Pachycephalosauria

Pachycephalosauria is a clade of ornithischian dinosaurs.

 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.Sullivan R.M. 2006. A taxonomic review of the Pachycephalosauridae (Dinosauria: Ornithischia). New Mexico Museum of Natural History and Science Bulletin 30 (35): 347–365.Longrich N.R; Sankey J. & Tanke D. 2010. Texacephale langstoni, a new genus of pachycephalosaurid (Dinosauria: Ornithischia) from the upper Campanian Aguja Formation, southern Texas, USA. Cretaceous Research 31 (2): 274. 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.Maryańska, Teresa; Chapman, Ralph E. & Weishampel, David B. 2004. Pachycephalosauria. In Weishampel, David B; Dodson, Peter & Osmólska, Halszka (eds) The Dinosauria. 2nd ed, Berkeley: University of California Press. pp. 464–477. .NBC News: "Boneheaded dinosaurs butted heads"

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

 References