What are the distinctive features of ornithopods?

Short Answer:

What are the distinctive features of ornithopods?

To what feature(s) might you attribute the success of ornithopods?

How did hadrosaurids resemble and differ from primitive ornithopods?

What features of the skeletons of different groups of ornithopods identify them as either bipeds or facultative bipeds?

What defensive strategies did ornithopods employ?

How do the jaw, tooth, and skull structures of the various ornithopods differ from each other?

Why do paleontologists think the crests and tubes of lambeosaurines functioned as signaling devices?