Adapted from Dinosaurs: The Very Latest Information and Hands-On Activities From the Museum of the Rockies, by Liza Charlesworth and Bonnie Sachatello-Sawyer. A Scholastic Professional Book.
Although some fans of carnivorous "Tyrannosaurs rex" and "Velociraptor" may find it a bit disappointing — the vast majority of dinosaurs were plant-eaters. Most plant-eating dinosaurs had peg-like or broad, flat teeth designed for snipping or stripping vegetation. While the diet of individual herbivores varied, it likely included a combination of leaves, twigs, and seeds — found in high treetops or close to the ground. Some plant-eaters, like "Apatosaurus," probably swallowed stones, which settled in their gizzards, helping to grind up the fibrous plant matter they consumed. These stomach stones, or "gastroliths," are sometimes found among dinosaur bones at dig sites.
Then there were the carnivores. These dinosaurs' teeth — long, sharp, and serrated — were designed for tearing through tough meat. The eating habits of individual species no doubt varies, but probably included fellow dinosaurs, lizards, insects, and early mammals. Were the carnivores as fierce as they have been portrayed in the past? Yes and no. Paleontologists theorize that fast-running meat-eaters, such as "Velociraptor," attacked and killed dinosaurs, then ate them. While others, like the infamous "Tyrannosaurus rex," were likely scavengers, feeding on animals that had been killed by other dinosaurs or had died from natural causes.
Did any of the dinosaurs eat both plants and meat? Probably. Fossils show that certain species had different kinds of teeth — some for grinding and others for tearing — which suggests that they may well have feasted on both types of food. That would make these dinosaurs "omnivorous," like us!