Parankylosauria
Parankylosauria is a group of armored thyreophoran dinosaurs known from the Cretaceous of South America, Antarctica, and Australia. Most analyses place parankylosaurs as a member of the Ankylosauria, in which case the group would have split from other ankylosaurs during the mid-Jurassic period, despite this being unpreserved in the fossil record. Another analysis has proposed that parankylosaurs are instead a distinct lineage of non-ankylosaurian armored dinosaurs with more ancestral anatomy. Several parankylosaurs are characterized by a distinctive frond-like tail weapon, made of several fused osteoderms projecting outward.
History of research
During the Mesozoic era, the southern continents 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 including would later be 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 later named in 1980 as Minmi paravertebra.Ankylosaurs from Gondwana have remained very mysterious. Fossil material continues to be scant, and southern taxa have been difficult to interpret in a phylogenetic context. In 1986 fragmentary ankylosaur remains were found on James Ross Island in Antarctica, and these were 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, which would make the genus based on a chimeric specimen. 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.
The discovery of the genus Stegouros in 2018, subsequently published and named in 2021, helped to clear up this uncertainty. The type specimen of the genus preserved enough of the skeleton to make it clear that there was a previously unrecognized monophyletic grouping of these southern ankylosaur taxa. Thus, 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, 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. Those of at least some species seem to have been proportionally large for the body. 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. This forms part of a relatively elongate snout. At the front of this snout was a large beak, 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 , a tooth with a high position compared to the , and a 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.
Torso
In general, parankylosaurs retain torso similar to the primitive armored dinosaur condition, creating a contrast 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 vertebrae have elongate and high, compressed similar to those of stegosaurs. The were fused into a short, U-shaped wedge; this is also shared with stegosaurs. Near the back of the dorsal column, the are also fused together. These back vertebrae were unfused to their respective ribs; fused ribs were traditionally considered a uniting trait of ankylosaurs, but are infact 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, 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 other ankylosaurs are made up of fused osteoderms that cover both the sacrum and the . None of the vertebrae of the tail associate with the sacrum to form sacrocaudals, unlike some other ankylosaurs. Like other ankylosaurs, some taxa possessed along the backbone. Those of Minmi, including some ending in wide bony sheets only otherwise known in Hungarosaurus, have been given term "paravertebrae" by some authors.
The limbs are parankylosaurs were relatively long and slender, as were the of the 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.
Tail
Tail vertebrae of parankylosaurs are highly distinct. Those near the base of the tail have a short, compressed shape from to back, and are instead expanded in width. A faint groove runs along the bottom of each vertebra. Those in the middle of the tail are simple and box-shaped. Most uniquely, the vertebrae near the end of the tail have a flattened shape, with a large ventral groove cleaving the shape of the vertebra into a "binocular" shape with two sides. The are have been reduced entirely into long ridges that reach to the end of the centrum. In other ankylosaurs, these processes are reduced to nubs at the back of the tail. Thus, from the front of the back of the tail the centra increase in length but decrease in height. At least some parankylosaurs, had shortened tails; Stegouros has an estimated count of 26 tail vertebrae, the lowest known in any armored dinosaur. Unlike derived ankylosaurids, which stiffen their tails using elongated, overlapping prezygapophyses, parankylosaurs possess short prezygapophyses on their tail vertebrae and stiffening is instead achieved through the surrounding cover of osteoderms. Likewise, the tails of parankylosaurs do not show vertebral fusion as is seen in ankylosaurids to support their large tail slabs.Similar to many armored dinosaurs, at least some parankylosaurs possessed weapons at the ends of their tails. While stegosaurs possessed spiked thagomizers and ankylosaurines possessed tail clubs localized to the end of the tail, parankylosaurs possessed a structure known as macuahuitl. This consisted of a frond-like pattern of several osteoderms arranged on each side of the entire back half of the tail. The flattened vertebrae from the end of the tail would have functioned to support this structure. The spines forming the macuahuitl, which have been described as "pup tent" shaped, have an elongated base with a large depression, a tapering shape ending with a sharp outward keel, and are uniformly flattened. Together, this gives the shape of a flat funnel, and the base fits in along each side of each tail vertebra. Similar osteoderms are known in Yuxisaurus, traditionally considered an early diverging armored dinosaur but potentially related to parankylosaurs. In Stegouros this macuahuitl structure begins after the fourteenth tail vertebra, and preserved osteoderms of Antarctopleta and Patagopelta indicate similar anatomy. The end of the tail is unknown in earlier parankylosaurs such as Kunbarrasaurus.