ranged around 1 - 2 m in length, with them increasing in size over time so that the largest armoured ornithischians were around 7.5 m and 9 tonnes, the largest horned ornithischians were around 8.5 m and 11 tonne, and the largest crested ornithischians were around 15 m and 13.5 tonnes.

Early ornithischians were relatively small dinosaurs, averaging about 1-2 meters in body length, with a triangular skull that had large circular orbits on the sides. This suggests that early ornithischians had relatively huge eyes that faced laterally.