putrefaction of a body occurs within a few hours to days after death (Chapter 7). Soon afterwards, a scavenger's sensitive nose, under the right wind conditions, can detect a decaying body over great distances, often more effectively than through listening for sounds of other scavengers or attempting to spot the body. Brain endo-casts and CT scans of Tyrannosaurus support this type of adaptation in the form of an enlarged olfactory bulb in the anterior portion of its brain. This and other evidence has led some paleontologists to suggest that this supposedly fierce predator may have been more like a six-tonne vulture. Other aspects of theropod senses used in predation can be speculated on the basis of modern predatory animals. For example, they may have been able to detect vibrations from the ground caused by herd movement. This presumably would have been easy with large herds of sauropods, ceratopsians, or hadrosaurs. They also could have "tasted" the air with their tongues, which is actually a form of smell used by some snakes and lizards. However, without any other corroborating evidence, both of these ideas remain speculation for now.

If a given theropod identified its potential prey, then a number of methods could have been used to kill it. The most often depicted way was the use of teeth and jaws to inflict fatal wounds, but in modern terrestrial carnivores this method is rarely used by itself. Modern land predators, such as large cats and bears, use fore limbs that are typically armed with sharp claws, which are combined with throwing their body weights against a prey animal while running at high speed. These techniques stop the prey long enough to deliver killing actions. Additionally, large cats will not necessarily slash with teeth or claws to kill an animal but will clamp their jaws around its neck to suffocate it. In fact, induced drowning is even possible. For example, a pair of cheetahs were once observed chasing an antelope into a water body and holding its head under water until it drowned. In this case and others, pack hunting is a very effective strategy, whereby multiple predators wear down their prey until it is too exhausted to offer any resistance to fatal wounding.

Some theropods certainly had teeth, jaws, and claws as available weapons. Unfortunately, the low preservation potential for soft-tissue wounds prevents independent confirmation of most hypotheses on theropod predation, but several features of a few theropods invoke only visions of death-dealing implements. Large, serrated teeth are one persuasive attribute, but huge, powerful arms that end in sharp unguals are another persuasive attribute of hunting. An intriguing find in the Upper Cretaceous of Mongolia was of 2.4-meter long forelimbs that ended in recurved unguals and phalanges. The rest of the body was never found, but the unusual arms were assigned to a new species, Deinochirus mirificus, which is currently interpreted as an ornithomimid. Like those of therizinosaurs, these arms with their prominent claws have been interpreted as possible tools for demolishing termite mounds, but they also would have served well for larger game.

Ambiguous as some skeletal evidence may be, little doubt is expressed about the most well-known feature of some dromaeosaurids, including the Late Cretaceous Velociraptor and Early Cretaceous Deinonychus and Utahraptor: a sharp, retracted ungual on digit II of the foot (Figs 9.9 and 9.15). This claw remained permanently above the ground surface with its point forward, strongly suggesting that it had an offensive purpose, such as disemboweling prey animals. Once again, because the preservation potential for soft-tissue damage is so low, independent verification of this claw as a killing feature is virtually absent. Nevertheless, one of the few examples in the geologic record of two dinosaur species directly interacting with one another as they died and, consequently, one of the most remarkable dinosaur discoveries ever found is a Velociraptor conjoined with its apparent intended prey, a Protoceratops (see Fig. 7.9). In this case, the digit II unguals of Velociraptor are clearly within the ventral (abdominal and throat) region of Protoceratops. The protoceratopsian also had the right forearm of the Velociraptor caught in its jaws, showing that it was

FIGURE 9.15 The formidable Utahraptor and its raptorial digit II ungual, skeletal mount with claw raised on left pes. Note its similarity to Deinonychus in both form and inferred function. College of Eastern Utah Prehistoric Museum, Price, Utah.

probably responding violently to its impending death. Amazingly, both animals had been killed at the same time by the depositional event that buried them. This deposition froze their positions until they were uncovered nearly 70 million years later in the Upper Cretaceous of Mongolia. This predator-prey tableau is appropriately nicknamed "The Fighting Dinosaurs."

