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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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