Parankylosauria

Parankylosaurs Temporal range: Cretaceous, 105–70  Ma PreꞒ Ꞓ O S D C P T J K Pg N
Fossil material of Stegouros , a parankylosaur
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Dinosauria
Clade:	† Ornithischia
Clade:	† Thyreophora
Clade:	† Ankylosauria (?)
Clade:	† Parankylosauria Soto-Acuña et al., 2021
Genera
† Antarctopelta † Kunbarrasaurus † Minmi † Patagopelta † Stegouros

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 (called a 'macuahuitl'), made of several fused osteoderms projecting outward.

History of research

Fossils remains of the parankylosaur Antarctopelta ; early discoveries of ankylosaurs in the Southern continents were often fragmentary.

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. ^[1] 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. ^[2] Much of this material including would later be shown to be misidentified and not belonging to ankylosaurs, including the named genera. ^{[2] [3] [4]} The first definitive ankylosaur to be recognized from Gondwana was discovered in Australia in 1964 and later named in 1980 as Minmi paravertebra . ^[2]

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. ^{[5] [6]} In 1986 fragmentary ankylosaur remains were found on James Ross Island in Antarctica, and these were later named as the genus Antarctopelta . ^{[7] [8] [9]} 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. ^[5] 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. ^{[10] [11]} Beyond these two genera, the record was limited to fragmentary remains in Australia and South America. ^{[11] [12] [13]} 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. ^{[6] [12] [14]}

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". ^[5] 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. ^[15] However, more recent studies have disputed this and suggested a parankylosaur affinity for Patagopelta. ^{[9] [16]}

Anatomy

Size of four named parankylosaurians compared to a human

Known members of Parankylosauria are all small animals of various sizes, ranging from 1.5–4.0 metres (4.9–13.1 ft). 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. ^{[5] [6]} 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. ^[6]

Skull

Skull diagram of Kunbarrasaurus

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. ^[6] Those of at least some species seem to have been proportionally large for the body. ^[5] 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 postorbital bone, but this is lacking in other genera such as Patagopelta. A mosaic sulci network divides this unornamented surface into various polygonal sections. ^[6]

A distinct, uniting trait of parankylosaurs are the narrow and elongate nasals , which are restricted to the top surface of the skull. This forms part of a relatively elongate snout. ^[6] At the front of this snout was a large beak, similar to that of other ankylosaurs. ^[11] The maxillae , 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. ^[6] Their teeth have large denticles (coarse ridges along the tooth), a tooth crown (the exposed peak of the tooth) with a high position compared to the root (anchor of the tooth in the jaw), and a cingulum (protrusion 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. ^{[6] [5] [9]}

Torso

Reconstructed models of Antarctopelta at the Chilean National Museum of Natural History

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 dorsal (back) vertebrae have elongate centra (the cylindrical center of the vertebra) and high, compressed neural arches (the structure mounted above the centrum, surrounding the spinal cord) similar to those of stegosaurs. The prezygapophyses (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 postzygapophyses (equivalent backward projections) 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. ^{[5] [6]}

Unlike other ankylosaurs, the sacrum , forming the centre of the pelvis, was small, simple, and narrow. On either side, the ilia 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 in euankylosaurs), the hip socket opened downwards, the forward-projecting preacetabular process was relatively short, and the backward-projecting postacetabular process 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 ilium (sides of the pelvis). None of the vertebrae of the tail associate with the sacrum to form sacrocaudals, unlike some other ankylosaurs. ^{[5] [6]} Like other ankylosaurs, some taxa possessed ossified tendons (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 term "paravertebrae" by some authors. ^{[5] [17] [18]}

The limbs are parankylosaurs were relatively long and slender, as were the metartarsals 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. ^{[5] [6] [9]} In Patagopelta the feet had hoof like nails, whereas in the earlier Kunbarrasaurus the feet possessed claws. ^[6]

Tail

Complete macuahuitl structure of Stegouros (left) and closeup of osteoderm structure and articulation with tail vertebrae in Stegouros and Antarctopelta (right)

Tail (or caudal ) 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 (more than twice as wide as tall), with a large ventral groove cleaving the shape of the vertebra into a "binocular" shape with two sides. The transverse processes (projections to either side from the top of vertebrae) have been reduced entirely into long ridges that reach to the end of the centrum. ^{[5] [6] [9]} In other ankylosaurs, these processes are reduced to nubs at the back of the tail. ^[14] Thus, from the front of the back of the tail the centra increase in length but decrease in height. ^{[5] [6]} 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. ^{[5] [6]}

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. ^{[5] [6]} Similar osteoderms are known in Yuxisaurus, traditionally considered an early diverging armored dinosaur but potentially related to parankylosaurs. ^[6] In Stegouros this macuahuitl structure begins after the fourteenth tail vertebra, and preserved osteoderms of Antarctopleta and Patagopelta indicate similar anatomy. ^{[5] [6]} The end of the tail is unknown in earlier parankylosaurs such as Kunbarrasaurus. ^{[6] [10] [11]}