Similar to this discovery, although less obvious in its interpretative value, is another interesting case of a possible predator-prey relationship interpreted from associated dinosaur skeletal remains. In this example, a Lower Cretaceous deposit in Montana contains the bones and teeth of at least four individuals of Deinonychus and one individual of the ornithopod Tenontosaurus. The unusual juxtaposition of a larger ornithopod with four small theropods of the same species, which normally are rare finds even as individuals, has been interpreted as the end result of pack hunting. In this scenario, the dromaeosaurids may have attacked the ornithopod and killed the much larger prey animal through a concerted and cooperative effort. The partial remains of the four predators are explained by a hypothesis that the prey animal used its superior bulk of nearly one tonne and accompanying strength to defend itself. As a result, several of the smaller predators, which weighed only about 50 to 100 kg each, may have suffered fatal injuries before the prey itself succumbed to their onslaught. Although the circumstances surrounding the remains of this presumed battle still raise more questions than answers, it is one of the few persuasive cases for pack hunting in some theropods. This idea has been long conjectured but not supported by much more than theropod trackways (mentioned earlier) and observations of pack-hunting behaviors in modern terrestrial carnivores. Further evidence supporting this predator-prey relationship of Deinonychus and Tenontosaurus are 15 localities discovered so far in the Lower Cretaceous of Montana, where Deinonychus teeth were found in the same vicinity as Tenontosaurus bones.

Of all theropods, tyrannosaurids are probably the dinosaurs best known for their eating habits, which are delineated by:

1 toothmarks and teeth in bones of their former meals;

2 contents of the coprolites attributed to them; and

3 probable former gut contents.

Of these lines of evidence, teeth and toothmarks are the most common, whereas gut contents are the most rare. Nevertheless, combined use of these data creates a remarkably complex picture of tyrannosaurid feeding preferences and their relationships to other dinosaurs. For example, Tyrannosaurus teeth were found in the fibula of Hypacrosaurus and a rib of Edmontosaurus, both ornithopods (Chapter 11). Probable tyrannosaurid toothmarks are also documented in bones of a theropod, the dromaeosaurid Saurornitholestes, as well as in bones of the ceratop-sian Triceratops (Chapter 13) and Edmontosaurus (Chapter 11). The toothmarks in Triceratops are on its ilium, so they were most likely not death-dealing marks but rather signs of feeding after the animal was already dead. Furthermore, the tooth-marks show both puncturing and scraping of the bone. This suggests that the tyran-nosaurid bit deeply into the hip region of the ceratopsian, with a calculated bite force of 13,400 N, the greatest known for any animal. The tyrannosaur then pulled meat from the bone, once again indicating a dead and non-struggling food item. However, tyrannosaurid toothmarks on the caudal vertebrae of a specimen of Edmontosaurus show signs of healing after they were inflicted, indicating that a tyran-nosaurid attempted to munch on a live prey. Other specimens of Edmontosaurus have toothmarks attributed to both Tyrannosaurus and Albertosaurus, showing that this herbivore was on the menu for more than one species of tyrannosaurid. Toothmarks from the dromaeosauridid Saurornitholestes also occur in Edmonto-saurus bones. This combination of body and trace fossil evidence permits the beginning sketch of a Late Cretaceous food chain: Edmontosaurus ate land plants, and was in turn eaten by both Saurornitholestes and Tyrannosaurus, but Tyrannosaurus also ate Saurornitholestes. Consequently, Tyrannosaurus was at the top of this food chain.

Coprolites and probable stomach contents augment tooth and toothmark data by showing what entered and exited at least a few of their gastrointestinal tracts. Two probable tyrannosaurid coprolites are interpreted on the basis of:

1 their stratigraphic occurrence in beds containing tyrannosaurid remains, the Upper Cretaceous Dinosaur Park Formation of Alberta, Canada;

2 their large size (44 and 64 cm long); and

3 their contents, as both consist of cylindrical masses of phosphatic rock that contain many small bone fragments.

