Palaeos		Ornithischia
Vertebrates		Overview

Ornithischia: Overview

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

The Predentata or Ornithischia are a very diverse group of dinosaurs distinguished by their bird-like hip bones and the presence of an extra bone (the predentary) in the front of the low jaw (hence the two names for them - "Ornithischia" is the older name).  Many types evolved ungulate (hoofed) mammal-like features like elaborate batteries of chewing teeth, and horns and other types of crests and head ornaments (used not so much against predators as in intra-specific rivalry - competition for mates, etc).

Originally only four suborders were distinguished - Ornithopoda, Stegosauria, Ankylosauria, and Ceratopsia.   More recently some of these groups have been shown to be related and hence united under larger sub-ordinal groups, while new groups are also required by modern discoveries. 

The base of the Ornithischia, like the base of many other big groups, is a mess of poorly known specimens which are impossible to classify with much accuracy.  In the case of the Ornithischia, this group is known collectively as the Fabrosauridae.  In addition, the three main (large) clades of Ornithischian (Predentata) dinosaurs are the:

1)  Thyreophora or armoured dinosaurs, including stegosaurs and ankylosaurs. 
2) Ornithopoda or mostly bipedal, typical plant-eaters: hypsilophodonts, iguanodonts and hadrosaurs.
3) Marginocephalia or horned and bone-head forms: pachycephalosaurs, psittacosaurs and ceratopsians.

Here is another, slightly more detailed breakdown, in the form of a cladogram with some synapomorphies (unique characters) of each group:

Ornithischia
|--"Fabrosaurs"  paraphyletic: no synapomorphies
`--Genasauria  cheeks
   |--Thyreophora  armor
   |  |--Stegosauria   armor as plates covering back
   |  `--Ankylosauria  armor as spikes or plates
   `--Cerapoda  uneven enamel
      &|--Ornithopoda  jaw joint below tooth row
      |  |--Heterodontosaurus  elongate hands
      |  `--+--Hypsilophodontidae  ridge on teeth
      |     `--+--Iguanodontidae  thumbs spike-like
      |        `--Hadrosauridae laterally expanded distal rostrum ("duck bill")
      `--Marginocephalia  back part of skull makes shelf
         |--Pachycephalosauria  domed head
         `--Ceratopsia  rostral bone
            |--Psittacosauridae reduced hands
            `--Neoceratopsia "frill"

We will discuss the fabrosaurs and each of the principle groups in more detail below.  MAK000117.

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