Armor

Skeleton of Kunbarrasaurus , showing rows of osteoderm along the torso

Like other armored dinosaurs, parankylosaurs had small bones known as osteoderms embedded in the skin across their necks, backs, limbs, and tails. While that along the end of the tail formed a tail weapon, other osteoderms adorned the body in armor and spikes. Larger osteoderms were arranged in patterns distinct to each species, whereas abundant arrays of tiny osteoderms known as ossicles filled in the space between them, particularly in the thorax and tail regions. Some of these ossicles possess a patern of striae (grooves) at right angles, unique to the ossicles of parankylosaurs. ^{[5] [6]} In Minmi and Kunbarrasaurus these ossicles were also present along the underside of the animals, whereas these were lacking in other genera such as Patagopelta. ^{[2] [6] [19]} The diversity of larger osteoderms also varied between species. Stegouros, a small and lightly built parankylosaur, had seemingly reduced armor, with limited flank armor and simple rows of small ovular osteoderms along the back. Rings of armor encircling the neck may have been absent in Stegouros and Antarctopelta, seemingly being secondarily lost compared to their inferred presence in ancestral ankylosaurs. ^{[5] [6]}

By comparison, some parankylosaurs such Patagopelta and related animals such as Minmi and Kunbarrasaurus possessed several different forms of osteoderm. Multiple rngs of bone were present around the neck, with multiple large osteoderms fused around each. An assortment of large elliptical, keeled, and spiked osteoderms were arranged along the top of the back and the sides of the torso. Behind this, a pavement of osteoderms covered the pelvic shield, with series of conical spikes fused to the same base. In Patagopelta, some of these form paired structures with a "binocular" base and two spikes, only otherwise observed in the euankylosaur Hungarosaurus. The largest spikes would have been directly over the hips, with smaller scutes present along the side. Also present around the hips and base of the tail were the largest osteoderms on the body. Elongated scutes would have run along the tail, while the largest osteoderms on the body were arranged along the side of the tail. These were flattened plate-like structures, with a triangular shape ending in a spike. Potentially overlapping with each other, these are reminiscent of structures found in euankylosaurs such as Sauropelta . ^[6]

Classification

Speculative life restoration of Stegouros

André Fonseca and colleagues in 2024 formally defined this clade in the PhyloCode as "the largest clade containing Stegouros elengassen , but not Ankylosaurus magniventris and Nodosaurus textilis ". This definition ensures both ankylosaurids and nodosaurids are excluded from Parankylosauria. ^[20] The following cladogram is reproduced from the phylogenetic analysis in the 2021 study by Sergio Soto-Acuña and colleagues: ^[5]

	Lesothosaurus Scutellosaurus Emausaurus Scelidosaurus Eurypoda Stegosauria Huayangosaurus Stegosauridae Ankylosauria Parankylosauria Kunbarrasaurus Antarctopelta Stegouros Euankylosauria Nodosauridae Liaoningosaurus Gobisaurus Shamosaurus Ankylosaurinae

In 2026, Agnolín and colleagues described several skeletal elements from the Allen Formation of Argentina that they referred to Patagopelta cristata . While this species was initially described as a nodosaurid in 2022, ^[15] later analyses and discussions preferred parankylosaurian affinities. ^[9] To test the relationships of Patagopelta, Agnolín et al. added it to the phylogenetic matrix of Raven et al. (2023), a dataset designed to test the relationships of all armored dinosaurs, ^[21] which had not previously sampled parankylosaurs in detail. Due to this lack of taxon and anatomical character sampling, earlier published iterations of this dataset, which included some traditional parankylosaur taxa (e.g., Patagopelta and Kunbarrasaurus), failed to recover them in a single monophyletic clade. ^[22] Using an updated version of this matrix, Agnolín et al. (2026) conducted their phylogenetic analysis under equal weighting (shown in the cladogram below) and implied weighting. Both versions recovered a monophyletic Parankylosauria, comprising Stegouros, Antarctopelta, Patagopelta, Kunbarrasaurus, and (for the first time) Minmi. Their implied weights analysis also recovered Yuxisaurus , a Jurassic thyreophoran from China, as the earliest-diverging member of the Parankylosauria, a result that has poor statistical support but is based on two shared anatomical characters. This is also consistent with earlier discoveries of relatives of Cretaceous dinosaur taxa from the Southern Hemisphere in Jurassic/Early Cretaceous rocks in China. More surprisingly, both analyses recovered Parankylosauria outside of its traditional placement within Ankylosauria. Instead, it was placed as the sister taxon to Eurypoda (ankylosaurs + stegosaurs) or in an unresolved polytomy with these clades and Yuxisaurus. The authors argued that Parankylosauria should be regarded as a lineage distinct from Ankylosauria, in part due to many plesiomorphic ('ancestral') traits in parankylosaur skeletons. ^[6]

Thyreophora	Emausaurus Scutellosaurus Scelidosaurus Eurypoda Yuxisaurus Stegosauria Ankylosauria Parankylosauria Stegouros Antarctopelta Kunbarrasaurus Minmi Patagopelta

In one of their analyses, Fonseca et al. (2024) recovered the enigmatic thyreophoran Jakapil as a basal ankylosaur, sister to both Parankylosauria and Euankylosauria. They noted that it took four steps in their analysis for Jakapil to fall within the Parankylosauria—a placement that should not be disregarded—though this is not the most parsimonious position. ^[20]

References

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