The 44-cm long specimen contains bone from a probable juvenile hadrosaur, and the larger specimen contains bone from an unidentified juvenile dinosaur. Finely-ground bone in a coprolite could be interpreted as the result of thorough chewing, but tyrannosaurid jaws were not amenable to their teeth repeatedly coming together in this way. An alternative hypothesis is that the bone consists of fragments accidentally scraped off with the flesh as the tyrannosaurid pulled with its anterior teeth. Indeed, the contents of the larger coprolite support gorging behavior rather than chewing, because it includes three-dimensional preservation of muscle tissue. Such preservation indicates a brief time for this food in the gut.

Probable former abdominal contents in another tyrannosaurid species, Daspletosaurus, provide even more information to augment the intriguing implications of the coprolite data. The skeletal specimen of Daspletosaurus, in the Upper Cretaceous Two Medicine Formation of Montana, had the vertebrae and a dentary from a juvenile hadrosaur in what was probably its gut. These hadrosaur bones show evidence of corrosion caused by partial digestion. This suggests that Daspletosaurus had an enzyme-producing proventriculus, the anterior portion of a two-part stomach followed posteriorly by a muscular gizzard. Such an anatomical arrangement is also seen in crocodiles and some birds, which leads to the hypothesis that some theropods shared this trait as a synapomorphy. However, much testing of this hypothesis is required because it is based on such scanty and preliminary results.

With reference to other feeding preferences, Baryonyx and other spinosaurs, such as Suchomimus, have been interpreted as piscivorous (fish-eating) theropods. This conclusion is made on the basis of:

1 fish scales found in association with a Baryonyx skeleton;

2 the crocodile-like skull and numerous small teeth of Suchomimus, well adapted for grabbing and holding fish; and

3 long arms with well-developed hook-like claws on both Baryonyx and Suchomimus.

The combination of many teeth, which are not as robust or lethal-looking as those of some other theropods, along with their unusual arms, led to the hypothesis that spinosaurs preferred a diet of fish derived from shallow aquatic environments. In this sense, these dinosaurs may have been like modern grizzly bears (Ursus arctos), which scoop fish out of streams.

No definitive single piece of evidence shows that a theropod actually killed another dinosaur, although the "fighting dinosaurs" of Velociraptor and Protoceratops persuasively shows the intent to kill in one instance. Nonetheless, the combination of data from functional morphology in theropod body plans and their toothmarks, trackways, and coprolites provide reasonable evidence that many, if not most, theropods were active predators. Lively debates have centered on whether some theropods were primarily predators or scavengers, and no dinosaur has received as much attention in this respect as Tyrannosaurus rex.

Tyrannosaurus is a prime example of a theropod that in some ways resembled a killer, but in other ways did not seem well-adapted to full-time hunting. One of the major objections to viewing Tyrannosaurus as a predator focuses on its ridiculously small arms, which were so short that they could not even reach its mouth. Moreover, these arms ended with two small digits, rather than the robust three-digit hands seen in other theropods. Because of this, some paleontologists suggest that Tyrannosaurus was primarily a scavenger. After all, such arms alone could not have possibly held on to a multi-ton struggling prey animal, and predation without the use of arms would have required a "land shark" approach. In this scenario, the large theropod would have waited for a prey animal to walk by, and then ambushed it by biting it. Of course, how a 13-meter-long, 6-tonne predator would remain unnoticed by a prey animal presents a problem for this scenario.

Modern terrestrial carnivores provide a guide to better understanding this problem. For these animals, part-time or full-time scavenging is a common means of obtaining meat. Indeed, some popularly assumed full-time predators, such as present-day large cats, have been observed chasing smaller predators away from kill sites to consume the corpse. One suggestion for Tyrannosaurus is that it was a part-time hunter that killed smaller animals, such as juveniles, a hypothesis supported by coprolite and toothmark evidence. However, it also may have used its large size to chase away smaller theropods from the scene of a successful hunt, an adaptation that would have been augmented considerably if more than one tyran-nosaur showed up at a kill site together. Whether such Mesozoic bullying was common or not is unknown, but the feeding behavior of modern terrestrial carnivores is complex and often opportunistic. Accordingly, theropods were probably no different, getting what they could by whatever means that worked, regardless of whether their food was alive or recently dead.

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