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PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002predators, there are a number of parallels. Bothradiations produced saber-toothed forms multipletimes—gorgonopsids and therocephalians in thePermo-Triassic, and creodonts, nimravids, felids,and even marsupials (Thylacosmilus (Fig. 8.4) ofSouth America) in the Cenozoic. Without a doubt,saber-like canines are advantageous, but theirabsence today frustrates our ability to explain theirobvious success. As noted above, it might enhancethe ability to take relatively large, thick-skinnedprey. Both radiations also produced short-snouted,powerful, biting predators (e.g., the ancientMoschorhinus and Arctognathus), although thisform is much more common among Cenozoicpredators, where it appears across a wide array offamilies, including oxyaenids, felids, nimravids,canids, ursids, and mustelids. The lower diversityof short-snouted species in the Permo-Triassic isunderstandable given that most species relied on aKI jaw system for killing and feeding.Both radiations of predators show a pattern ofdynasty replacement. That is, one or a few cladesevolve large size and seem to dominate thecarnivore guild for several million years, but thendecline and are replaced by new taxa. In theCenozoic, most of the dynasties appear to be madeup of a more diverse collection of taxa than thoseof the Permo-Triassic. That is, the Cenozoicdynasties include two to three families whereas thePermo-Triassic dynasties include a single suborderor infraorder (e.g., gorgonopsids). In some cases,such as the gorgonopsids (Fig. 4), a fairly diversearray of ecomorphs is included within the group,rivaling that seen in the Cenozoic. However, inmost cases there does seem to have been lessecological diversity in feeding types within Permo-Triassic predator guilds, based on their similaritiesin tooth form and jaw mechanics.In both the Permo-Triassic and Cenozoic,dynasty replacement is associated with considerableevolutionary convergence, as declining clades seemto leave ecological vacuums that are subsequentlyfilled by new taxa. This is especially apparent in theCenozoic record. For example, during the NorthAmerican “cat gap” (23–17 Ma), several families(e.g., canids, ursids, mustelids) produced relativelyshort-snouted species with large canines and reducedpost-carnassial teeth. A similar trend towardhypercarnivory is apparent in South AmericanPleistocene canids and also occurred underconditions of very low felid diversity (VanValkenburgh, 1991). In this instance, the nearabsence of cat-like species was a result of a priorextinction of the sabertooth marsupialThylacosmilus, and an apparently slowerimmigration of felids than canids into SouthAmerica. Convergence among Permo-Triassicsynapsids is less common, but is evident betweentherocephalian sabertooths and the gorgonopsidsthat replace them, as well as between derivedgorgonopsids, which went extinct at the end of thePermian, and moschorhinid therocephalians,which did not. Another example is that of theconvex dorsal profile that is seen in thesphenacodont pelycosaur-grade synapsidDimetrodon and some gorgonopsids such asArctops. Functional convergence may also underliethe broadly similar skull proportions of thesphenacodont Secodontosaurus and somedolichorostral therocephalians such as Lycideops.Within both ancient and Cenozoic predatorclades, there is frequently a tendency towardincreasing body size and hypercarnivory over time.For example, therocephalians, gorgonopsids, anddinocephalians all tend to get larger in body size,enlarge their canine teeth, and strengthen their jawsand snouts over their history. Among Cenozoicmammals, the tendency toward large size is usuallyaccompanied by a reduction in snout length and postcarnassialmolars. This trend is apparent among allthree subfamilies of canids (Van Valkenburgh et al.,in press), oxyaenids (Gunnell, 1998), hyaenodontids(Mellet, 1977), amphicyonids (Viranta, 1996; Hunt,1998b), and hyaenids (Werdelin and Solounias,1996). The prevalence of this pattern suggests thatit is driven by aspects of the large carnivore niche.Levels of interspecific competition for food arerelatively high within guilds of extant carnivorousmammals (Eaton, 1979; Palomares and Caro, 1999;Van Valkenburgh, 1985, 2000). Because largerindividuals tend to dominate inter- and intraspecificconflicts, such as occur over carcasses,284