The name of this group of dinosaurs is associated with a number of anatomical features in which small and large bony shields fused together, completely covering their back and sides.

The Latin name Ankylosauria is derived from the Greek ἀγκύλος [ankylos] — "curved", "bent" with the anatomical meaning "hard" or "fused" and σαῦρος [sauros] — "lizard".

[6] In the 1908 description of the genus Ankylosaurus, Barnum Brown described the family Ankylosauridae as a group of representatives with a "rigid spine", but noted the wide, curved shape of the ribs, suggesting a "strongly curved" back[7] (an error based on the alleged similarity to stegosaurs and glyptodonts, as ankylosaurs have flat backs).

[6] Ankylosauria and Stegosauria together form the two major subgroups of Thyreophora, a group of armoured dinosaurs distinct from ornithopods and marginocephalians.

[4] Historically used for forms lacking large vertical plates, Kenneth Carpenter proposed in 1997 the first informal definition of the group, as all ornithischians closer to Ankylosaurus than Stegosaurus.

As some analyses, like that of Carpenter from 2001 or David B. Norman in 2021 find Scelidosaurus and possibly other early forms like Emausaurus and Scutellosaurus to fall closer to Ankylosaurus than Stegosaurus, Carpenter and later Norman suggested redefining Ankylosauria to limit it to the two subclades Nodosauridae and Ankylosauridae, creating the new clade Ankylosauromorpha for all taxa closer to Ankylosaurus than Stegosaurus.

Scelidosaurus Stegosauria Kunbarrasaurus Antarctopelta Stegouros Cedarpelta Dongyangopelta Gargoyleosaurus Gastonia Hylaeosaurus Mymoorapelta Panoplosaurus Peloroplites Polacanthus Hoplitosaurus Sauroplites Taohelong Nodosaurinae Aletopelta Chuanqilong Liaoningosaurus Gobisaurus Shamosaurus Ankylosaurinae A 2023 review of Thyreophora rejects the traditional Ankylosauridae-Nodosauridae split, instead finding "nodosaurids" to be referrable to three separate families: Panoplosauridae, Polacanthidae, and Struthiosauridae.

[12] Sarcolestes Paw Paw juvenile Animantarx Cedarpelta Gargoyleosaurus Hoplitosaurus Niobrarasaurus Polacanthus Texasetes Patagopelta Hylaeosaurus Gastonia burgei Gastonia lorriemcwhinneyae Peloroplites Sauropelta Tatankacephalus Invictarx Silvisaurus Taohelong Europelta Struthiosaurus transylvanicus Struthiosaurus languedocensis Struthiosaurus austriacus Hungarosaurus Pawpawsaurus Tianchisaurus Panoplosaurus Denversaurus Edmontonia longiceps Nodosaurus Edmontonia rugosidens Anoplosaurus Dracopelta Borealopelta Mymoorapelta Stegopelta Vectipelta Dongyangopelta Zhejiangosaurus Ahshislepelta Aletopelta Talarurus Gobisaurus Kunbarrasaurus Liaoningosaurus Chuanqilong Shamosaurus Jinyunpelta Tsagantegia Nodocephalosaurus Zaraapelta Akainacephalus Pinacosaurus mephistocephalus Pinacosaurus grangeri Crichtonpelta Ziapelta Ankylosaurus Anodontosaurus Tarchia kielanae Saichania Minotaurasaurus Tarchia teresae Antarctopelta Zuul Euoplocephalus Dyoplosaurus Scolosaurus

They may have split up during the Aalenian period, more than 170 million years ago, but they were definitely in Africa by the Bathonian due to the presence of Spicomellus in Morocco.

An incomplete radius and ulna from the Isle of Skye in Scotland are known, the exact affiliation of which to ankylosaurs or stegosaurs is not established.