Ankylosauria

Ankylosauria is an extinct group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are first known to have appeared in the Early Jurassic Period of China, and persisted until the end of the Cretaceous Period. They have been found on every continent except Africa. The first dinosaur ever discovered in Antarctica was the ankylosaurian Antarctopelta, fossils of which were 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 stegosaurs, armored dinosaurs known for their combination of plates and spikes.

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

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

The polacanthines, an intermediate group sometimes placed in their own family, 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 polacanthines include, among others, Hylaeosaurus, Polacanthus and Mymoorapelta. Opinions range from them belonging to their own family, the Polacanthidae, distinct from the other two or being a subfamily of the Ankylosauridae, to some researchers questioning their status as a discrete group.

The traditional ankylosaurids are from later in the Cretaceous. They had much wider bodies and have even been discovered with bony eyelids. The large clubs at the end of their tails may have been used in self-defense (swung at predators) or in sexual selection. This family included Ankylosaurus, Euoplocephalus, and Pinacosaurus.

In 1997, Carpenter defined the clade Ankylosauria as all thyreophorans closer to Ankylosaurus than to Stegosaurus (a definition followed by most paleontologists today, including Sereno, 2005). This "stem-based" definition means that the primitive armored dinosaur Scelidosaurus, which is slightly closer to ankylosaurids than to stegosaurids, is technically a member of Ankylosauria. Upon the discovery of Bienosaurus, Dong Zhiming (2001) erected the family Scelidosauridae for both of these primitive ankylosaurs.

Taxonomy
A simplified version of one possible classification follows:


 * Order Ornithischia (the "bird hipped" dinosaurs)
 * Suborder Thyreophora (armored herbivores)
 * Infraorder Ankylosauria (the tanks)
 * Family Scelidosauridae
 * Scelidosaurus
 * Bienosaurus
 * Minmi
 * Antarctopelta
 * Family Nodosauridae
 * Family Ankylosauridae
 * ?Subfamily Polacanthinae
 * Subfamily Ankylosaurinae (tail clubs)