A new species of ankylosaur discovered in subantarctic Chile provides fresh insights into the origin and early evolution of these armoured dinosaurs. The findings, presented in Nature, reveal that Stegouros elengassen evolved a large tail weapon unlike those seen in other dinosaurs.
Ankylosaurs (Ankylosauria) from Laurasia, the northern landmass of what was once the Pangaea supercontinent, are diverse and well-studied. However, those from southern Gondwana — believed to be likely to include the earliest kinds of ankylosaur — are rare and poorly understood.
Alexander Vargas and colleagues describe the well-preserved, mostly complete skeleton of a small ankylosaur, roughly two metres in size, from the late Cretaceous period (around 71.7 to 74.9 million years ago) discovered at Magallanes in southernmost Chile. The authors reveal that the skeleton belonged to a new species of ankylosaur, named Stegouros elengassen. Stegouros has distinctive skull features like those of other ankylosaurs, but the rest of its skeleton is believed to be largely primitive, with some stegosaur-like characteristics. Stegouros also possessed a large tail weapon composed of seven pairs of flattened, bony deposits fused together in a frond-like structure across the distal part of its tail, differentiating it from the paired spikes and clubs of other armoured dinosaurs. Phylogenetic analyses — equivalent to constructing the ankylosaur family tree — allowed the team to recognise Stegouros as an ankylosaur, specifically related to Kunbarrasaurus from Australia and Antarctopelta from Antarctica.
In light of their findings, the authors conclude that different branches of the ankylosaur family tree may have existed in Laurasia and Gondwana following the final separation of these supercontinents in the late Jurassic period. This proposition, they state — alongside other possibilities raised by the discovery of Stegouros — reiterates how much we have yet to learn about the evolution of armoured dinosaurs, particularly in Gondwana.
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