Tyrannosaurid Paleobiology

15 New Evidence for Predation by a Large Tyrannosaurid

279 15 New Evidence for Predation by a Large Tyrannosaurid Nate L. Murphy, Kenneth Carpenter, and David Trexler A partial skeleton of a hadrosaur believed to be Brachylophosaurus canadensis shows evidence of a failed attack by a large theropod, possibly Daspletosaurus sp. The injury consists of damage to the neural spines of the last sacral vertebra and first two caudal vertebrae. Remodeled bone, even at the site of the traumatic amputation of the caudal neural spine, demonstrates that the individual survived the attack. In addition, the attack came from the rear, suggesting that the Brachylophosaurus was fleeing the attacker. This specimen adds to the growing body of knowledge that tyrannosaurids were capable of active predation. The relative importance of predation and scavenging by tyrannosaurid theropods has been controversial. Horner and Lessem (1993) have argued that Tyrannosaurus was an obligate scavenger because of the large body size, short arms, large eyes, and large olfactory lobes. However, Carpenter (2000) described a failed attack on an adult hadrosaur, Edmontosaurus. Carpenter argued from morphological features that the bite was most parsimoniously ascribed to Tyrannosaurus. The features of Tyrannosaurus that Horner and Lessem (1993) cited as evidence for scavenging apply equally to other tyrannosaurids as well. By inference then, the other tyrannosaurids (e.g., Gorgosaurus, Daspletosaurus and Albertosaurus) were scavengers in the absence of evidence to the contrary. That contrary evidence is now slowly emerging. Wegweiser et al. (2004) report evidence of an attack on a lambeosaurine hadrosaur by an unidentified tyrannosaurid. That specimen consists of a partially healed rib bearing the impression of a tooth. And now, newly discovered articulated caudal vertebrae of the hadrosaurid Brachylophosaurus cf. canadensis provides evidence that another tyrannosaurid, probably Daspletosaurus, was an active predator as well. The specimen (Fig. 15.1) consists of dorsals 15–18, sacrals 1–9, and caudals 1–74 with associated chevrons and ossified tendons, the ilia, proximal ends of the left and right pubes, iliac peduncle of the left and right ischia, and proximal ends of the left and right femora. Because the tail is complete to the terminal caudal, we believe the specimen was originally complete and that the missing parts were lost to erosion. The specimen shows trauma to the tail (Fig. 15.2) that we believe demonstrates a failed attack similar to the specimen described by Carpenter (2000). Abstract Introduction Institutional Abbreviations DMNH, Denver Museum of Natural History (now Denver Museum of Nature and Science), Denver, Colorado; JRF, Judith River Foundation/ Judith River Dinosaur Institute, Malta, Montana. Murphy, Carpenter, and Trexler 280 15.2. Close-up of the bitemarked region in JRF 1002. A) Bitten off neural spine (arrow) at the base of the tail. B) Traumatic amputation of the neural spine of the second caudal (arrows). C) Detail of the traumatized region of the caudals. Note exostosic bone at the site of the traumatic amputation and on the neural spines of sacral neural spines 8 and 9 and caudal 1. Evidence for Predation by a Large Tyrannosaurid 281 The specimen, JRF-1002, was found in the lower portions of the Judith River Formation in Phillips County, along the Little Cottonwood drainage in northeastern Montana. The specimen was found 138 feet above the marine Claggett Formation. The lithology of the site is composed of loosely consolidated, fine-grained sandstone containing some clay clasts and some laminates of carbonaceous plant material; the depositional environment is believed to be meandering river. The depositional environment and taphonomy is similar to that of other Brachylophosaurus specimens recovered from the same general area (see Murphy et al. 2006). The specimen, JRF-1002, has a total length of 354 cm, of which the sacrum is 46 cm and the tail 297 cm; the left ilium is 67 cm long. From these measurements, the individual is estimated to have been 4.35 m long and 1.4 m tall at the hips. The sutures between the centra and neural arches are still visible, indicating that the individual was not fully grown. The neural spine of the last sacral (S-8) and of caudals 1, 2, and 3 show trauma (Fig. 15.2) that we interpret to have been caused by a bite most likely inflected by a large tyrannosaurid. In addition, the ossified tendons are missing in this region but are otherwise in situ elsewhere on the specimen . The damaged sacral neural spine (S-8) shows localized exostosis on the lateral surface (Fig. 15.2C), as...