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FARLOW AND HOLTZ— PREDATION IN DINOSAURShave relied on their powerful jaws alone.Another difference between the faunal types isthe overlap of theropod body sizes. In assemblagespossessing multiple lineages of large-bodiedtheropods there is commonly great overlap in thesize of the carnivores. For example, in the MorrisonFormation the carnosaur Allosaurus, the basaltetanuran Torvosaurus, and the neoceratosaurCeratosaurus would all include individuals of1 tonne or greater body mass. Similarly, theBahariya Formation’s spinosaurid Spinosaurus,carnosaur Carcharodontosaurus, and basalcoelurosaur Deltadromeus all exceeded 2 tonnesin mass (the first two by a considerable margin).The presence of comparable-sized predatorssuggests the possibility of competition among thesetaxa for food, perhaps mitigated by some form ofmorphologically mediated niche partitioning(Henderson, 2000). Similar size overlap occursbetween the adults of medium-sized theropods inthese assemblages, which would additionally havebeen in potential competition with immatureindividuals of the largest-bodied species.In marked contrast, all the larger carnivorousdinosaurs in Late Cretaceous assemblages of westernNorth America and eastern and central Asia aretyrannosaurids, and among these there is typicallyjust one or two species present in potential sympatry.Furthermore, there is often a large discontinuity inadult sizes between the tyrannosaurids and the nextlargest unquestionably carnivorous dinosaurs in thefauna (generally dromaeosaurids), rather than thegradational distribution of adult sizes seen in nontyrannosaurid-dominatedfaunas.Macroecology of carnivorous dinosaurs.—Thesingle most noteworthy feature of most dinosaurs,of course, is their large size. Body size affects or iscorrelated with numerous physiological andecological features of animals (Brown, 1995; Brownand West, 2000). Large animals have bigger homeranges than do smaller species, and carnivoresrequire more habitat space than herbivores (Kelt andVan Vuren, 2001). Farlow (2001) used publishedregressions of home range area against body massin extant predatory mammals, birds, and lizards tospeculate that the home range size of the 2500-kgcarnosaur Acrocanthosaurus would haveencompassed hundreds or thousands of squarekilometers. Kelt and Van Vuren (2001), however,suggested that there may be some upper limit tohome range area in mammals, regardless of bodysize and diet. If true, and if this upper limit holds forother terrestrial vertebrates, it raises the question ofhow gigantic predators like large theropods couldhave survived on relatively small (as compared withindividual animal size) home ranges.Because an individual animal’s home rangearea becomes larger with increasing body size,population density (number of individuals / habitatarea) must decrease (Damuth, 1987; Brown, 1995;Smallwood, 2001), which in turn mandates largegeographic ranges if big-bodied species are to berepresented by enough individuals for long-termviability (Calder, 2000). For trophodynamicreasons carnivores must have lower populationdensities than herbivores, and so the problem ofsufficient habitat space should be particularly acutefor enormous carnivores (Farlow, 1993; Burnesset al., 2001). The huge sizes routinely achieved bycarnivorous dinosaurs are therefore ecologicallypuzzling. Conceivably, theropod gigantism wasfacilitated by a combination of lower foodrequirements than expected for elephantinemammalian meat-eaters, along with elevated ratesof biological productivity under the greenhouseconditions of the Mesozoic Era (Farlow, 1993;Farlow et al., 1995; Burness et al., 2001). In anotherscenario (Carrano and Janis, 1991) the greaterreproductive capacity of herbivorous dinosaursrelative to placental mammals (due to oviparity ofthe former) would allow for more available“packages” of dinosaurian meat that could beconsumed by theropods while still allowing for aviable sustainable population of prey.POSSIBLE DIRECTIONS FORFUTURE RESEARCHAlthough progress in understanding predatorpreyinteractions in dinosaur communities willdepend in large part on fortuitous discoveries ofparticularly informative specimens or assemblages,259