Dinosaur Predatorprey Interactions

Theropod food preferences and the intensity of predation.—Because predation by and on dinosaurs often resulted in the destruction of prey items, it is difficult to quantify the food preferences of theropods, or to assess the intensity of their predation on herbivorous dinosaurs, in the way that can sometimes be done for marine invertebrates (e.g., by determining the relative frequency of drilled bivalve or brachiopod shells). However, some inferences can be made by examining bite marks and coprolites.

In Late Cretaceous skeletal assemblages from Alberta and Montana, the incidence of tooth-marked bone ranges from a few percent to about 14%; the number is higher for isolated bones than for bones within bonebeds (Fiorillo, 1991; Jacobsen 1997, 1998, 2001; Jacobsen and Ryan, 1998). There is no indication that tyrannosaurids deliberately crushed bones, in the manner of some mammalian carnivores, even though their teeth and jaws were strong enough to handle bone-breaking (Farlow et al., 1991; Erickson et al., 1996; Molnar, 1998; Hurum and Currie, 2000); bone-biting seems to have been incidental to feeding on meat. Hadrosaur bones more frequently show bite marks than do the bones of other herbivorous dinosaurs and theropods.

The presence of juvenile ornithischian bones in tyrannosaurid gut regions and coprolites (Chin et al., 1998; Varricchio, 2001) invites speculation that these large predators preferred to take young as opposed to fully grown individuals of plant-eating dinosaurs. This would have involved less risk of injury to the predator than tussles with large and perhaps dangerous prey. Given that even the largest dinosaurs would have had the capacity to produce large clutches of eggs every year (as opposed to placental mammals, in which larger body size is associated with longer gestation periods; Carrano and Janis, 1991), a stable population size of dinosaurs would have required a high rate of infant mortality. It seems quite likely that a substantial fraction of these juvenile dinosaurs disappeared down the gullets of theropods.

Trackway evidence.—Fossilized trackways offer clues to predatory behavior by theropod dinosaurs. Thulborn and Wade (1984) described a mid-Cretaceous tracksite in Queensland, Australia, in which a host of small bipedal dinosaurs panicked and fled during the approach of a much larger bipedal dinosaur, most likely a large theropod. Whether the bigger dinosaur was actually hunting the smaller animals is uncertain, but at one point it made a sharp change in its direction of travel consistent with the hypothesis that it was trying to drive them in a particular direction.

In 1940 Roland T. Bird collected segments of the trackways of a sauropod and a large theropod dinosaur in the Lower Cretaceous Glen Rose Limestone at what is now Dinosaur Valley State Park near Glen Rose, Texas (Bird, 1985). The theropod

(very likely Acrocanthosaurus; Farlow, 2001) repeatedly stepped into and deformed the prints made by the sauropod, and the trails of both animals made a turn at the same point, suggesting that the meat-eater was close behind and following the big herbivore (Farlow, 1987; Thomas and Farlow, 1997).

Dinosaur tracksites suggest that at least some dinosaurs were gregarious some of the time (Ostrom, 1972, 1986; Currie, 1983; Lockley et al., 1986; Thulborn, 1990; Lockley, 1991; Lockley and Hunt, 1995; Lockley and Meyer, 2000), corroborating interpretations about dinosaur sociality based on skeletal assemblages (Coombs, 1990; Horner, 1997; Farlow, 2000; Eberth et al., 2001). Conceivably, herding behavior on the part of herbivorous dinosaurs was an anti-predator strategy (Day et al., 2002), while group hunting by theropods may have permitted them to kill prey too large for a single hunter to take (Farlow, 1976; Maxwell and Ostrom, 1995).

The Paluxy River sauropod trackway collected by R. T. Bird was one of at least a dozen sauropod trails that seem to have been made by a group of the huge plant-eaters. Bird further believed that a group of theropods was following this herd— rather than just one carnivore tracking a single herbivore. Regrettably, the trackway evidence at Dinosaur Valley State Park does not clearly support Bird's interpretation, but neither does it falsify it (Farlow, 1987).

Predation vs. scavenging.—Perhaps the best known predatory dinosaur, Tyrannosaurus, has been suggested to have been an obligate scavenger (Horner, 1994; Horner and Lessem, 1993; Horner and Dobb, 1997). Horner (1994) argues that several morphological features of Tyrannosaurus would have precluded a predatory lifestyle: 1) relatively small size of the eye that would have prohibited spotting prey at a distance; 2) limb proportions indicative of slow top running speeds, which would have prevented Tyrannosaurus from chasing and capturing prey; 3) disproportionately tiny forelimbs that would have been useless for holding prey; 4) relatively broad teeth that depart from the expected blade-like configuration for teeth of a predator.

