Ankylosauria is a clade of herbivorous, armored ornithischian dinosaurs subdivided in two major clades: the Ankylosauridae and the Nodosauridae. The most derived members of this group are characterized by shortened skulls, pyramidal squamosal horns, and tail clubs. Fossil evidence of armored dinosaurs from Gondwana is scarce. They were present primarily in Asia and North America. Stegouros elengassen, a new specimen from the Late Cretaceous Dorotea Formation of southern Chile, offers new evidence that contributes to the understanding of the relationships among the ankylosaurs from Gondwana.
Stegouros lived about 72 to 75 million years ago, and reached 2 meters in lenght (six feet). The generic name is derived from the the Greek word “stego” (roof ) and the Greek word “uros”
(tail) in reference to the covered tail. The specific name “elengassen” comes from an armoured
beast in the mythology of the Aónik’enk people.
The holotype (CPAP–3165), represented by a nearly complete skeleton, semi-articulated, was discovered in 2017 at the lower section of the Dorotea Formation. The new specimen exhibits an unusual combination of characters. The skull and teeth, are classically Ankylosauria, but the pelvis and slender limb bones resemble Stegosauria. The most striking feature of Stegouros is the short, bizarre tail covered by seven pairs of large osteoderms, that are fused into a flat composite unit resembling a battle axe.
Phylogenetic analyses with five different datasets indicate that Stegouros was closer to Ankylosauria than to Stegosauria. It was grouped with the basal ankylosaurs Kunbarrasaurus (from the late Lower Cretaceous of Australia) and Antarctopelta (from the Late Cretaceous of Antarctica) forming a monophyletic clade that split earliest from all other Ankylosauria. The study, led by Chilean paleontologists, proposes the clade Parankylosauria “to include the first ancestor of Stegouros—but not Ankylosaurus—and all descendants of that ancestor“ (Soto-Acuña et al., 2021).
Soto-Acuña, S., Vargas, A.O., Kaluza, J. et al. Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile. Nature (2021). https://doi.org/10.1038/s41586-021-04147-1