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.
History of research
During the Mesozoic era, the southern continents (South America, Antarctica, Australia, and Africa in addition to India and Zealandia) were unified into a supercontinent known as Gondwana. This was in contrast to the supercontinent of Laurasia in the Northern Hemisphere, with both originating from the breakup of Pangaea. Gondwana itself gradually split apart over the course of the Jurassic and Cretaceous eras. Ankylosaurian dinosaurs from Laurasia have historically been far more extensively recorded and studied. Reports of the group in Gondwana date back to 1904, with a specimen from Australia and include referrals of Loricosaurus, Lametasaurus, and Brachypodosaurus to group among assorted fragmentary material. Much of this material was later shown to be misidentified and not belonging to ankylosaurs, including the named genera. The first definitive ankylosaur to be recognized from Gondwana was discovered in Australia in 1964 and named in 1980 as Minmi paravertebra. Ankylosaurs from Gondwana have remained mysterious. Fossil material continues to be scant, and southern taxa have been difficult to interpret in a phylogenetic context. In 1986, a fragmentary ankylosaur specimen was found on James Ross Island in Antarctica, and was later named as the genus Antarctopelta. Its vertebrae were poorly preserved and so foreign compared to those of euankylosaurs that it was at times questioned if they might instead belong to a marine reptile, in which case the specimen would be a chimera. In 1989 a better preserved Australian ankylosaur specimen was discovered that would initially be considered another specimen of Minmi before being named as the distinct genus Kunbarrasaurus in 2015. Beyond these two genera, the record was limited to fragmentary remains in Australia and South America. The possibility of a biogeographic connection between South America and ankylosaurs in Australia was raised, though based on conjecture. The concept that these southern ankylosaurs formed a distinct lineage from Northern ones as part of an endemic Gondwanan fauna had been suspected, but difficult to confirm due to a lack of material. A more complete specimen discovered in 2018 and described as the new genus Stegouros in 2021 showed that there was a previously unrecognized monophyletic grouping of these southern ankylosaur taxa. The study naming the genus, by Sergio Soto-Acuña and colleagues, coined Parankylosauria based on Stegouros, Antarctopelta, and Kunbarrasaurus. The name, referencing its parent group, means "at the side of Ankylosauria". Whether or not all known Gondwana ankylosaurs belong to the group remains unclear. Patagopelta, for example, was described from Argentina in 2022 as a member of the North American lineage Nodosauridae. This would suggest that in addition to the more ancient Parankylosauria, more derived euankylosaurians also inhabited South America, having migrated from North America as part of a biotic interchange during the Campanian. However, more recent studies have disputed this and suggested a parankylosaur affinity for Patagopelta.
Anatomy
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