Theropod trackways not only contribute to hypotheses about their locomotion, but also directly relate to interpretations of their feeding behavior. As mentioned in the previous section, stalking, pack hunting, and smaller theropods running away from a larger carnivore have all been suggested by their tracks. Nevertheless, the starting point of discussion of theropod feeding behavior typically involves describing their teeth.

Most often, theropod teeth are categorized as ziphodont, a plesiomorphic trait in which they curved posteriorly, are serrated, conical (pointed), and have carinae, which were well adapted for slicing through soft animal tissue. Thicker and more robust examples, such as the teeth of tyrannosaurids, such as Albertosaurus, were also capable of punching through bone (Fig. 9.14). The meat-eating interpretation of theropod teeth is also well supported by numerous examples of toothmarked bones where the marks match known theropod teeth (Chapter 14). In addition, skeletal remains in the body cavities of a few theropods may well represent consumed animals. Probable theropod coprolites containing bone fragments, and in one case fossilized muscle tissue, were documented. These numerous lines of evidence thus corroborate hypotheses, derived originally from just teeth, that most theropods were undoubtedly carnivores. Interestingly, few theropods, such as some ornithomimids (Ornithomimus and Struthiomimus) and oviraptorids (Oviraptor and Ingenia), had no teeth or any sign of tooth sockets. Consequently, they must have lost them as an inheritable trait in their preceding evolutionary history. Although Ornithomimosauria is typically thought of as a group of toothless dinosaurs, one ornithomimid species (Pelecanimimus) had about 200 teeth, the most of any thero-pod. As a result, exceptions arise for each generalization about theropods.

One such exception is the hypothesis that some theropods may not have been carnivores. For example, therizinosaurs, such as the Early Cretaceous Alxasaurus in China and Therizinosaurus, Segnosaurus, and Erlikosaurus of the Late Cretaceous in Mongolia, represent mysteries in how they fed and what they ate. An inventory of their meal-gathering tools shows some strange traits for theropods. For example:

1 Alxasaurus, Erlikosaurus, and Segnosaurus had poorly-developed, leaf-shaped teeth;

2 teeth were absent from the anterior part of the premaxilla of Erlikosaurus; and

FIGURE 9.14 The Late Cretaceous tetanuran and tyrannosaurid Albertosaurus, showing its jaws filled with prominent, recurved, and serrated teeth ideally suited for slicing and dicing flesh and bone. Royal Ontario Museum, Toronto, Ontario, Canada.

FIGURE 9.14 The Late Cretaceous tetanuran and tyrannosaurid Albertosaurus, showing its jaws filled with prominent, recurved, and serrated teeth ideally suited for slicing and dicing flesh and bone. Royal Ontario Museum, Toronto, Ontario, Canada.

Royal Ontario Museum Dinosaurs

3 Therizinosaurus had unguals on its manus that measured up to 70 cm long, greater than its forearm length.

Such prodigious unguals, analogous to the fictional character Edward Scissorhands, arguably could have been used for defense. However, they were certainly too unwieldy to be used in any effective way against other large, predatory theropods. Instead, they are interpreted as adaptations for feeding.

These and other therizinosaur traits are the reason why they are often considered the oddest of all theropods. With their relatively long prosauropod-like necks and stout torsos, they clearly were not well-suited for hunting and meat-eating. As a result, some paleontologists have proposed that they were best adapted for browsing on vegetation. In this hypothesis, the claws are interpreted as implements for raking tree branches. Giant ground sloths, herbivores that lived only about 12,000 years ago in North America, had similar adaptations, so this might be an example of convergent evolution. An alternative hypothesis is that such claws were used for eating insects, much like modern anteaters that use their comparable armature to tear apart termite mounds. Trace fossil evidence shows that termites had developed mounds by the Late Jurassic that resembled modern ones in Africa and Australia. Therizinosaurs show up in the geologic record by the end of the Early Cretaceous, so from an evolutionary standpoint it is appropriate that they could be termite-eaters. However, most therizinosaurs were huge in comparison to a typical modern anteater, which means that they would have had to tear apart many termite nests to gain sufficient nutrition for sustenance. Like many problems in evolution, no single solution provides a complete answer to these enigmatic theropods.

Another group of theropods recently suspected of herbivory is the ornithomi-mosaurs. This hypothesis is based on the following criteria:

1 ornithomimosaurs have less robust skulls than similarly-sized theropods;

2 these skulls often lack teeth and instead are assumed to have been beaked;

3 they are more common than undoubted carnivorous theropods in some Late Cretaceous assemblages; and

4 some specimens of the ornithomimosaurs, Sinornithomimus and Shenzhousaurus, have gastroliths in the areas of their abdominal cavities.

A light skull lacking teeth argues for adaptations to foodstuff that was either not fighting back or was softer than most prey animals. Although modern predatory birds do not need teeth to kill or tear into their prey animals, they also have strong skulls, high speeds aided by powered flight that can cause killing impacts, and sharp talons. Ornithomimosaurs had none of these compensating traits for effective predation. Relative abundance can also be a clue to their supposed herbivory as ecological communities tend to have many more herbivores than carnivores. Finally, the documentation of gastroliths in Sinornithomimus and Shenzhousaurus lends more credence to their having been herbivorous, because gastroliths are present largely in other undoubted herbivorous dinosaurs, presumably as an aid to grinding plant material (Chapters 10 and 11).

As far as most flesh-eating theropods were concerned, anatomical data provide clues about their sensory abilities used for discerning and acquiring either prey or corpses. Many theropod lineages, such as allosaurids, troodontids, and tyran-nosaurids, have orbits positioned toward the front of the skull rather than laterally. This suggests that they had developed stereoscopic vision, also known as binocular vision. Prey also could have been detected through sound, and Troodon shows sufficient cavities in its skull for organs that would have allowed for sound location. Olfactory sensations were yet another way for theropods, which could have used a sense of smell to find food, particularly already-dead animals. Recall that

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