The pachycephalosaurus or "thick headed lizard" lived in the Cretaceous period approximately 70 million years ago. It measured between 4 and 5 m in length and weighed almost 2 tonnes. Its main characteristic was without a doubt the large hump in the form of a dome on its head. This was made of very thick and extremely solid bone and constituted a sort of "helmet", the role of which is uncertain. We can only suppose that the males fought with one another by striking with their heads. These dinosaurs had small curved teeth at the front of the mouth and also had a beak.