View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002million years. The 170-million-year hiatus betweenthe last diverse synapsid carnivore community(Middle Triassic cynodonts) and the first Cenozoicpredators (creodonts) was the result of dinosaurprevalence during that interval (Benton, 1998).Predatory non-mammalian synapsids wereprofoundly dominant in their niches throughout thePermo-Carboniferous, a pivotal period in Earthhistory for tetrapod evolution. At this time, thefundamental amniote groups—anapsids, diapsids,and synapsids—had their origin and firstdiversification. In addition, ancient amphibians—chiefly temnospondyls—radiated tremendouslyand commanded inland freshwater aquatic andsemi-aquatic habitats (Carroll, 1988; Benton,1998). Carnivores in any ecosystem applyconsiderable ‘evolutionary pressure’ and areimplicated in such adaptations among prey speciesas crypsis, armor and cranial adornments, tusks,increased cursorial capacity, and greater size(Gittleman, 1989, 1996). This was as true forancient terrestrial ecosystems as it is today. Gaininginsights into the structure and composition ofancient predator guilds should shed light ontetrapod evolution during the Late Paleozoic.Predators, as defined in this overview, areanimals that regularly kill and eat other vertebrates(Savage, 1977). In the interest of brevity, andbecause the fossil record is more complete for largerthan for smaller vertebrates, we will largely restrictour review to terrestrial species of large size—thatis, jackal-size (about 7 kg) and larger. Our primaryfocus will be on heads and teeth and hence killingand feeding behavior, rather than on limbs andlocomotion. This reflects the fact that cranial anddental material is much more abundant in the fossilrecord than are limbs, especially in the case of thePermo-Triassic synapsids. The review begins witha look at overall similarities and differences in theanatomy and feeding mechanics of the two widelyseparated radiations of carnivores. This is followedby a summary of the evolutionary history withineach group and then by an attempt at a synthesis,noting parallel trends in morphology and guilddynamics between the Permo-Triassic and Cenozoicsynapsid predators.NON-MAMMALIAN ANDMAMMALIAN SYNAPSIDCARNIVORES COMPAREDAll but the latest Permo-Triassic synapsidcarnivores differed from Cenozoic synapsids in jawmechanics. Reconstructing the fine details ofmuscles in fossil vertebrates is problematic, butgeneral directions of muscle forces (vectors) canbe estimated. In addition, the overall morphologyof the skull and jaws provides some indication offunction (Parrington, 1955; Olson, 1961; Kemp,1969, 1982). Based on their anatomy, most ancientcarnivores used a predominantly Kinetic-Inertial(KI) system of jaw closure in which the jaws wererapidly slammed shut, disabling the prey by themomentum and ultimately the kinetic energy ofthe jaws and teeth (Olson, 1961). The major jawclosing muscles were the anterior pterygoideus andtemporalis (Fig. 1). In more derived amniotes, suchas modern mammals, the KI system wassupplemented (and supplanted) by the Static-Pressure (SP) system. In SP systems, a strong bitewith nearly closed jaws serves as the killing bite(Olson, 1961). The pterygoideus musculature isreduced; instead, the muscles driving the closureare the temporalis and the masseter (Fig. 1).Modern examples of predators that use KI orSP systems offer insights into differences in jawmusculature and function that can be applied tofossil tetrapods. For example, the rapid and highlydamaging jaw adduction of crocodilians is a goodexample of a KI system combined with a significantSP bite (Cleuren and de Vree, 2000). Incrocodilians, the fast KI bite is generated by anexpanded anterior pterygoideus musculaturewhich, when the jaws are wide open, lies at 90 o tothe long axis of the lower jaw. This is the optimalalignment for the input of forces (muscular) to alever (lower jaw). The temporalis musculature incrocodilians is aligned at 90 o to the long axis ofthe lower jaws when they are almost closed andthus provides the SP component of the bite. Largefelids such as lions use a more purely SP system inwhich the jaws are closed more slowly but withgreat force (Biknevicius and Van Valkenburgh,268