Scientists have made speculations and deductions about omnivorous dinosaurs, which ate both plants and animals, but there is no undisputed evidence. Some dinosaur fossils exhibited birdlike beaks, suggesting they may have evolved such structures to consume both animal and plant products. Omnivorous diets may have occurred as a transition from a carnivorous, or meat-based, diet to an herbivorous, or plant-based, one. Scientists can only guess, based on comparisons of teeth, jaw and limb structures to those of living animals, whether a dinosaur ate meat, plants or both.
Fossils of Heterodontosaurus are rare, but paleontologists theorise that it was a small dinosaur not much bigger than a turkey. Given a name meaning "lizard with different teeth," Heterodontosaurus had several kinds of teeth in its jaw, including pointed canines for puncturing and robust back teeth for grinding. The variety of teeth types implies that the dinosaur had a varied diet, possibly consisting of both vegetation and meat sources. Because of its ability to replace teeth only once in its lifetime and the assortment of teeth, Heterodontosaurus seem more closely related to mammals than reptiles.
Gallimimus resembled birds in appearance, but at the end of the Gallimimus' fingers protruded a set of long claws. The dinosaur used these to procure fruit from trees or capture prey. Larger meat-eating dinosaurs such as Tyrannosaurus and Tarbosaurus lived at the same time, about 70 million years ago during the Cretaceous Period, and would not have hesitated to place Gallimimus on their menus.
Ornithomimus looked more menacing than it actually was. Although a relatively large dinosaur, ornithomimus could outrun its enemies, reaching 65 miles per hour on its powerful hind legs. Its mouth resembled a bird's toothless beak and Ornithomimus, like Gallimimus, appeared very much like a large ostrich at a distance. Because of the absence of teeth, scientists hypothesise that Ornithomimus ate a variety of vegetation and small animals.
After locating a fossil of an Oviraptor, archaeologists were surprised to observe that the remains were hunched over a nest of eggs, presumably in a predatory fashion. Only years later, after analysing the contents of the eggs and finding an Oviraptor embryo inside, did scientists realise that the Oviraptor was guarding its eggs when it died. The Oviraptor's jaw houses no teeth, but was able to clamp down with immense force using its powerful jaw muscles. This jaw would have been inefficient at getting at the interior of an egg, but ideal for chomping on vegetation and crushing the shells of clams.
The Therizinosaurs possessed fearsome claws on its forelimbs, in addition to a beaklike jaw absent of teeth. Scientists theorise that these claws were used for ripping apart insect nests and not to slash at predators or prey. The Therizinosaur could stretch its neck, similar to a giraffe, to pull off the leaves of trees or cut off foliage with their claws. These dinosaurs, called "scythe lizards," were relatives of the Oviraptors.
Although the teeth of Troodons have the saw-like quality typical of meat eaters, Troodon dentition most resembled that of the Therizinosaur and Ornithomimus. The ragged edges of the Troodon tooth were coarser than those of typical carnivores and indicated a closer relationship to herbivores. Thomas Holtz, a geologist from the University of Maryland, and his colleagues suggest that these dental characteristics may have evolved to accommodate a diet that included eggs, worms, insects and plants. Troodon's claws and the proximity of some of its teeth to nests of eggs also pointed to a partly carnivorous diet.