Yaverlandia, from the Early Cretaceous of England, is the oldest known pachycephalosaur. But the only known specimen is a small, thick, skull-cap with two bony lumps that shows little about what the animal may have looked like or its relationships. The dome-headed dinosaurs were scarce, small to medium-size animals of the Cretaceous Period. Most lived in the northern hemisphere but one genus, Majungatholus, is known from the Late Cretaceous in Madagascar.
The back of the head of dome-headed dinosaurs was broadened into a shelf that often had bony lumps or short spikes. One family, the Homalocephalidae, had bones on the top of the skull that were flat and thick (as in Homalocephale and Goyocephale from Mongolia).
In the other family, the Pachycephalosauridae, the bones were raised into a very thick, high dome that was the main feature of the animal's appearance and even grew over the shelf (as in Stegoceras). Pachycephalosaur skeletons are rare, but their skull-domes, since they were solid bone, often were fossilized. They are quite common in some places.
Pachycephalosaurs had broad, chubby bodies. They were bipedal plant-eaters. As an animal grew, its dome got larger. The thick domes were used to compete for mates in head-butting contests between males or to fight predators by butting them in the side.
The Ceratopsia were the horned dinosaurs and their relatives. They were different from the pachycephalosaurs-and all other dinosaurs-because they had a special bone, the rostral, which formed part of a large, parrotlike beak.
Family: Psittacosauridae: The oldest and most primitive ceratopsians belong in this family of small, bipedal runners such as Psittacosaurus from the Early Cretaceous of China and Mongolia.
Family: Protoceratopsidae: In this family, the back of the skull was expanded into a wide frill over the back of the neck. Microceratops was a bipedal animal, like its possible psittacosaurid ancestors. The other protoceratopsids walked on all fours, making their larger heads easier to support.
Family: Ceratopsidae: The Ceratopsidae had the shortest range of any dinosaur family. They arose during the Late Cretaceous in western North America. They quickly evolved into many unusual forms and lived until the end of the Mesozoic Era. From cow to elephant size, the quadrupedal (four-legged) ceratopsids had horns and frills on their heads.
They had powerful jaws with hundreds of teeth for slicing tough plants. Triceratops had the most powerful jaw muscles of any land animal. The horns were used as weapons. The frills may have protected the neck and may have been brightly colored for mating season. The frills may also have helped keep its body temperature even.