DINOSAUROMORPHA |--+--SAUROPODOMORPHA | `--THEROPODA Ornithischia |--Thyreophora | |--Stegosauria | `--Ankylosauria `--Cerapoda |--Ornithopoda | |--Hypsilophodontidae | `--Hadrosauridae `--Marginocephalia |--Pachycephalosauria | |--Homalocephalidae | `--Pachycephalosauridae `--Ceratopsia |--Psittacosauridae `--+--Protoceratopsidae `--+--Centrosaurinae `--Ceratopsinae
The Marginocephalia or "fringed heads" were a specialised group of Ornithischians defined by a small shelf or frill at the back of their skull. There are two main types, the dome heads and the horned dinosaurs, which evolved from a common ancestor during the early Cretaceous. With the exception of one or two questionable Gondwana forms, the group appears to be limited to Laurasia.
Suborder 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.
Pentaceratops sternbergi - (drawing copyright Øyvind M. Padron)
Suborder Ceratopsia - horned dinosaurs, small to large parrot-beaked dinosaurs, divided into the primitive early Psittacosauridae - parrot-beaked dinosaurs, stocky animals about about 1.5 meters long, and the Neoceratopsia or horned dinosaurs, including both horned and hornless types, all equipped with a bony frill, and the medium-sized to larger ones quadrapedal. Included here area number of famous late Cretaceous types like Styracosaurus, Triceratops, etc.
The Ceratopsians were the last of the major dinosaurian groups to evolve; first appearing during the Aptian or Albian (Early Cretaceous II). They are also among the most geographically limited, none being known outside the Asiamerican (eastern Asia + western North America) landmass. Reputed "Neoceratopsia" from Mid-Cretaceous southern Australia are probably not of this group. They may pertain to an unknown lineage of herbivorous quadrapedal Gondwanaland Predentata. Only the very most specialised Ceratopsia (the family Ceratopsidae, known only from the Senonian of western North America) developed horns and grew to a large size. (Triceratops horridus was about the size of a modern elephant).
Range: Early Cretaceous (perhaps much earlier) to Late Cretaceous of China & North America.
Phylogeny: Cerapoda: Ornithopoda + *: Pachycephalosauria + Ceratopsia.
Characters: Narrow parietal shelf; post squamosal shelf; short post maxillary plate; vomers contact maxilla (maxillae meet on midline) rather than premaxilla; premaxillae excluded from internal nares; no obturator process on ischium; pubis short, with loss of pubic symphysis.
Links: DinoData: Marginocephalia; Introduction to the Marginocephalia; Marginocephalia - EnchantedLearning.com; New Page 6; The Natural History Museum's Dino Directory; Pachycephalosaurs and Ceratopsians (November 5). ATW011227.
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 shortned 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.
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.
Ceratopsia: ~Ceratopia Marsh
Range: Early to Late Cretaceous of Asia & North America.
Phylogeny: Marginocephalia: Pachycephalosauria + *: Psittacosauridae + (Protoceratopsidae + (Centrosaurinae + Ceratopsinae).
Characters: Unique single rostral bone, usually with rugose surface indicated horn beak; maxilla at least 2/3 as tall as length; broad, inflexible mandibular symphysis (i.e. where mandibles grow together); jaw typically narrow, with flaring jugal (so skull triangular in dorsal view); tall snout w relatively broad premaxilla. Commonly parietal shelf (not necessarily a frill) overhanging occipital skull; often nasal & supraorbital horns. Teeth replaced very rapidly; achieved scissor-like vertical shear & high coronoid process gave additional leverage on lower jaw. Shelf may have started as giving increased length to mandibular adductor muscles. Presumably served in more derived species for display and possibly defense.
Psittacosauridae: Psittacosaurus (= Protiguanodon).
Range: Early Cretaceous of Asia.
Phylogeny: Ceratopsia: (Protoceratopsidae + Centrosaurinae + Ceratopsinae)) + *.
