View - Kowalewski, M. - Virginia Tech

VAN VALKENBURGH AND JENKINS—HISTORY OF SYNAPSID PREDATORSappearance of ambush predators with retractileclaws and all three canine tooth types: scimitar,dirk, and conical.The nimravids persisted in North America untilabout 23 Ma, filling the ecological roles held nowby bobcat- to jaguar-sized felids, while thehyaenodontids filled more canid-like roles.Between 23 Ma and 17 Ma, specialized cat-likepredators were absent from North America forabout 5 million years until felids arrived from theOld World (Fig. 7). Notably, nimravids eventuallyreappeared in North America in the late Miocene(circa 11 Ma) in the form of the most bizarre andlast of all nimravids, Barbourofelis. Barbourofelis(Fig. 8.3) had enormously elongate dirk-shapedupper canines and highly reduced cheek teeth. Thecoronoid process on the lower jaw was very smallin order to allow the jaws to open wide enough toprovide space between upper and lower canine tips.Late Oligocene to mid-Miocene (approximately28–15 Ma): Canids, Amphicyonids, HemicyonineUrsids.—In the latter half of the Oligocene, theformerly dominant nimravids and creodontsdeclined in diversity for reasons that are unclear.They were replaced by species that evolved in NorthAmerica (canids, amphicyonids) as well as by newimmigrant taxa (ursids, amphicyonids, mustelids)from the Old World. The elimination of cat-likenimravids around 23 Ma seems to have promotedthe evolution of hypercarnivory in several taxa.Among the canids, short-snouted forms such asEnhydrocyon appeared before the nimravidsdisappeared but were smaller in body size than thesabertooths. During the “cat” gap in the record, alarge, leopard-sized mustelid (Megalictis), as wellas several wolf-like ursids (e.g., Phoberocyon) andamphicyonids (e.g., Daphoenodon), filled thehypercarnivorous niche (Van Valkenburgh, 1991).With the exception of the large mustelid, all thesehypercarnivores appear to have been more cursorialthan either Hyaenodon or any of the Oligocenecanids (Hunt, 1998a, 1998b). Although several ofthese species converged on the cat pattern in havingshorter snouts and reduced postcarnassial molars, asabertooth ecomorph did not evolve among them.One additional group outside the orderCarnivora should be mentioned as possiblepredators and scavengers of this period: theentelodonts, non-ruminant artiodactyls of large size(150–170 kg) with formidable canines, incisors andcrushing premolars (Joeckel, 1990). These large,somewhat boar-like ungulates exhibit heavily wornteeth suggestive of bone-cracking behavior, andthey may have played a somewhat hyena-like rolein the late Oligocene–early Miocene.Mid–Late Miocene (approximately 15–5 Ma):Borophagine Canids and Felids.—Cat-likecarnivorans reappeared in the New World about 18Ma with the immigration of the Old World felidPseudaeleurus (Martin, 1998b). Pseudaeleurus hadconical canines but was joined by the scimitartoothedand larger felid Nimravides and the dirktoothednimravid Barbourofelis around 11 Ma. Thegeneric diversity of felids reaches a maximum ofsix in North America in the latest Miocene (about6 Ma) at which time there were dirk-toothed (tribeHomotheriini) and scimitar-toothed (tribeSmilodontini) sabercats as well as at least oneconical-toothed genus. Alongside the rise in feliddiversity, there were declines in diversity in all otherlarge hypercarnivorous taxa, including hemicyonineursids, amphicyonids, and borophagine canids (VanValkenburgh, 1999). Of these three, the borophaginecanids declined slowest and persisted into thePliocene. The borophagine canids of the middle andlate Miocene were of two types—large cursorialwolf-like predators (e.g., Epicyon) and less cursorialspecialized hyena-like bone-crackers (e.g.,Osteoborus) (Munthe, 1998). Both types are likelyto have hunted in packs, taking down prey as largeas or larger than themselves (Van Valkenburgh etal., in press). The bone-cracking borophagines arevery similar to living hyaenids in cranialmorphology, with dome-like skulls that provideextensive areas for jaw adductor musculature, aswell as short snouts to increase bite force at thecanines (Fig. 9). However, whereas hyenas use theirrobust premolars to crack bones, borophagines usedboth their carnassials and premolars (Werdelin,1989). This activity blunted their carnassials andlikely inhibited their ability to use them as manyspecies do today, to slice through tough skin toaccess muscle and viscera (Van Valkenburgh, 1996).281