FIGURE 9.1 Cladogram for Theropoda and Herrerasauridae, showing relationships of major clades and outgroups. Note that an alternative cladogram would have herrerasaurids as an outgroup sharing a common archosaur ancestor with dinosaurs, but outside of the Dinosauria.

adaptation to increased mobility but later aided in the development of flight in some theropod lineages. Related to this lightening of limb bones is another common theropod characteristic, pneumatic (air-filled) bones, which are found in the skull, vertebrae, and costae. Air-filled spaces in the theropod skeleton allowed room for soft-tissue air sacs, which were filled from a theropod's lungs. Air sacs in the skull functioned as lightweight support without adding extra bony tissue. This adaptation put less strain on the cervical vertebrae, which allowed some theropods to develop extremely large skulls in comparison to the rest of their bodies. Examples of such theropods include allosaurids and tyrannosaurids. (Contrast this head/ body ratio to that of sauropods: Chapter 12.) Although pneumatization is not restricted to theropods, they expressed it more than any other dinosaur clade.

Large, recurved, and serrated teeth, designed for cutting through and consuming flesh, constitute a common anatomical attribute of theropods and probably represent an ancestral condition. However, they are not found in all theropods, and some species, such as Oviraptor and Struthiomimus of the Late Cretaceous, actually lack teeth. These theropods may have had beaks or other anatomical traits covering the bones that did not fossilize. They also may have been omnivorous, herbivorous, or insectivorous, but compelling evidence from stomach contents or coprolites favoring either mode is still lacking. Gastroliths, which normally are found with herbivores (Chapter 14), occur in a few theropods. These trace fossils and oddly-shaped teeth in a few theropods have led to rethinking about the previously definitive statement that all theropods were meat-eaters. Nevertheless, all dinosaur toothmarks

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