Coelurosaurs were one of the strangest groups of dinosaurs. In addition to the famous predators Tyrannosaurus and Velociraptor, the coelurosaurs included the small, fuzzy Sinosauropteryx; "ostrich-mimics" such as Struthiomimus; the long-necked, sickle-clawed giant Therizinosaurus; the tiny, ant-eating Albertonykus; the bird-beaked oviraptorosaurs like Citipati; and birds. Within the past decade, especially, new discoveries have radically changed our understanding of this group of dinosaurs. Now a study shows that, even though this group contained some of the most famous predators of all time, many of these dinosaurs were herbivorous.
Traditionally, dinosaur diets seemed to break down along neat evolutionary lines. The long-necked sauropods and all the ornithischian dinosaurs (ankylosaurs, ceratopsians, hadrosaurs, etc.) were herbivores, whereas all the theropods were carnivorous. This is no longer the case. Coelurosaurs were theropods, and in a review of their evolution just published in PNAS by Lindsay Zanno and Peter Makovicky, the Field Museum scientists found that relatively few coelurosaurs had an exclusively carnivorous diet.
Zanno and Makovicky determined the different dietary habits of coelurosaurs by looking for gut contents, fossil feces and other evidence that would indicate whether a particular dinosaur was a strict carnivore or a herbivore. (They are just labels that are useful for categorization, of course. Alligators sometimes eat fruit, and cows sometimes eat other animals, and so even a primarily carnivorous dinosaur could have eaten plants sometimes and primarily herbivorous dinosaurs may have eaten meat on occasion.) These pieces of evidence, paired with what the authors called "putatively herbivorous traits" in the skeleton, allowed them to more rigorously test ideas about which coelurosaurs might have been herbivores. The ornithomimosaur Sinornithomimus, for example, had toothless, beaked jaws, and specimens have been found with evidence of a gastric mill (small stones in the stomach which would have ground up food), confirming that it ate a significant amount of plant food.
Zanno and Makovicky concluded that there is good evidence for herbivory in 44 known coelurosaur species spanning six groups: the ornithomimosaurs, therizinosaurs, oviraptorosaurs, alvarezauroids, several early birds, and the single troodontid Jinfengopteryx. (The carnivorous dinosaurs consisted of the compsognathids, the tyrannosaurs and most of the dromaeosaurs.) In other words, the coelurosaurs appear to have been "dietary opportunists" in which multiple lineages shifted to herbivorous diets or had more varied diets than the tyrannosaurs and raptors. No two lineages made the shift to herbivory in exactly the same way. Even though many herbivorous coelurosaurs shared similar traits, such as toothless beaks and long necks, these traits evolved independently and in different orders, and so this convergence might indicate some evolutionary constraints which shaped the herbivorous coelurosaurs in similar ways.
Viewed as a whole, the coelurosaurs were a diverse group of dinosaurs that also had an array of diets. In fact, most coelurosaur subgroups show adaptations to eating plant food, meaning that, contrary to what we might suppose, the hypercarnivorous species are actually the oddballs among the group. Further study will be required to better resolve the diets of individual species, but for now it is apparent that the coelurosaurs were the most varied group of dinosaurs to have ever evolved.
Yoshitsugu Kobayashi and Jun-Chang Lü (2003). A new ornithomimid dinosaur with gregarious habits from the Late Cretaceous of China Acta Palaeontologica Polonica, 48 (2), 235-259
Zanno, L., & Makovicky, P. (2010). Herbivorous ecomorphology and specialization patterns in theropod dinosaur evolution Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1011924108