideus musculature offelids is greatly reduced so there is little KIcomponent, but the mechanical advantage (distancefrom fulcrum to effort divided by distance fromfulcrum to bite point) of the expanded temporalismuscles is great because it has a very long momentarm (the coronoid process). Moreover, much of thetemporalis is aligned close to 90 o relative to thisstructure, enhancing muscle force when the jawsare nearly closed (Russell and Thomason, 1993).The SP bite system may have replaced the KIsystem in synapsids because impact stresses onteeth and bones were lessened during the killingbite. In addition, the SP system allowed a firmergrip on prey and better control of teeth at occlusion.The masseter musculature enhanced the bite forceof slicing cheek teeth and reduced the need for largeinternal pterygoideus musculature that couldobstruct air and food pathways.In addition to jaw mechanics, Permo-Triassicmammalian predators differ from their Cenozoicequivalents in tooth morphology. With the exceptionof some very derived, nearly mammalian,cynodonts, Permo-Triassic synapsid predatorsdisplayed little differentiation in dental morphologyalong the tooth row, except in size (Figs. 1, 2).Numerous simple conical to blade-shaped teeth linedthe jaws. Large caniniform teeth were often presenton both upper and lower jaws, thus separating thetooth row into incisors, canines, and cheek teeth.Unlike modern carnivores, there was no tooth-totoothocclusion among the cheek teeth and no preciseshearing between the upper and lower teeth. As aresult, the diversity of dental types among nonmammaliansynapsid carnivores was much lowerthan that expressed among mammalian carnivores.In mammals, the evolution of precise occlusion andside-to-side jaw movements (in association with theevolution of the masseter musculature) allowedmammals to produce more effective grinding as wellas slicing actions. Cenozoic carnivores includespecies with teeth specialized for herbivory (e.g.,Giant Panda), omnivory (e.g., Brown Bear),carnivory (e.g., felids), and bone-cracking (e.g.,hyaenids). By contrast, the radiation of nonmammaliansynapsid carnivores produced onlyspecies specialized for meat-eating, with littleevidence of adaptation for consuming bone or nonvertebratefoods. Included among these were the firstsabertooth predators, foreshadowing an ecomorphthat was to appear repeatedly among mammals.269