Parankylosauria

Parankylosauria is a group of armored dinosaurs that inhabited the Southern Hemisphere during the Cretaceous period. First proposed as a distinct clade in 2021, they are thought to be an early diverging branch of the ankylosaur lineage. While most ankylosaurs (members of Euankylosauria) are found in the Northern Hemisphere, ankylosaur discoveries in the Southern hemisphere are rare and often fragmentary. They have been found in South America, Antarctica, and Australia, which were partially united into the supercontinent of Gondwana at the time. The 2018 discovery of a relatively complete skeleton later named as the genus Stegouros was a key development in understanding the evolutionary link between Gondwanan ankylosaurs.

Compared to other ankylosaurs, parankylosaurs were rather small in size (1.5–4.0 metres (4.9–13.1 ft)) and retained a number of primitive traits shared with early armoured dinosaurs and stegosaurs but lost in euankylosaurs. Though only a few species are known, they seem to show anatomical diversity, with some species being small and lightly built and others being more robust and heavily armoured. Their skulls, like other ornithischians, possessed both beaks and teeth, and had relatively elongate snouts with distinct narrow nasal bones. The complex ornamentation and horns of other ankylosaur skulls appear absent in parankylosaurs. Their bodies were more primitive than their skulls, with vertebral characteristics shared with stegosaurs and simple, small pelvic regions distingushing them from euankylosaurs. Parankylosaur legs were relatively long and their feet were slender, with some species possessing hoof-like nails and others claws. Their tail vertebrae were wide and flat, and some parankylosaurs possessed shortened tails surrounded by large osteoderms that formed a weapon structure called a "macuahuitl", in reference to the Mesoamerican weapon. Smaller osteoderms were embedded across the body to form armor, with some species possessing prominent spikes and others having reduced their armor to simply rows of small oval structures.

Before the discovery of Stegouros, the relationships of Gondwanan ankylosaurs to other ankylosaurs was unclear, and inconsistent between analyses. When Parankylosauria was proposed in 2021, it was found to represent the first lineage to branch off from the rest of Ankylosauria. Consequently, Parankylosauria must have originated during the Middle Jurassic despite their fossil record being restricted to the Cretaceous period. It has been cladistically defined as "the largest clade containing Stegouros elengassen, but not Ankylosaurus magniventris and Nodosaurus textilis". Some studies have disputed the existence of Parankylosauria as a natural grouping, instead considering its proposed members to be distantly related. However, several studies have accepted the clade and the 2021 results have been replicated in multiple distinct phylogenetic analyses using distinct datasets. Uncertainties remain as to whether Parankylosauria truly belongs in Ankylosauria or is instead an earlier branching part of the armoured dinosaur lineage.