Characters: 1-2m early ceratopsian. Enamel primarily on one side of teeth, for upper teeth, thickened side is on the outside, reverse on lower teeth, cerating self-sharpening effect; crowns of even hatchlings show shear wear; tall, parrot-like snout with neomorphic unpaired rostral bone inserted between premaxillae (opposes predentary); nares high; anteriorly vaulted palate; distance from orbit to front of skull <40% of skull length, shortest of all dinosaurs (even oviraptors??); nasal extends as processes ventral to nares; premaxilla forms lateral surface of snout, broadly separating maxilla from nares; antorbital fenestrae and fossae absent; elongated jugal; slight parietal frill; post-cranial skeleton similar to basic Ornithischian pattern, especially Hypsilophodonts; moderately long, outstretched tail stiffened by (sometimes ossified) tendons along spine; arms ~58% length of legs; likely facultative biped; $ 2 outer digits of manus reduced; manual unguals expanded & slightly hoof-like; tibia » femur (cursorial); 4 functional digits on pes; gastroliths common (unlike derived Ceratopsians); skin impressions broadly similar to other Ornithischia.
Links: DinoData: Psittacosauridae; Psittacosaurus- ZoomDinosaurs.com; Lectures 12: Early Cretaceous; Psittacosaurus mongoliensis; New Page 6; DINOBASE, Sibbick's dinosaur pictures; ?????? Vertebrates (Chinese); ORNITHISCHIA; Biologybase: Checklist of the Non-Avian Dinosaurs; Psittacosaurus The Natural History Museum's Dino Directory. ATW020118.
Protoceratopsidae: Leptoceratops, Protoceratops.
Range: Late Cretaceous of Asia & North Amweica.
Phylogeny: Ceratopsia:: (Centrosaurinae + Ceratopsinae) + *.
Characters: Unclear whether clade or grade. No orbital or developed nasal horns; Rare in NAm, common in Asia.
Links: Paleo Mont Park; Fighting Dinosaurs.
Range: Late Cretaceous of North America.
Phylogeny: Ceratopsia::: Ceratopsinae + *.
Characters: Long nasal horns, hooks & processes on parietals; short, square squamosals; bone "finger" projecting into back of nares; in some, orbital & nasal horns appear as irregular, thickened bone pads (for horn spikes?); divergent ornamentation developed as approaching adulthood -- young of all species appear the same. Bone-beds common -- perhaps herds.
Links: DinoData: Centrosaurinae; Centrosaurus nasicornis.
Ceratopsinae (= Chasmosaurinae): Chasmosaurus, Torosaurus, Triceratops.
Range: Late Cretaceous of North Amweica.
Phylogeny: Ceratopsia::: Centrosaurinae + *.
Characters: Up to 8m (generally larger than centrosaurs), with skull up to 2.5 m. Complex, large narial opening with many fenestrae and bone processes; short nasal and long orbital horns; snout relatively short; neotenous epinasal bone on ant of nasal horn; some (e.g. Triceratops) had frontal sinus between horns and braincase, possibly used as shock absorber in combats (as in bovids) or as part of extensive system of vascularized cavities in thermoregulation; large conical epijugal horn on flared cheek; spikes absent from parietal (? Triceratops had epoccipitals as shown); frill longer than basal length of skull, with bend in middle (secondary reduction in Triceratops); squamosal concave and extends full length of frill; no known bone beds found (solitary?).
Links: DinoData: Chasmosaurinae; Biologybase: Checklist of the Non-Avian Dinosaurs; Abstract: Marcot; Chasmosaurinae; Literature - Ceratopsia; Ceratopsinae -- The Dinosauricon; The Evolution of Dinosaurs; GEOL 104 Dinosaurs- A Natural History; Torosaurus Fact Sheet - EnchantedLearning.com; GEOL 104 Lecture 20- Marginocephalia- That's using your head!; Adrian's Dinosaur Notes Page ( For Dr. Masons g180 Class ); Figure 5; Triceratops. ATW030125.