Ankylosauria

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{{Taxobox | color = pink | name = Ankylosaurians | fossil_range = Cretaceous | regnum = Animalia | phylum = Chordata | classis = Sauropsida | superordo = Dinosauria | ordo = Ornithischia | subordo = Thyreophora | infraordo = Ankylosauria | infraordo_authority = Osborn, 1923 | subdivision_ranks = Families | subdivision =

}} The Ankylosauria, less formally known as the ankylosaurians, were a group of ornithischian ("bird-hipped") dinosaurs that lived in the late Cretaceous period. They are characterised by bony plates, which protected them against predators.

There have been found on every continent except Africa. The first dinosaur ever discovered in Antarctica was a nodosaurid recovered from Ross Island in 1986.

Ankylosauria was first described by Henry Fairfield Osborn in 1923. In the Linnaean classification system, the group is usually considered a suborder or an infraorder. It is contained within the group Thyreophora, which also includes the stegosaurians. Thyreophora is either considered a suborder, or is left unranked.

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Classification

Ankylosauria is split into two families, Nodosauridae (the nodosaurids) and Ankylosauridae (the ankylosaurids). The big difference is that most ankylosaurids (except the "polacanthids") have bony clubs at the end of their tails, which nodosaurids lack.

The nodosaurids were earlier and more primitive. They had narrow heads, and frequently had large spikes protruding from their bodies. This group traditionally includes Nodosaurus, Edmontonia, and Sauropelta.

An intermidate group sometimes placed in their own family, the polocanthids, were once placed in the Nodosauridae due to their lack of tail clubs. More recently, it has been found that they were more closely related to the ankylosaurids, and by most cladistic definitions, are members of that family. The polocanthids include Hylaeosaurus, Polacanthus, and Mymoorapelta.

The traditional ankylosaurids are from later in the Cretaceous. They had much wider bodies, and thicker armor. They have even been discovered with bony eyelids. The large clubs at the end of their tails were swung at predators. This family included Ankylosaurus, Euoplocephalus, and Pinacosaurus.

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Taxonomy

A simplified version of one possible evolutionary tree (cladogram) follows:

Order Ornithischia (the "bird hipped" dinosaurs)
- Suborder Thyreophora (armored herbivores)
  - Infraorder Stegosauria (spikes or plates down their back)
  - Infraorder Ankylosauria (the tanks)
    - Family Nodosauridae (no tail clubs)
    - Family Ankylosauridae
      - Subfamily Polocanthinae
      - Subfamily Ankylosaurinae (tail clubs)