Pachycephalosaurs probably first appeared during the the Jurassic period (when they originated remains a subject of research and debate), and survived until the end of the Cretaceous period. Some Pachycephalosaurs would have been among the last non-avian dinosaurs which died out during the mass extinction which took place about 65 million years ago.
All Pachycephalosaurs were herbivores (plant-eaters) and were bipedal (walked on two legs).
The feature that Pachycephalosaurs are best known for, and the defining characteristic of the group is the bony roofs of their skulls. All Pachycephalosaurs had skulls like this, although the exact size and shape did vary between species. In some species of Pachycephalosaurs, the skull has a bony dome several inches thick, in some the skull is flat or wedge-shaped, and in several species their are bony knobs/horns on the skull.
The purpose of Pachycephalosaurs' skull adaptations have been a subject of much debate - it has in the past been asserted that animals must have engaged in head-on butting contests for mates (like thick-headed sheep and other animals do today), but the structure of the neck (which could not have been orientated in a straight line would have made it poor for transmitting force) and the shape of the domes (rounded domes would have resulted in glancing blows which transmitted force poorly) seems to refute this hypothesis. If not used for head-on butting, the bony domes could perhaps have been used for flank butting - the structure of the horns in some species of Pachycephalosaurs seems to be consistent with this hypothesis.
Pachycephalosaurs first appeared during the Jurassic, perhaps about 150 million years ago, and survived until the end of the Cretaceous period, about 65 million years ago
Types of Pachycephalosaurs
Here is a list of some Pachycephalosaurs:
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