View - Kowalewski, M. - Virginia Tech

VAN VALKENBURGH AND JENKINS—HISTORY OF SYNAPSID PREDATORSNON-MAMMALIANSYNAPSID PREDATORS:LONG-TERM TRENDSThe long-term evolutionary trends seen in thecrania of carnivorous non-mammalian synapsidpredators are essentially those documented for thewhole of the Permo-Triassic Synapsida (Romer,1966; Kemp, 1982; Hopson and Barghusen, 1986).They include an increase in solidification of thebraincase wall, reduction in the number of skullbones, expansion of the temporalis musculature anddevelopment of the masseteric musculature,reduction of the pterygoideus component, increasedheterodonty, formation of an osseous secondarypalate, increased brain size, and reduction of thepostdentary bones, which evolve into bones of themammalian middle ear (Kemp, 1982; Sidor, 2001).In terms of skull mechanisms relating to acarnivorous habit, the KI bite system is replaced bythe SP bite system, and the saber-toothed adaptationprevalent in the Late Permian is not seen aftergorgonopsids until the advent of Cenozoiccreodonts. In temporal sequence, convex-skulled,step-jawed sphenacodonts were followed bypachyostotic (thick-boned) skulls and gigantismamong anteosaurs. These in turn were succeededby a variety of saber-toothed forms, both earlytherocephalians and gorgonopsids, which dominatedLate Permian ecosystems. Smaller, rather ‘mammallike’carnivores of the Triassic cynodont cladesupersede the Late Permian saber-toothed predatorcommunity. These animals were close to mammalancestry and very similar to modern mammals intheir skull design (Kemp, 1982).THE EVOLUTIONARY HISTORYOF CENOZOICSYNAPSID PREDATORSSynapsids relinquished the role of dominantterrestrial predator to theropod dinosaurs for theremainder of the Mesozoic. Then, following thedemise of the dinosaurs about 65 Ma, about tenmillion years passed before the first appearance ofa large, specialized meat-eater (e.g., Oxyaena;Gunnell, 1998). Thus, the period to be covered inthis part of the review, the last 55 million years, ismuch shorter than that spanned by the abovedescribednon-mammalian synapsids (80–100million years). Nevertheless, because of its relativerecency, the fossil record of carnivorous mammalsis much richer in detail and diversity than that ofthe ancient synapsids. Here we can provide only abrief summary, but there are several recent reviewsof carnivore history that offer more information (e.g.,Martin, 1989; Hunt, 1996; Werdelin, 1996; VanValkenburgh, 1999). Our emphasis will be onnotable morphological innovations and repeatedpatterns in the history of carnivorous mammals,including convergence on similar ecotypes anddynasty replacement (as was described for Permo-Triassic synapsids). Most of the discussion relieson examples from the excellent North American andEurasian fossil records.Today, the large, terrestrial predators of theworld all belong to a single placental order, theCarnivora. Until very recently, there was onemarsupial predator of reasonable size, the Australianthylacine (Thylacinus cynocephalus), but itsextinction has left the carnivorans (members of theorder Carnivora) as sole occupants of this adaptivezone. In the early part of the Cenozoic, two nowextinctorders, Mesonychia and Creodonta,dominated predator guilds. The mesonychids wereperhaps the first Cenozoic predators, although theirdentition is not highly specialized for meat-eating,and they may have been quite omnivorous.Mesonychids, such as the 62 Ma coyote-sizedDissacus, looked somewhat dog-like externally, buttheir teeth differed markedly from those of canids.The lower tooth row of Dissacus includes a seriesof premolar-like teeth with blunt cusps and weaklydeveloped cutting blades (Fig. 6.1) that seem lessspecialized for carnivory than those of the Triassiccynodont Cynognathus (Fig. 5.3). The firstunquestionable Cenozoic hypercarnivores(specialists on vertebrate prey) were among thecreodonts. Unlike mesonychids, creodonts evolvedspecialized slicing molar teeth, in which upper andlower molars occluded like scissors and created largeshear facets on opposing teeth (Fig. 6.2).277