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Stalking of prey, interpreted from a trackway in east Texas made by a large Early Cretaceous theropod (probably Acrocanthosaurus), possibly following the tracks of a large sauropod.

An Early Cretaceous stampede in Queensland, Australia of a group of small theropods and ornithopods, most running in the opposite direction from a much larger therapod.

FIGURE 9.13 Large theropod trackway from the Upper Jurassic Morrison Formation of Colorado. (A) Overview of trackway showing pace and stride lengths; dinosaur ichnologist for scale. (B) Close-up of one track in sequence with pressure-release structure evident in left upper (outer) edge of track, indicating a pushing of the sediment by the theropod as it shifted from the left to the right foot.

FIGURE 9.13 Large theropod trackway from the Upper Jurassic Morrison Formation of Colorado. (A) Overview of trackway showing pace and stride lengths; dinosaur ichnologist for scale. (B) Close-up of one track in sequence with pressure-release structure evident in left upper (outer) edge of track, indicating a pushing of the sediment by the theropod as it shifted from the left to the right foot.

Apparent pack hunting, where tracks left by a herd of Late Jurassic sauropods were followed by tracks of a group of large theropods. A similar pack-like configuration of large theropods is inferred from multiple and equally-spaced trackways on the same bedding plane from the Early Cretaceous of Mongolia.

Early Jurassic theropods in the western and eastern USA (Utah and Massachusetts, respectively) that stopped to sit down, leaving metatarsal and posterior body impressions.

Limping, presumably from a limb injury, which might be expected for an animal subjecting itself to risky behavior, such as running too fast and tripping.

Probably the single most important insight gained from theropod trackway information is that theropods were apparently the most active of all dinosaurs. Despite their relative scarcity as body fossils in comparison to other dinosaurs, theropod tracks are the most abundant of all dinosaur tracks. In most places where dinosaur tracks are found, they outnumber the tracks of all other dinosaur clades combined. For example, Middle Jurassic shoreline deposits in northwestern Wyoming show thousands of theropod tracks, but not one track attributable to an ornithopod, sauro-pod or thyreophoran. The high activity level and mobility indicated by this wealth of data, along with their paleobiogeographic distribution, has been used to infer that theropods were physiologically different enough from other dinosaurs in that they were endothermic (Chapter 8). This is a requirement in modern terrestrial animals that stay active for long periods of time. In contrast, few modern ectother-mic terrestrial animals are active on a regular basis and must spend large amounts of time soaking up sunlight. The physiological considerations of theropods, including whether they were endothermic, ectothermic, or perhaps a combination of the two in various stages of their lives, is also a subject pertinent to theropod-bird interconnections (Chapter 15).

Interestingly, recent discoveries of small, feathered theropods from the Early Cretaceous of China suggest that not all theropod locomotion was on the ground. At least two species show adaptations for tree climbing and two others were capable of either gliding or powered flight. Epidendrosaurus ninchengensis and Scanso-riopteryx heilmanni are actually similar enough that they may represent one species, and both show the following features indicative of an arboreal lifestyle:

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