Ankylosaurs Temporal range: Middle Jurassic—Late Cretaceous,
Collection of ankylosaurs. From top left to right: Liaoningosaurus, Edmontonia, Tianzhenosaurus, Gargoyleosaurus, Scolosaurus, Denversaurus, Gastonia, Borealopelta and Akainacephalus.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Thyreophora
Clade:	†Eurypoda
Suborder:	†Ankylosauria Osborn, 1923
Subgroups
Parankylosauria Euankylosauria Soto-Acuña et al., 2021[1] Ankylosauridae Nodosauridae Genera of uncertain affinity Cryptosaurus Dracopelta Mymoorapelta Sarcolestes Serendipaceratops? Sinankylosaurus Spicomellus Stegosaurides? Tianchiasaurus Vectipelta
Synonyms[2]
Ankylosauromorpha Carpenter, 2001

Ankylosauria is a group of herbivorous dinosaurs of the clade Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are known to have first appeared in the Middle Jurassic, and persisted until the end of the Cretaceous Period. The two main families of Ankylosaurs, Nodosauridae and Ankylosauridae are primarily known from the Northern Hemisphere, but the more basal Parankylosauria are known from southern Gondwana during the Cretaceous.

Ankylosauria was first named by Henry Fairfield Osborn in 1923.^[3] In the Linnaean classification system, the group is usually considered either 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.

Etymology

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. On the skull these shields fused with the underlying bones, and the dorsal ribs fit snugly to the vertebrae. The Latin name Ankylosauria is derived from the Greek ἀγκύλος [ankylos] — "curved", "bent" with the anatomical meaning "hard" or "fused" and σαῦρος [sauros] — "lizard".^[4]

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^[5] (an error based on the alleged similarity to stegosaurs and glyptodonts, as ankylosaurs have flat backs). Therefore, "rigid lizard" and "curved lizard" could be additional meanings applied to the name of ankylosaurs.^[4]

Classification

Ankylosauria and Stegosauria together form the two major subgroups of Thyreophora, a group of armoured dinosaurs distinct from ornithopods and marginocephalians.^[2] 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.^[6] This definition was further refined by Paul Sereno in 2005 to specify Ankylosaurus magniventris and Stegosaurus stenops, the type species of both genera, a definition that was followed by Madzia and colleagues in 2021 when the group name and definition was formalized following the PhyloCode.^[2] Phylogenetic and morphological studies have differed on the inclusion of certain early taxa into Ankylosauria, especially the armoured Early Jurassic form Scelidosaurus. 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.^[2][7][8] However, as historically even these primitive forms were considered ankylosaurs if they were more derived than Stegosaurus, Madzia and colleagues considered a redefinition of Ankylosauria to be undesirable, instead preferring to abandon Ankylosauromorpha as a same-definition junior synonym of Ankylosauria. The informal clade Euankylosauria, suggested by Norman as an alternative to redefining Ankylosauria, was discussed as potentially being useful in future for the typical group of nodosaurids and ankylosaurids,^[2] and was defined in 2021 by Soto-Acuña and colleagues as such as the result of their novel phylogenetic results, which grouped Antarctopelta, Kunbarrasaurus (Minmi sp. of earlier analyses) and their new taxon Stegouros as a unique group of non-euankylosaurian ankylosaurs, which they gave the name Parankylosauria. The result of their phylogenetic analysis is below.^[1]

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.^[9]

Paleobiology

Armour of the nodosaurid Edmontonia

Possible neonate-sized ankylosaur fossils have been documented in the scientific literature.^[19]

References