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PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002Carnivorans also evolved specialized slicing teeth,but unlike creodonts, they have only a single pair,the upper fourth premolar and lower first molar(Fig. 6.3). This single carnassial pair is a keydiagnostic character of the order Carnivora. In atypical canid, the carnassials lie about midway alongthe tooth row, separating somewhat conicalpremolars from more flattened rear molars. Thisvariety of tooth form has allowed great evolutionaryversatility within the order. By enhancing one set ofteeth over another, species can evolve towardomnivory (larger rear molars for grinding; e.g.,FIGURE 6—Lateral view of the lower jaws of 1,Dissacus, a mesonychid; 2, Hyaenodon crucians,a creodont; and 3, Hesperocyon gregarius, a canidcarnivoran. Arrows indicate molar teeth specializedfor cutting. Dissacus is modified from Fig. 1 ofO’Leary and Rose (1995b); the others are fromcolor transparencies.bears), carnivory (larger carnassials; e.g., cats), orbone-cracking (larger, bulbous premolars; e.g.,hyenas) (Ewer, 1973).In both Eurasia and North America, theCenozoic history of mammalian carnivorescomprises a series of five dynasties (VanValkenburgh, 1999). Each dynasty is defined bythe prevalence in body size and diversity of one ormore groups. The taxa are not identical on bothcontinents, but there is considerable overlapreflecting the fact that the two have been connectedseveral times over the last 65 million years. InNorth America, the sequence of large predatordynasties was: 1) creodonts; 2) creodonts andnimravids; 3) canids, amphicyonids, hemicyonineursids; 4) borophagine canids and felids; and 5)canine canids and felids. In Eurasia, the pattern isvery similar, except that hyenas fulfill many of thesame roles filled by canids up until the Pliocene(Werdelin and Solounias, 1991, 1996). We nextreview each of these dynasties, highlightingsignificant morphological advances.Early to middle Eocene (approximately 55–40Ma): Creodonts.—Creodonts appear in the latePaleocene (about 57 Ma) and reach their maximumdiversity in North America in the early to middleEocene (48–54 Ma). Early forms were civet-like,preying on relatively small mammals, birds, andperhaps arthropods (Gunnell, 1998). Over time,creodonts expanded into more hypercarnivorousforms, including the first sabertooth predator sincethe Permian. This early sabertooth, Macheroides, ispoorly represented but appears to have been aboutthe size of a bobcat (Lynx rufus), which is muchsmaller than typical sabercats, though comparableto some Late Permian gorgonopsids (Dawson et al.,1986). There were also a few very short-facedcreodonts in the Eocene, with deep jaws and robustpremolars, that may have been bone-crackers, suchas Patriofelis and Paleonictis. This diverse array ofcreodonts ranged in body size from that of foxes tothat of leopards and they were the dominantcarnivores of the Eocene. They coexisted withsmaller species, feliforms and caniforms that representthe earliest members of the order Carnivora (Flynn,1998), as well as a few omnivorous mesonychids.278