Ornithischia
Ornithischia, as the name indicates, was coined for the birdlike pelvic girdle, although they are not the ancestors of birds.
The ornithischian skull and dentition was modified very early by a herbivorous diet. Lesothosaurus separated early, but the skull of Lesothosaurus already shows such adaptations, with broad proportions, a less flexible upper jaw, and a more mobile connection for the lower jaw.
The major clades were already established by the early Jurassic. The ornithischians divided into armoured thyreophorans and unarmoured ornithopods and marginocephalians.
Read more about this topic: Evolution Of Dinosaurs