Physical evidence of predatory behavior in Tyrannosaurus rex

Feeding strategies of the large theropod, Tyrannosaurus rex, either as a predator or a scavenger, have been a topic of debate previously compromised by lack of definitive physical evidence. Tooth drag and bone puncture marks have been documented on suggested prey items, but are often difficult to attribute to a specific theropod. Further, postmortem damage cannot be distinguished from intravital occurrences, unless evidence of healing is present. Here we report definitive evidence of predation by T. rex: a tooth crown embedded in a hadrosaurid caudal centrum, surrounded by healed bone growth. This indicates that the prey escaped and lived for some time after the injury, providing direct evidence of predatory behavior by T. rex. The two traumatically fused hadrosaur vertebrae partially enclosing a T. rex tooth were discovered in the Hell Creek Formation of South Dakota.One of the most daunting tasks of paleontology is inferring the behavior and feeding habits of extinct organisms. Accurate reconstruction of the lifestyle of extinct animals is dependent on the fossil evidence and its interpretation is most confidently predicated on analogy with modern counterparts (1–6). This challenge to understanding the lifestyle of extinct animals is exemplified by the controversy over the feeding behavior of the Late Cretaceous theropod Tyrannosaurus rex (3, 7–17). Although predation and scavenging have often been suggested as distinct feeding behavior alternatives (3, 7–9, 11–17), these terms merit semantic clarification. In this study, predation is considered a subset of feeding behavior, by which any species kills what it eats. Although the term “predator” is used to distinguish such animals from obligate scavengers, it does not imply that the animal did not also scavenge.Ancient diets can be readily reconstructed on the basis of the available evidence, although their derivation (e.g., predation or scavenging behavior) often remains elusive. Speculation as to dinosaur predation has ranged from inferences based on skeletal morphology, ichnofossils such as bite marks, coprolites, stomach contents, and trackways and, by more rarely, direct predator–prey skeletal associations (3, 4, 18–23).Direct evidence of predation in nonavian dinosaurs other than tyrannosaurids has been observed in rare instances, such as the Deinonychus–Tenontosaurus kill site of the Cloverly Formation where the remains of both were found in close association along with shed teeth (9, 24), and the “fighting dinosaurs” from the Gobi Desert, in which a Velociraptor and Protoceratops were found locked in mortal combat (9, 17). The evidence on tyrannosaurids is more limited. Putative stomach contents, such as partially digested juvenile hadrosaur bones, have been reported in association with tyrannosaurid remains (3, 12, 18). This latter instance only represents physical evidence of the last items consumed before the animal’s death, an indicator of diet but not behavior.Mass death assemblages of ornithischians frequently preserve shed theropod teeth (6, 22, 24). Lockley et al. (23) suggest such shed teeth are evidence of scavenging behavior. It is widely argued that T. rex procured food through obligate scavenging rather than hunting (11, 14, 25–27) despite the fact that there is currently no modern analog for such a large bodied obligate scavenger (26). Horner (25) argued that T. rex was too slow to pursue and capture prey items (14) and that large theropods procured food solely through scavenging, rather than hunting (11, 25). Horner also suggested that the enlarged olfactory lobes in T.rex were characteristic of scavengers (25). More recent studies (28, 29) determined the olfactory lobes of modern birds are “poorly developed,” inferring that enlarged olfactory lobes in T. rex are actually a secondary adaptation for predation navigation “to track mobile, dispersed prey” (30). T. rex has a calculated bite force stronger than that of any other terrestrial predator (7), between 35,000 and 57,000 Newtons (30, 31), and possible ambulatory speeds between 20 and 40 kph (7, 15, 16), documenting that it had the capability to pursue and kill prey items.Healed injuries on potential prey animals provide the most unequivocal evidence of survival of a traumatic event (e.g., predation attempt) (3, 32, 33), and several reports attribute such damage to T. rex (4, 17, 19, 20). These include broken and healed proximal caudal vertebral dorsal spines in Edmontosaurus (17) and healed cranial lesions in Triceratops (4, 19). Although the presence of healed injuries demonstrates that an animal lived long enough after the attack to create new bone at the site of the damage (a rare occurrence in the fossil record) (19), the healing usually obliterates any clear signature linking the injury to a specific predator. Bite traces (e.g., raking tooth marks on bone and puncture wounds in the bones of possible prey animals) attributed to T. rex (2, 4, 19) are ambiguous, because the damage inflicted upon an animal during and after a successful hunt mirrors feeding during scavenging. This makes distinction between the two modes of food acquisition virtually impossible with such evidence (3, 34–38).Tooth marks, reported from dinosaur bone-bearing strata worldwide (e.g., 2–4, 8, 19, 20, 39, 40), are further direct evidence of theropod feeding behavior, attributed by some to specific theropod groups (2, 4, 19, 20). Happ (19) and Carpenter (17) identified theropods to family and genus by matching spaces to parallel marks (traces) with intertooth distance. Happ (19) described opposing conical depressions on a left supraorbital Triceratops horn that was missing its distal third (tip), attributing them to a bite by either a T. rex or a crocodilian. Happ (19) stated that the spacing of the parallel marks present on the left squamosal of the same individual matched the intertooth distance of tyrannosaurids. The presence of periosteal reaction documents healing. This contrasts with the report by Farlow and Holtz (3) and again by Hone and Rauhut (20) of the same Hypacrosaurus fibula containing a superficially embedded theropod tooth. Absence of bone reaction precludes confident attribution to predation.Two coalesced hadrosaur (compare with Edmontosaurus annectens) caudal vertebrae were discovered in the Hell Creek Formation of Harding County, South Dakota (40). Archosaur fauna identified in this site include crocodiles, dinosaurs, and birds (41). Physical evidence of dental penetration and extensive infection (osteomylitis) of the fused vertebral centra and healing (bone overgrowth) document an unsuccessful attack by a large predator. A tooth crown was discovered within the wound, permitting identification of the predator as T. rex. This is unambiguous evidence that T. rex was an active predator, fulfilling the criteria that Farlow and Holtz (3) advanced. As T. rex comprises between 1% and 16% of the Upper Cretaceous dinosaurian fauna in Western North America (41–45), its status as a predator or obligate scavenger is nontrivial and could have significant implications for paleoecological reconstructions of that time period. The present contribution provides unique information demonstrating the ecological role for T. rex as that of an active predator. Despite this documentation of predatory behavior by T. rex, we do not make the argument that T. rex was an obligate predator. Like most modern large predators (27, 45) it almost certainly did also scavenge carcasses (9, 16).