View - Kowalewski, M. - Virginia Tech

FARLOW AND HOLTZ— PREDATION IN DINOSAURSnumber is higher for isolated bones than for boneswithin bonebeds (Fiorillo, 1991; Jacobsen 1997,1998, 2001; Jacobsen and Ryan, 1998). There is noindication that tyrannosaurids deliberately crushedbones, in the manner of some mammaliancarnivores, even though their teeth and jaws werestrong enough to handle bone-breaking (Farlow etal., 1991; Erickson et al., 1996; Molnar, 1998;Hurum and Currie, 2000); bone-biting seems to havebeen incidental to feeding on meat. Hadrosaur bonesmore frequently show bite marks than do the bonesof other herbivorous dinosaurs and theropods.The presence of juvenile ornithischian bonesin tyrannosaurid gut regions and coprolites (Chinet al., 1998; Varricchio, 2001) invites speculationthat these large predators preferred to take young asopposed to fully grown individuals of plant-eatingdinosaurs. This would have involved less risk ofinjury to the predator than tussles with large andperhaps dangerous prey. Given that even the largestdinosaurs would have had the capacity to producelarge clutches of eggs every year (as opposed toplacental mammals, in which larger body size isassociated with longer gestation periods; Carranoand Janis, 1991), a stable population size ofdinosaurs would have required a high rate of infantmortality. It seems quite likely that a substantialfraction of these juvenile dinosaurs disappeareddown the gullets of theropods.Trackway evidence.—Fossilized trackwaysoffer clues to predatory behavior by theropoddinosaurs. Thulborn and Wade (1984) described amid-Cretaceous tracksite in Queensland, Australia,in which a host of small bipedal dinosaurs panickedand fled during the approach of a much largerbipedal dinosaur, most likely a large theropod.Whether the bigger dinosaur was actually huntingthe smaller animals is uncertain, but at one point itmade a sharp change in its direction of travelconsistent with the hypothesis that it was trying todrive them in a particular direction.In 1940 Roland T. Bird collected segments ofthe trackways of a sauropod and a large theropoddinosaur in the Lower Cretaceous Glen RoseLimestone at what is now Dinosaur Valley State Parknear Glen Rose, Texas (Bird, 1985). The theropod(very likely Acrocanthosaurus; Farlow, 2001)repeatedly stepped into and deformed the printsmade by the sauropod, and the trails of both animalsmade a turn at the same point, suggesting that themeat-eater was close behind and following the bigherbivore (Farlow, 1987; Thomas and Farlow, 1997).Dinosaur tracksites suggest that at least somedinosaurs were gregarious some of the time(Ostrom, 1972, 1986; Currie, 1983; Lockley et al.,1986; Thulborn, 1990; Lockley, 1991; Lockley andHunt, 1995; Lockley and Meyer, 2000),corroborating interpretations about dinosaursociality based on skeletal assemblages (Coombs,1990; Horner, 1997; Farlow, 2000; Eberth et al.,2001). Conceivably, herding behavior on the partof herbivorous dinosaurs was an anti-predatorstrategy (Day et al., 2002), while group hunting bytheropods may have permitted them to kill prey toolarge for a single hunter to take (Farlow, 1976;Maxwell and Ostrom, 1995).The Paluxy River sauropod trackway collectedby R. T. Bird was one of at least a dozen sauropodtrails that seem to have been made by a group ofthe huge plant-eaters. Bird further believed that agroup of theropods was following this herd—rather than just one carnivore tracking a singleherbivore. Regrettably, the trackway evidence atDinosaur Valley State Park does not clearlysupport Bird’s interpretation, but neither does itfalsify it (Farlow, 1987).Predation vs. scavenging.—Perhaps the bestknown predatory dinosaur, Tyrannosaurus, hasbeen suggested to have been an obligate scavenger(Horner, 1994; Horner and Lessem, 1993; Hornerand Dobb, 1997). Horner (1994) argues that severalmorphological features of Tyrannosaurus wouldhave precluded a predatory lifestyle: 1) relativelysmall size of the eye that would have prohibitedspotting prey at a distance; 2) limb proportionsindicative of slow top running speeds, which wouldhave prevented Tyrannosaurus from chasing andcapturing prey; 3) disproportionately tiny forelimbsthat would have been useless for holding prey; 4)relatively broad teeth that depart from the expectedblade-like configuration for teeth of a predator.We do not find these arguments persuasive. The255