Dinosaur diets and feeding behavior varied widely throughout the clade, including carnivorous, herbivorous, and omnivorous forms. Ornithopoda Hadrosauridae While studying the chewing methods of hadrosaurids in 2009, the paleontologists Vincent Williams, Paul Barrett, and Mark Purnell found that hadrosaurs likely grazed on horsetails and vegetation close to the ground, rather than browsing higher-growing leaves and twigs. This conclusion was based upon the evenness of scratches on hadrosaur teeth, which suggested the hadrosaur used the same series of jaw motions over and over again. As a result, the study determined that the hadrosaur diet probably consisted of leaves, and lacked the bulkier items such as twigs or stems, which might have required a different chewing method and created different wear patterns. However, Purnell said these conclusions were less secure than the more conclusive evidence regarding the motion of teeth while chewing. The hypothesis that hadrosaurs were likely grazers rather than browsers appears to contradict previous findings from preserved stomach contents found in the fossilized guts in previous hadrosaur studies. The most recent such study before the publication of the Purnell study was conducted in 2008, when a team led by University of Colorado at Boulder graduate student Justin S. Tweet found a homogeneous accumulation of millimeter-scale leaf fragments in the gut region of a well-preserved partially grown Brachylophosaurus. As a result of that finding, Tweet concluded in September 2008 that the animal was likely a browser, not a grazer. In response to such findings, Purnell said preserved stomach contents are questionable because they do not necessarily represent the usual diet of the animal. The issue remains a subject of debate. Coprolites (fossilized droppings) of some Late Cretaceous hadrosaurs show that the animals sometimes deliberately ate rotting wood. Wood itself is not nutritious, but decomposing wood would have contained fungi, decomposed wood material and detritus-eating invertebrates, all of which would have been nutritious. Theropoda Tyrannosaur tooth marks are the most commonly preserved feeding traces of carnivorous dinosaurs. It is usually not possible to identify tooth marks on bone made by small predatory dinosaurs due to similarities in the denticles on their teeth. However, there are exceptions, like an ornithomimid caudal vertebra that has tooth drag marks attributed to Saurornitholestes and a partial Troodon skeleton with preserved puncture marks. Small bones of small theropods that were preyed upon by larger ones may have been swallowed whole and digested frequently enough to affect their