Palaeos		Ornithischia
Vertebrates		Pachycephalosauria

Ornithischia: Pachycephalosauria

DINOSAUROMORPHA
|--+--THEROPODA
|  `--SAUROPODOMORPHA
|
ORNITHISCHIA
|--Thyreophora
|  |--Stegosauria
|  `--Ankylosauria
`--Cerapoda
   |--Ornithopoda
   |  |==Hypsilophodontidae
   |  `--Hadrosauridae
   `--Marginocephalia 
      |--Pachycephalosauria 
      |  |--Homalocephalidae 
      |  `--Pachycephalosauridae 
      `--Ceratopsia 

1. Pachycephalosauria X
2. Pachycephalosauridae X

This page is being revised

Pachycephalosauria

Dome headed dinosaurs, superficially resemble the ornithopods; bipedal, small to medium-sized, with a bony "battering ram" head.  It has frequently been supposed that this peculiar skull feature was used in fights over territory or mates.  However, the assumption has been strongly questioned based on the relative weakness of the neck.  

These creatures walked on their hind legs and looked much like Ornithopd dinosaurs (with which they were originally clasisfied).  It is now known that they are much more closely related to horned dinosaurs (Ceratopsians).  Pachyephalosaurs (literally "thick headed lizards") evolved skulls ornamented with knobs and spikes, the purpose of which is still not completely clear.  There were two main groups: the homalocephalids, which had heads that were flat, and the pachycephalosaurids, in which the top of the head was rounded, like a bony bowling ball.  It used to be thought that the pachycephalosaurs rammed each other at high speed, but the rounded skulls would have made this impractical.  Perhaps the ornamentation served some sort of intraspecific, probably sexual display and rivalry purpose.

Descriptions

Pachycephalosauria: Yaverlandia. Pachycephalosaurus > Triceratops. 

Range: Early to Late Cretaceous of China & North America. 

Phylogeny: Marginocephalia: Ceratopsia + * : Pachycephalosauridae + Homalocephalidae. 

Characters: Dentition heterodont & small; maxillary & posterior (distal) dentary teeth mediolaterally compressed, with leaf-shaped crowns bearing denticles; premaxilla retains teeth (primitive); frontal and parietal thickened dorsoventrally; external surfaces of skull strongly ornamented, usually tubercles on postorbital & squamosal; basicranium foreshortened; dorsal vertebrae strongly consolidated with double ridge-groove between zygapophyses of adjacent vertebrae; sacral and anterior caudal vertebrae have long ribs; intertwined ossified tendons on distal (?) caudals; scapula slender and much longer than shortened humerus; pubes small & excluded from acetabulum; pelvic girdle broad; no obturator process on ilium. Head-butting? Unsettled issue – certainly butted something. 

Image: Stegoceras by Matt Celeskey, Hairy Museum of Natural History.

Links: DD: Pachycephalosauria; Pachycephalosaurus and Pterodactylus; WMNH - Pachycephalosaur dorsal vertebra; FPDM : Pachycephalosauria; Pachycephalosauria after Sereno, 2000; Literature - Pachycephalosauria; Lecture 19: Late Cretaceous I.  ATW011130.

Homalocephalidae: flat-heads.

Range: Early Cretaceous of China. 

Phylogeny: Pachycephalosauria: Pachycephalosauridae + *. 

Characters: Skull evenly thickened & flat dorsally with numerous pits; supratemporal fenestrae present & relatively large.

Links: link; link (with rotatable skull).

Pachycephalosauridae: More specialized Pachycephalosaurs? Pachycephalosaurus, Prenocephale, Stegoceras, Stygimoloch.

Range: Early to Late Cretaceous of China & North America. 

Phylogeny: Pachycephalosauria: Homalocephalidae + *. 

Characters: Frontals & parietals greatly thickened & fused into single dome-like structure; no upper temporal fenestra; base of skull shortened in some; ridge-and-groove articulation between dorsal vertebrae possibly provided greater rigidity to backbone; long tail; very broad pelvis; long, low ilium; widely positioned femora; tail with the basketwork of ossified tendons. 

Image: of Pachycephalosaurus (Graves Museum) courtesy of Michael Corriss. 

Links: DinoData: Pachycephalosauridae; Biologybase: Checklist of the Non-Avian Dinosaurs; Pachycephalosaurus- Enchanted Learning Software; Pachycephalosauridae; Homalocephale; pachycephalosauria; Prenocephalae; pachycephalosauria cladogram; The Natural History Museum's Dino Directory; New Page 6; DINOBASE, Sibbick's dinosaur pictures; Pachycephalosauridae (photo of vertebra, but otherwise unremarkable); Pretty Butte Paleontology- Catalog of Pachycephalosauridae ... (images of isolated dome!); 7 (M. Shiraishi's gallery).  ATW030718.