New dinosaur: Armoured herbivore had a tail shaped like a fern frond

Stegouros elengassen, unearthed in Chile, had a strange flattened tail that looked like a fern frond – a feature never seen before in an ankylosaur dinosaur


1 December 2021

Stegouros elengassen

An artist’s impression of Stegouros elengassen

Lucas Jaymez

A new species of ankylosaur found in Chile had a unique tail unseen in any other member of this dinosaur family. The discovery sheds light on the mysterious origins of ankylosaurs in the southern hemisphere.

Ankylosaurs were quadrupedal, herbivorous dinosaurs that roamed Earth throughout the Jurassic and Cretaceous periods. They were covered in bony deposits on their skin called osteoderms, which provided protection, much like a turtle’s shell. Ankylosaurs from the northern portion of what used to be the supercontinent Pangaea have been widely studied. However, those from southern Pangaea have been much harder to come by and are poorly understood.

Alexander Vargas at the University of Chile in Santiago and his colleagues have reported the discovery of a new ankylosaur, whose almost-complete skeleton was found in the Río de Las Chinas valley in southern Chile. They called the new species Stegouros elengassen.

The team found that the skeleton had a mix of traits from known ankylosaurs and from stegosaurs, a related group of four-legged plant-eating dinosaurs. In fact, the pelvis of S. elengassen was almost identical to that of a stegosaur, but the jawbones that carried its upper tooth row were clearly ankylosaurian.

The skeleton also had a flat, weapon-like tail, with seven pairs of broad, laterally facing blades, making the tail end look like the frond of a fern. This is unlike anything seen in other ankylosaurs, which typically had large, club-shaped tails. The dinosaur also appeared to be less armoured and more slender-limbed than other ankylosaurs.

“This is our first good look at a South American armoured dinosaur, and it is not like any armoured dinosaur you’ve ever seen before.” says Vargas. “It has a tail weapon that is a new category – all we knew [before] was tail spikes and tail clubs, now we have this weird frond-like thing.”

After analysing its features and comparing them with other ankylosaurs, the team determined that S. elengassen belonged to one of the deepest branches of the ankylosaur evolutionary tree.

These findings revolutionise our understanding of southern ankylosaurs, says Vargas. It was previously presumed that ankylosaurs spread from northern to southern Pangaea relatively late in the group’s evolution. But because S. elengassen lacks many of the typical body features found in northern ankylosaurs, Vargas and his colleagues now think the southern branch of ankylosaurs must have split off much earlier in prehistory.

“We don’t quite understand what the evolutionary driving forces are for these ankylosaurians to remain morphologically more ancestral compared to the ankylosaurian dinosaurs from the northern hemisphere,” says Jelle Wiersma at James Cook University in Australia. “But this study certainly highlights that there is still a lot to learn about this particular group of dinosaurs.”

Journal reference: Nature, DOI: 10.1038/s41586-021-04147-1

Sign up to Wild Wild Life, a free monthly newsletter celebrating the diversity and science of animals, plants and Earth’s other weird and wonderful inhabitants

More on these topics: