Pachycephalosauria

Pachycephalosauria is a clade of ornithischian dinosaurs.

Pachycephalosaurs Temporal range: Upper Cretaceous 92–66 mya
Pachycephalosaurus, a type of pachycephalosaur
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Order:	†Ornithischia
Suborder:	†Pachycephalosauria
Family:	†Pachycephalosauridae

Skeletal mount of Pachycephalosaurus

Overview

Along with Ceratopsia, it makes up the clade Marginocephalia.

With the exception of two species, most pachycephalosaurs lived during the Late Cretaceous Period, dating between about 85.8 and 65.5 million years ago.

They are exclusive to the Northern Hemisphere, all of them being found in North America and Asia.They were all bipedal, herbivorous/omnivorous animals with thick skulls.

Skulls can be domed, flat, or wedge-shaped depending on the species, and are all heavily ossified.The domes were often surrounded by nodes and/or spikes.

Partial skeletons have been found of several pachycephalosaur species, but to date no complete skeletons have been discovered.

Often isolated skull fragments are the only bones that are found.

Fossil works

In some species, the skull roof is domed and several inches thick; in others it is flat or wedge-shaped.The flat-headed pachycephalosaurs are usually regarded as distinct species or even families, but they may be juveniles of dome-headed adults.^[1][2] The domes were often surrounded by nodes and/or spikes.

There is evidence of head-butting in pachycephalosaurs. Of 100 domes studied, 20 percent showed signs of healed injuries. The pathologies included pitting where the bone had become infected from wounds that originated in the skin. These findings suggest that pachycephalosaurids may have been using their heads for both display and defence as many modern animals do.^[3][4]

Competition for mates is the usual reason why adult males fight. Competition for territory is another possibility. There are many examples of both in mammalian fighting. And the head could be used as defence against predators.

Incidentally, these thick head shields are called the parietosquamosal shelf.

Genera

Family Pachycephalosauridae

• Acrotholus
• Amtocephale
• Dracorex
• Gravitholus
• Micropachycephalosaurus
• Pachycephalosaurus
• Prenocephale
• Sphaerotholus
• Stegoceras
• Stygimoloch
• Tylocephale
• Wannanosaurus

Tribe Pachycephalosaurini

• Pachycephalosaurus