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FARLOW AND HOLTZ— PREDATION IN DINOSAURS(which orcas and crocodiles will find surprising).Because the morphology of Tyrannosaurus matchesthe predictions of his hypotheses, Horner concludesthat Tyrannosaurus could not have been a predator,without first testing those hypotheses.The brain of Tyrannosaurus had respectablylarge olfactory bulbs (Brochu, 2000), suggestingthat the sense of smell was quite acute in thisdinosaur. Horner and Dobb (1997) argued that thiswould have allowed Tyrannosaurus to detect theodor of rotting carcasses from afar. This isunquestionably true, but it is also true that a keensense of smell would have been useful for pickingup the scent of live prey, or for behaviors unrelatedto food acquisition (Brochu, 2000).We agree with Horner that Tyrannosaurus isunlikely to have engaged in extended, Hollywoodstylebattles with other large dinosaurs (or hugeapes, for that matter). However, surprise, hit-andrunattacks on healthy victims (Paul, 1988), orculling of sick, injured (Carpenter, 2000), or veryyoung dinosaurs, would seem quite likely. In short,we suspect that Tyrannosaurus and othercarnivorous theropods were, like most extantpredators, opportunistic carnivores, eagerlysearching for carrion (in which activity the largebody sizes of many theropods may have been anadvantage; Farlow, 1994), but also killing preywhenever possible.DINOSAUR FAUNASComposition.—Dinosaurs began as minorcomponents of Late Triassic large-tetrapod faunas(cf. Parrish, 1993; Rogers et al., 1993), but by thebeginning of the Jurassic Period had become thedominant terrestrial large vertebrates. Over theremainder of the Mesozoic Era the taxonomiccomposition of herbivorous and carnivorousspecies in dinosaur faunas varied across time andspace, but two particularly noteworthy faunal suitescan be recognized. One of these has the herbivorousdinosaur component strongly influenced or evendominated by sauropods (e.g., the Morrison andWessex Formations) (Table 1); this faunal type ischaracteristic of much of the world during theJurassic and Cretaceous Periods. Sauropods areabsent or rare and ornithischians dominant in thesecond faunal type (e.g., the Dinosaur ParkFormation), which occurred in the Late Cretaceousof western North America and eastern and centralAsia (Table 1).Neoceratosaurs, basal tetanurans, andcarnosaurs (Holtz, 2000) are the dominant theropodgroups in the first faunal type, and coelurosaurs inthe second. Medium-sized and large theropods inthe first faunal suite come from a variety oflineages, but all large-bodied taxa in the secondtype are tyrannosaurids. In both faunal typespredatory dinosaurs individually are far lessabundant than plant-eaters (Farlow, 1997)Although we do not know which carnivorousdinosaur species specialized on which herbivorousspecies, the marked differences between the twokinds of faunas suggest the possibility of majordifferences in predator-prey interactions betweenthem. For example, adult sauropods wereconsiderably bigger than the largest theropods, whilemost big ornithischians were much closer totyrannosaurids in body size. Even if tyrannosaurspreferred to attack immature individuals of preyspecies, it is easy to imagine a single tyrannosaurkilling an adult hadrosaur or ceratopsian. It is muchharder to visualize a single allosaur slaying an adultapatosaur or brachiosaur. Did large theropods insauropod-dominated faunas attack only immaturesauropods and ignore fully grown adults, or did theyengage in group hunting to haul down big sauropods,or did they mainly scavenge sauropod carcasses?Theropod species in the multi-taxon predatorassemblages typical of sauropod-dominated faunasshow interesting morphological differences fromtyrannosaurids that suggest differences in the styleof predation between carnivores in thesecommunities. In the multi-taxon assemblagesseveral large-bodied theropod taxa (basal tetanurans,spinosaurids, and carnosaurs) possessed verypowerfully built forelimbs terminating in large talons.It is quite likely that these predators employed theirforelimbs as weapons of prey acquisition. In contrast,tyrannosaurids are characterized by greatly reducedforelimbs, and so their style of prey acquisition would257