Psittacosaurus (Cretaceous)

Psittacosaurus is a herbivorous dinosaur of Ceratopsia group from the Early Ceratopsia. Its fossils were first discovered by a group of archaeologists from the American Museum of Natural History in 1922, in Mongolia. The Psittacosaurus’ skull was shaped like a parrot's beak.
Mature Psittacosaurus was 1.5 metres long and weighed up to 100 kilograms. It had small forelimbs with tiny digits and strong hind limbs for locomotion. All limbs had four digits. Psittacosaurus’ skull was characterized by jaws without incisors, a robust beak and bony crest at the top where the jaw muscles were attached. Later this decoration evolved into the bony neck frill and armour for later ceratopsians.
Like other herbivores, Psittacosaurs were used to swallowing small stones to help with digesting tough fibrous plants. . Unlike other ancient reptiles, its jaws did not move up and down. It ground the food like millstones - the lower jaw slid forward and backward on the joint.