There were more major changes in the cerapods during the Cretaceous period than in the Thyreophora. In particular, the ornithopods became more diversified and the marginocephalians evolved even more over time.

Overall, the Cretaceous period ornithopods still carried the characteristics that made them ornithopods, but they did increase in number and diversify. They were still medium to large plant eaters and, in general, mostly bipedal; they ranged from less than 3 feet (1 meter) tall and six feet (two meters) long to as big as about 23 feet (7 meters) tall and 66 feet (20 meters) long. They further developed their teeth, cheeks, and ability to chew over the millions of years of the Cretaceous.

The animals also seemed to change certain physical characteristics. In particular, the ornithopods could walk, or in some cases trot, on all four feet; at higher speeds, they were probably mostly bipedal. Their feet also started to change, with some evolving into more hoof-like shapes. Others developed hands that could probably grasp vegetation. As they grew in size, some changed their structures to support more weight, including the number of back vertebrae connecting the pelvis to the backbone.

In the Cretaceous period, the ornithopods included such dinosaurs as the hypsilophodonts, heterodontosaurids, iguanodontids, and hadrosaurs. This list includes one of the earliest discovered dinosaurs, the Iguanodon, as well as the famous crested and “duck-billed” hadrosaurs, which were the most diverse and successful groups under the ornithopods.