View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002The last of the Borophaginae, Borophagus, wasextinct by the start of the Pleistocene (1.8 Ma), anda bone-cracking ecomorph did not reappear in NorthAmerica, despite the fact that a hyaenid didimmigrate from the Old World about 3.5 Ma. Thisspecies, Chasmoporthetes ossifraga, did not havethe enlarged bone-cracking premolars of extanthyenas. Instead, it was more wolf-like in itsdentition, except that unlike wolves and like hyenas,Chasmoporthetes had no postcarnassial molars(Berta, 1981). This hypercarnivore is relatively rarein the North American record, vanishing about750,000 years ago.Pliocene to Recent (approximately 5 Ma topresent): Felids and Canine Canids.—This finalturnover event within the carnivore guild isassociated with a major extinction event amongterrestrial mammals that occurred near the Miocene-Pliocene boundary (8–5 Ma). In both the Old andNew Worlds, more than 60 percent of the generawent extinct (Webb, 1983, 1984; Savage andRussell, 1983) at a time when there was significantclimatic shift towards increased aridification.Associated with this was a decline in mesicwoodlands and expansion of seasonally aridgrasslands (Janis, 1993). In North America, thelosers among the predators were the wolf-likehypercarnivorous borophagine canids. In Eurasia,this was paralleled by a significant decline in thediversity of wolf-like hyaenids as well as sabertoothcats. In both the Old and New Worlds, the bonecrackingcanids and hyaenids were less affected bythe extinction event, suggesting that their ability tomore fully exploit carcasses was advantageous.In North America, wolf-like borophagine canidswere replaced by members of the extant subfamilyCaninae. Canines dispersed from North America toAsia about 7–8 Ma and there replaced the wolf-likehyaenids of the Old World. In both regions, thecanines coexisted with sabertooth and conicaltoothedfelids. Over the past 5 million years,sabertooth cats decreased in diversity as conicaltoothedbig cats increased. Although sabertoothnimravids and felids were never very diverse—mostpaleocommunities included two species—they werepresent in both the Old and New Worlds throughoutmost of the past 40 million years, and it is not clearwhy this widespread ecomorph declined toextinction in the Pleistocene. They disappeared firstfrom Africa (circa 1 Ma), then Eurasia (circa 0.5Ma), and finally North America (circa 0.01 Ma),which hints that humans may have played somerole in their demise, but the mechanism is as yetunclear. Because we don’t understand fully thefunctional significance of having elongate flattenedcanines as opposed to shorter, stouter canines, it isdifficult to determine what factors, such as extinctionof a particular size or type of prey, might have had anegative impact on sabertooths. It does seemprobable that sabertooth cats, almost all of whichFIGURE 9—Lateral view of the skulls of twoCenozoic bone-crackers drawn to the same size.1, the extant Spotted Hyena, Crocuta crocuta, and2, the Pliocene borophagine canid, Borophagussecundus (drawn from Matthew and Stirton, 1930).282