Psittacosaurus measured about 2 metres (6.5 feet) long and was probably bipedal most of the time. The skull was high and narrow and is characterized by a small bone (rostral) that forms the upper beak. The anterior region of the skull was shaped very much like a parrot's (parrot) beak in that the upper jaw curved over the lower, hence the dinosaur's name (psittac being derived from the Latin term for parrot). Apart from these unusual features, Psittacosaurus appears likely to have evolved from bipedal ornithopod dinosaurs sometime in the Late Jurassic (Jurassic Period) (154 million to 144 million years ago) or very Early Cretaceous Period (144 million to 99 million years ago).