The earliest ornithischian dinosaur was Pisanosaurus, a three-foot-long, two-legged (bipedal) plant-eater from the late Middle Triassic of Argentina. All ornithischians were plant-eaters.
Later ornithischians split into three advanced groups: heavy, armored plant-eaters that walked on all fours; specialized dome-headed dinosaurs and horned dinosaurs; and two-legged plant-eaters that included Iguanodon and duckbilled dinosaurs.
Family: Fabrosauridae: This family is found in Late Triassic through Late Jurassic rocks on several continents. Other primitive ornithischians are usually classified in this family. It was named after Fabrosaurus, from the Early Jurassic of South Africa. All fabrosaurids were small bipedal plant-eaters. The best-known fabrosaurid is Lesothosaurus, also from the Early Jurassic of southern Africa. The pelvis of Lesothosaurus shows some features that put it at the bottom of ornithischians. Scutellosaurus from the Early Jurassic of western North America was protected by small bony plates resembling the larger plates of later armored ornithischians.
Stegosaurs were the main armored dinosaurs of the Jurassic Period; ankylosaurs remained in the background. But ankylosaurs replaced stegosaurs in the Cretaceous Period. Scelidosaurus was very much like both groups. It was from the Early Jurassic of England and it walked on all four limbs (quadrupedal). It had stegosaurlike teeth, an ankylosaurlike pattern of armor plates and spines, and a pelvis like Scutellosaurus.
Family: Stegosauridae: These armored dinosaurs probably evolved in China during the Early Jurassic. By the Late Jurassic, they were in Europe (Dacentrurus), North America (Stegosaurus), and Africa (Kentrosaurus).
The armor was a double row of large bony plates that ran along the back from behind the head to the tail. Sharp spines on the end of the tail were used as a weapon against predators.
During the Early Cretaceous, ankylosaurs replaced stegosaurs everywhere except in India. Ankylosaurs were different from stegosaurs because they had flexible body armor rather than a double row of tall plates. They were also closer to the ground, with only a slight arch, if any, to their backs. The heads of stegosaurs were long and narrow, but ankylosaurs had short, board skulls. Instead of tail spines, ankylosaurs had shoulder spines or tail clubs for defense.
Family: Nodosauridae: The more primitive ankylosaurians, including all Jurassic and southern-hemisphere genera, belong in this family. Some nodosaurids had large, cone-shaped spines along the neck and shoulders for protection.
Family: Ankylosauridae: This family may have arisen during the Early Cretaceous from a nodosaurid ancestor. Ankylosaurid skulls had horns projecting from the back, giving them a triangular shape when viewed from above. All ankylosaurids had massive, bony tail clubs for defense.