PACHYCEPHALOSAURIA

INTRODUCTION

     Pachycephalosauria are members of the Marginocephalia, along with the Ceratopsia (the horned dinosaurs).  All members of the Marginocephalia have a frill projecting from the back of the skull (an extension of the parietal and squamisal bones of the skull).  In the Pachycephalosauria, this frill is very small as compared to the Ceratopsisa.  The Pachycephalosauria were bipedal, small to fairly large (1.5 to 4.5 meters) marginocephalians with greatly thickened bones of the skull roof (thickening of the frontal and parietal bones of the skull).  They lived primarily during Late Cretaceous time, but two Late Jurassic genera are known from China.  Dinosaur paleontologists speculate that the "males" had larger, more thickened bony domes that were used for butting (of other "males") during mating rituals.  They may have lived in upland areas in small herds and occupied a niche similar to mountain sheep and goats.

Royal Tyrrell Museum Tour: Pachycephalosaurs (From: http://tyrrell.magtech.ab.ca/tour/pachyces.html)
 

GENERAL CHARACTERISTICS OF THE PACHYCEPHALOSAURIA

• 1) Extremely thickened frontal and parietal bones in the roof of the skull (but brain no larger).
• 2) Hindlimbs much longer than forelimbs (indicating probable bipedal stance).
• 3) Ossified tendons in the tail region and relatively long tail.
• 4) Six to eight sacral vertebrae.
• 5) Expansion of the back of the skull and expansion of the parietal and squamosal bones into a shelf at the back of the skull.
• 6) Head set at an angle to the vertebral column.
• 7) Tongue and groove articulation of the the vertebrae to stablize the back (to possibly take the stress of butting activities).
• 8) Greatly reduced pubic bone (prepubic process much larger than the pubic bone).
• 9) bony spines and knobs on the external surfaces of the skull
• 10) development of osteoderms on the rim of the skull roof
• 11) Small leaf-shaped teeth (except for front peg-like teeth in some and small canines in some) with triangular crowns and coarse serrations on the front and back margins.

     Most dinosaur paleontologists divide the Pachycephalosauria into two monophyletic clades (families), the Homalocephalidae and the Pachycephalosauridae.  However, Paul Sereno, University of Chicago, disagrees and has pretty much disbanded the Homalocephalidae (see the discussion in Fastovsky and Weishampel).  In this course, we will consider there to be two types of pachycephalosaurians, the Homocephalidae (flat headed pachycephalosaurians) and the Pachycephalosauridae (the dome headed pachycephalosaurians).

Characteristics of the Homalocephalidae

• 1) Flattened top of skull.
• 2) Frontal and parietal bones separated by a suture.
• 3) Fairly even thickness of top of skull from one side to another.
• 4) Skull pitted dorsally.
• 5) Short canine teeth in both upper and lower jaws.
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• A-D, Homalocephale calathocercos; skull in lateral (A), dorsal (B), palatal (C) and occipital (D) views (after Maryanska and Osmolska 1974) (from Maryanska, 1990, p. 567; in The Dinosauria; Weishampel, Dodson, and Osmolska, eds.; University of California Press; Berkeley, Los Angeles, and London; 731 p.) (Scale is 9 cm)

• 1) Dome-like thickening of the top of the skull (extreme thickening of the frontal and parietal bones into a dorsally convex dome).
• 2) Parietal and frontal bones are fused to form the dome.
• 3) The upper temporal fenestra (supratemporal fenestra) is closed over.
• 4) Skull smooth dorsally.
• 
• Thickened skull dome of Stegoceras (From Lucas, 1997).

Pachycephalosaur Biogeography

     Pachycephalosaurs are primarily known from deposits of Laurasia (except for one genus which has been found in Madagascar).  The more primitive forms are primarily known from Asia (Mongolia and China) and probably first arose there.  Remember by late Jurassic time, Laurasia and Gondwanaland were significantly separated, particularly North and South America.

                      Distribution of Pachycephalosauria remains (From Lucas, 1997)

Pachycephalosaur Paleoecology and Paleobiology

• 1) In North America, probably lived in upland regions in small herds, much like mountain goats and sheep (most of the material found in North America is water worn skull cap, although Stegoceras is known from a well preserved skull and partial postcranial skeleton and Pachycephalosaurus is known from an almost complete skull).
• 
• Reconstructed skeleton of Stegoceras (From Lucas, 1997)
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• 2) In Mongolia and China, better preservation.  Climate was more arid there and the desert conditions and stream environments must have been more favorable for preservation.
• 3) Broad rib cage and enlargement of the digestive towards the rear of the animal.  All this suggesting a large vat-like digestive system for the chemical processing of food (their dental hardware was not as good for masticating food as with other ornithischians).
• 4) Heads used as battering rams (based on studies of functional morphology and stress that would be present in the head and the postcranial skeleton - see Fastovsky and Weishampel).  May have practiced head-on butting, but more lilely body butting and head pushing.
• 5) Stegoceras skull domes found in Upper Cretaceous rocks of Canada (where they are fairly common), suggest sexual dimorphism.  Perhaps the adult males had a larger and more convexly domed skull than did the adult females.  Of course, the purpose would be to impress the female and to drive off competing males.  Thus, there would have been sexual selection for males with prominent domes, knobs, and spikes on the head.