Their major evolutionary advantage was the progressive development of a chewing apparatus that became the most sophisticated ever developed by a reptile. They had beaks, multiple teeth rows, cheek pouches and true chewing. Efficient chewing means faster digestion, as the material is broken into smaller pieces.
Ornithopoda means "bird feet", from the Greek. It refers to their three-toed feet (though many early forms had four toes). They had no armour, a horny beak, and various other features. Some ornithopods and cerapods had thin cartilaginous plates along the outside of the ribs. In some cases, these plates mineralized and so were fossilized. The function of these plates is unknown.