Pachycephalosaurs were among some of the last Dinosaur groups to appear. They lived 75-65 mya.
They are named after one of the best-known members of the group, Pachycephalosaurus.
Pachycephalosaurus means "thick-headed reptile," due to the domed and hugely thickened bone on the hàng đầu, đầu trang of its skull--like a cyclist's crash helmet.
About 15 feet long from nose to tail, Pachycephalosaurus lived in Montana, South Dakota, and Wyoming.
Stegoceras, which also lived in North America, was about 8 feet long, with a body the size of a goat.
Homalocephale was about 10 feet long and had a flatter skull. It lived in East Asia.
Pachycephalosaurs may have defended themselves bởi loweringtheir heads and charging at their enemies.
At breeding time, the males may have engaged in head-butting contests, as some con cừu, cừu and goats do today.