One of the most dramatic of these evolutionary changes occurred in body size. From their small ancestors, some dinosaurs reached sizes exceeding 35 meters in length and 50 tons in weight. In fact, most dinosaurs were relatively large—the average size of a Mesozoic dinosaur was about 100 kilograms, quite large compared with the average size of a Cenozoic mammal (about two to five kilograms). The earliest dinosaurs were among the smallest. Aside from birds, dinosaurs rarely evolved to small sizes. Instead, they appear to have followed what is often called “Cope’s Rule,” the maxim that most animals tend to get larger over time. Not only did some dinosaurs reach immense sizes, but nearly every group of dinosaurs got larger over time. The earliest ceratopsians, pachycephalosaurs, ornithopods, and thyreophorans were all small compared with their descendants. The one significant exception occurred in predatory dinosaurs. Although some theropods were quite large (reaching five tons or so), many evolved to become very small, culminating in the lineage leading to the first birds.

Although the first dinosaurs were carnivores, two groups (sauropodomorphs and ornithischians) developed the ability to process plant material. The earliest dinosaurian herbivores eventually produced a diverse array of descendants, some with well-developed abilities to consume vegetation. In early ornithopods, for example, the individual teeth show wear facets that indicate they were being used to grind food in the mouth, probably aided by thin cheeks. Later ornithopod species show the development of more and more teeth, packed so tightly that they formed a single large surface for cutting and grinding food. Similar “dental batteries” are also found in ceratopsians, although like ornithopods their earliest species had much simpler chewing systems.