View - Kowalewski, M. - Virginia Tech

VAN VALKENBURGH AND JENKINS—HISTORY OF SYNAPSID PREDATORSwere leopard-sized and larger, killed relatively largeprey, and their knife-like canines may have made iteasier for a solitary individual to succeed.The last five million years witnessed theevolution of a number of highly cursorialcarnivorans among a variety of families. Forexample, most of the Caninae are characterized byslender, relatively inflexible limbs, compact feet,and the ability to run tirelessly. Within the felids,long-legged genera arose in both Eurasia (thecheetah Acinonyx) and North America (the cheetahlikepuma Miracinonyx) (Van Valkenburgh et al.,1990). Running hyenas, such as Chasmoporthetes,were present in both the Old and New Worlds, andthere was even a long-legged running bear,Arctodus, in North America. This burst of largecursorial predators in the Plio-Pleistocene is notassociated with a similar trend among theirungulate prey. Instead, the ungulates evolved theircursorial adaptations some 20 million years earlier(Janis and Wilhelm, 1993). Consequently, it seemslikely that the predators were responding to a globalchange in vegetation structure that favored longdistancepursuit over ambush, such as a dramaticreduction in vegetative cover. In any case, the Plio-Pleistocene guilds of large carnivorans display agreater diversity of locomotor types than anyprevious guild.MAMMALIAN PREDATORS:LONG-TERM TRENDSThe long-term trends in morphological changeobserved among the Cenozoic synapsids are notnearly as profound or dramatic as those of thePermo-Triassic. As noted above, the evolutionaryhistory of these ancient synapsids documents thetransition from a reptilian type of craniodental andjaw muscle architecture to that of mammals. Nosuch major advances were made by Cenozoicsynapsids. Instead, the history is more one of earlydiversification into a variety of feedingmorphologies followed by repeated iterations ofthese types by different groups over time. Forexample, among Eocene creodonts, there werespecies similar to cats (both sabertooth and conicaltoothed),bone-cracking hyaenids, wolves, andcoyotes. These fundamental carnivorous feedingtypes evolved multiple times over the last 55million years. The absence of secular trends in thedentition and feeding mechanics of the Cenozoicpredators suggests that the synapsid craniodentalarchitecture that evolved in the Mesozoic was verysuccessful and required little or no tinkering toproduce capable meat-eaters. In addition, the lackof a trend reflects the fact that the material propertiesof the food (prey) probably changed little over theCenozoic; skin, muscle, and bone are assumed tohave been much the same in the Paleocene as thepresent. By contrast, Cenozoic herbivorousmammals do exhibit long-term trends in dentalmorphology that reflect an environmental shifttoward cooler, more arid habitats and associatedtougher vegetation (e.g., grass) (cf. Janis, 1993).However, the Cenozoic cooling trend did havean impact on predators. It produced more openhabitats and favored the evolution of cursorialityin both predator and prey. Thus, over the course ofthe past 55 million years, there are repeated trendstowards longer limbs and more compact joints incarnivores. However, it is not a steady progression;instead, it occurred in steps, with the most obviousanatomical advances made in the early Miocene,and then in the Plio-Pleistocene (Janis andWilhelm, 1993). Whether these anatomical changesactually corresponded to improvements in runningability is not known. It is very difficult to ascertaineither top speed or endurance ability frommammalian skeletons, in part because species withvery different builds, such as spotted hyenas andwolves, display similar locomotor capabilities(Bakker, 1983; Janis and Wilhelm, 1993).Nevertheless, in general, extant predators are moregracile in form than their Eocene and Oligocenecounterparts and this probably reflects a shifttoward speed over brawn.PATTERNS IN THEEVOLUTIONARY HISTORY OFSYNAPSID PREDATORSDespite the striking differences in anatomybetween Permo-Triassic and Cenozoic synapsid283