We do not find these arguments persuasive. The size of the orbit of Tyrannosaurus relative to its skull size is in fact rather large for a reptile of its size (Fig. 3). Furthermore, the dimensions of the orbit suggest that Tyrannosaurus had a big eye in absolute terms, which would have increased its light-gathering capacity and thus its acuity (Walls, 1942; Dusenberry, 1992). Even though Tyrannosaurus lacks the cursorial hind limb proportions of smaller theropods, and was probably not as good a runner as sometimes portrayed (Farlow et al., 1995b, 2000; Christiansen, 1999; Hutchinson and Garcia, 2002), its metatarsus/femur or tibia/femur length ratios indicate that it was likely as fleet, or faster, than other big theropods, and certainly faster than the herbivorous dinosaurs that were its likely prey (Gatesy, 1991; Holtz, 1995).

Horner's last two arguments strike us as begging the question. Without explicitly saying so, he is hypothesizing that grasping forelimbs are a necessity for killing prey (which will be news to wolves, seriemas, and secretary birds), and that animals with broad-based teeth are unable to kill prey with them

FIGURE 3—Relationship between skull length (occipital condyle to tip of snout) and anteroposterior diameter of the orbit in tyrannosaurids (Gorgosaurus, Daspletosaurus, Tyrannosaurus), theropods other than tyrannosaurids {Eoraptor, Herrerasaurus, Coelophysis, Dilophosaurus, Syntarsus, Abelisaurus, Carnotaurus, Ceratosaurus, Acrocanthosaurus, Allosaurus, Giganotosaurus, Monolophosaurus, Sinraptor, Yangchuanosaurus, Dromaeosaurus, Velociraptor, Erlikosaurus, Ingenia, Ornitholestes, Saurornithoides, Dromiceiomimus, Gallimimus, Garudimimus, Struthiomimus), extant crocodylians (Alligator, Caiman, Melanosuchus, Paleosuchus, Crocodylus, Osteolaemus, Tomistoma, Gavialis), the extinct crocodylian Deinosuchus, the extinct crocodylomorph Sarcosuchus, and several extant species of the varanid lizard genus Varanus{acanthurus, bengalensis, dumerili, exanthematicus, gouldii, griseus, indicus, komodoensis, niloticus, olivaceus, prasinus, rudicollis, salvator, timorensis). Note that tyrannosaurids (including Tyrannosaurus itself, represented by the three biggest tyrannosaurid specimens) have orbits (and therefore presumably eyes) as large or larger relative to skull size than those of other carnivorous reptiles.

FIGURE 3—Relationship between skull length (occipital condyle to tip of snout) and anteroposterior diameter of the orbit in tyrannosaurids (Gorgosaurus, Daspletosaurus, Tyrannosaurus), theropods other than tyrannosaurids {Eoraptor, Herrerasaurus, Coelophysis, Dilophosaurus, Syntarsus, Abelisaurus, Carnotaurus, Ceratosaurus, Acrocanthosaurus, Allosaurus, Giganotosaurus, Monolophosaurus, Sinraptor, Yangchuanosaurus, Dromaeosaurus, Velociraptor, Erlikosaurus, Ingenia, Ornitholestes, Saurornithoides, Dromiceiomimus, Gallimimus, Garudimimus, Struthiomimus), extant crocodylians (Alligator, Caiman, Melanosuchus, Paleosuchus, Crocodylus, Osteolaemus, Tomistoma, Gavialis), the extinct crocodylian Deinosuchus, the extinct crocodylomorph Sarcosuchus, and several extant species of the varanid lizard genus Varanus{acanthurus, bengalensis, dumerili, exanthematicus, gouldii, griseus, indicus, komodoensis, niloticus, olivaceus, prasinus, rudicollis, salvator, timorensis). Note that tyrannosaurids (including Tyrannosaurus itself, represented by the three biggest tyrannosaurid specimens) have orbits (and therefore presumably eyes) as large or larger relative to skull size than those of other carnivorous reptiles.

(which orcas and crocodiles will find surprising). Because the morphology of Tyrannosaurus matches the predictions of his hypotheses, Horner concludes that Tyrannosaurus could not have been a predator, without first testing those hypotheses.

The brain of Tyrannosaurus had respectably large olfactory bulbs (Brochu, 2000), suggesting that the sense of smell was quite acute in this dinosaur. Horner and Dobb (1997) argued that this would have allowed Tyrannosaurus to detect the odor of rotting carcasses from afar. This is unquestionably true, but it is also true that a keen sense of smell would have been useful for picking up the scent of live prey, or for behaviors unrelated to food acquisition (Brochu, 2000).

We agree with Horner that Tyrannosaurus is unlikely to have engaged in extended, Hollywoodstyle battles with other large dinosaurs (or huge apes, for that matter). However, surprise, hit-and-run attacks on healthy victims (Paul, 1988), or culling of sick, injured (Carpenter, 2000), or very young dinosaurs, would seem quite likely. In short, we suspect that Tyrannosaurus and other carnivorous theropods were, like most extant predators, opportunistic carnivores, eagerly searching for carrion (in which activity the large body sizes of many theropods may have been an advantage; Farlow, 1994), but also killing prey whenever possible.

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