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 () 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 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
Known members of Parankylosauria are all small animals of various sizes, ranging from . Compared to euankylosaurians, they retained traits seen in primitive thyreophorans as well as the related stegosaurs. Their bodies, in particular, have a number of primitive traits, whereas typical ankylosaur skull traits were acquired early in the group's evolution and thus shared between lineages. Proportionally they were characterized by longer limbs and in some cases shorter tails than those seen in other ankylosaurs. Like other ankylosaurs they possessed armor across their bodies, made up of various small osteoderms (bones embedded in the skin), and a reinforced sheet of bone known as a pelvic shield over the hips, and a tail weapon. The latter in parankylosaurs is known as a macuahuitl, named after the mesoamerican weapon of the same name, and consisted of a series of osteoderms along both sides of the tail rather than a singular localized club or thagomizer at the end of the tail. Multiple different bodyplans appear to have been present within the group. Genera like Stegouros and Antarctopelta show smaller size and a lighter build with reduced armor. Others like Patagopelta and Kunbarrasaurus were most robust, more heavily adorned in armor and covered in a more diverse array of osteoderms.
Skull
As with other ankylosaurs, the detailed skull anatomy of parankylosaurs is difficult to appreciate due to bone sheets depositing over the sutures between skull elements. The skulls of at least some species seem to have been proportionally large. Unlike other ankylosaurs, skulls of parankylosaurs are composed of largely unfused bones which are rather thin. Unlike early armored dinosaurs, their skulls are textured with various ridges, pits, and grooves. However, they lack the more elaborate skull ornamentation seen in many other ankylosaurs. Known parankylosaurs lack spines, horns, bosses, or pyramidal protrusions, instead having simplistic unadorned skulls. Kunbarrasaurus does possess thick eye ridges formed by the bone, but this is lacking in other genera such as Patagopelta. A mosaic sulci network divides this unornamented surface into various polygonal sections.
A distinct, uniting trait of parankylosaurs are the narrow and elongate , which are restricted to the top surface of the skull. These form part of a relatively elongate snout. At the front of the snout was a large , similar to that of other ankylosaurs. The , holding the teeth in the upper jaw, are oriented so as to strongly diverse in the back, unlike other ankylosaurs. The teeth themselves are, in general, dissimilar to those of ankylosaurids and instead very similar to those of nodosaurs. Their teeth have large (coarse protuberances along the cutting edges), a (the exposed peak of the tooth) with a high position compared to the (anchor of the tooth in the jaw), and a (shelf-like bulge near the base of the tooth) that was large and asymmetrically shaped. This anatomy differs somewhat between species, with the earlier genus Kunbarrasaurus having larger, more coarse and deeply grooved denticles and the Late Cretaceous genus Patagopelta having finer, shorter ones. These teeth do not occupy a distinct morphotype from those of other ankylosaurs.
Torso
In general, the torso of parankylosaurs is similar to the primitive armored dinosaur condition, contrasting with the derived anatomy seen in euankylosaurs. Several traits are shared with stegosaurs, the sister group of ankylosaurs, rather than with other ankylosaurs. This may be indicative of ancestral traits of the two lineages, retained in stegosaurs and parankylosaurs bust lost in other ankylosaurs. The (back) vertebrae have elongate (the cylindrical center of the vertebra) and high, compressed (the structure mounted above the centrum, surrounding the spinal cord) similar to those of stegosaurs. The (forward projections to attach one vertebra to the next) were fused into a short, U-shaped wedge; this is also shared with stegosaurs. Near the back of the dorsal column, the (equivalent backward projections) are also fused together. These back vertebrae were not fused to their respective ribs; fused ribs were traditionally considered a uniting trait of ankylosaurs, but are in fact unfused in some basal forms.
Unlike other ankylosaurs, the , forming the centre of the pelvis, was small, simple, and narrow. On either side, the were primitive and similar to stegosaurs and unlike other ankylosaurs in several respects. The overall shape was long and low (rather than large and subrectangular as in euankylosaurs), the hip socket opened downwards, the forward-projecting was relatively short, and the backward-projecting was elongate. A thin layer of dermal bone covers the sacrum, reinforcing the pelvis with a "pelvic shield". This is superficially similar to that seen in stegosaurs, formed by the sacral vertebrae themselves, while the pelvic shield of other ankylosaurs is made up of fused osteoderms that cover both the sacrum and the (sides of the pelvis). None of the vertebrae of the tail associate with the sacrum to form sacrocaudals, unlike some other ankylosaurs. Like other ankylosaurs, some taxa possessed (tendons turned to bone) along the backbone. Those of Minmi, including some ending in wide bony sheets (ossified aponeuroses) only otherwise known in Hungarosaurus, have been given the term paravertebrae by some authors.
The limbs of parankylosaurs were relatively long and slender, as were the of the hind feet. Both stegosaurs and euankylosaurs, contrastingly, have stout limbs and broad feet. Instead, the parankylosaur condition is more similar to very early armored dinosaurs. In Patagopelta the feet had hoof-like nails, whereas in the earlier Kunbarrasaurus the feet possessed claws.