Two of the earliest known ornithischian dinosaurs were Lesothosaurus and Heterodontosaurus, both of whose remains were discovered in Late Triassic to Early Jurassic rock of the Stormberg Group in southern Africa. The Lesothosaurus was a small, slender biped about three feet (one meter) long, with long hind legs, delicate, short arms and a five-fingered hand. Although this dinosaur had the characteristic ornithischian hip structure, it lacked a deeply inset tooth row, meaning its cheeks were not well developed. Therefore, paleontologists suggest that the Lesothosaurus was the most primitive known ornithischian dinosaur; it is often associated with the Genasauria division, Neornithischia.

The Heterodontosaurus, another dinosaur about three feet (one meter) long, differed from Lesothosaurus in the skull and hands. Its skull had a deeply inset tooth row, the tooth pattern was more complicated, and it had thicker outside enamel on the upper and inside of the lower teeth. The Heterodontosaurus was more advanced than Lesothosaurus, exhibiting characteristics some shared by later ornithischian dinosaurs.