Scientists writing in the journal “Nature” have announced the discovery of a new species of Late Cretaceous ankylosaur from southernmost Chile. The dinosaur named Stegouros elengassen evolved a large tail weapon, unlike any other tail weapon known in the Dinosauria. Stegouros had a flat, frond-like structure formed by seven pairs of laterally projecting osteoderms. This formidable tail weapon superficially resembles a macuahuitl, a frightening war club used by Aztec warriors.

Stegouros life reconstruction
A life reconstruction of the newly described Gondwanan ankylosaur Stegouros elengassen. Picture credit: Mauricio Álvarez.

A Combination of Stegosaur and Ankylosaur Anatomical Characteristics

The researchers from the University of Chile (Universidad de Chile) in Santiago report on an articulated, nearly complete fossil specimen representing a single animal that was excavated by a field team in February 2018 from sediments relating to the Dorotea Formation in the province of Ultima Esperanza in southern Chile. Stegouros was very small for an ankylosaur, measuring around two metres in length and weighing approximately 100 kilograms.

It shows ankylosaurian cranial characters, but a largely ancestral postcranial skeleton, with some stegosaur-like characters.

A phylogenetic analysis places Stegouros in the Ankylosauria and the research team postulate that it is closely related to other ankylosaurs known from southern Gondwana such as Kunbarrasaurus from Australia and Antarctopelta, fossils of which were found on James Ross Island off the Antarctic Peninsula.

Stegouros fossil excavation.
Field team members jacket the fossilised remains of Stegouros. Picture credit: Universidad de Chile.

The Parankylosauria Clade

The researchers propose the establishment of a new clade of ankylosaurs – the Parankylosauria which consists of basal ankylosaurs that split from the main ankylosaurian lineage during the Middle Jurassic, although the fossil evidence for this is currently lacking with Stegouros, Antarctopelta and Kunbarrasaurus forming part of a ghost lineage of basal ankylosaurs with origins back in the mid-Jurassic.

Stegouros fossil remains.
The posterior of the Stegouros specimen with the armoured tail clearly shown. Picture credit: Universidad de Chile.

A Transitional Ankylosaur

Commenting on the significance of this fossil discovery, lead author of the scientific paper, Sergio Soto, a researcher at the University of Chile stated that the fossilised remains of S. elengassen represent a “Rosetta Stone” for deciphering the fragmentary fossils of other ankylosaurs from southern Gondwana.

He added:

“Stegouros is an evolutionary link between ankylosaurs and other older lineages of armoured dinosaurs. Stegouros has only some of the features normally found in ankylosaurs, particularly the skull, but many others are absent. It also has some traits similar to stegosaurs, inherited from a common ancestor with them, but that other ankylosaurs lost in evolution.”

A close-up view of the armoured tail of Stegouros elengassen.
A close-up view of the armoured tail of Stegouros elengassen. The tail club served as a defensive structure perhaps used in intraspecific combat or to deter attacks from predators. Picture credit: Universidad de Chile.

Stegouros roamed the Chilean Patagonia around 74 million years ago. The genus name translates as “roofed tail”, whilst the species name comes from a mythical armoured monster from the folklore of the indigenous Tehuelche people.

The scientific paper: “Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile” by Sergio Soto-Acuña, Alexander O. Vargas, Jonatan Kaluza, Marcelo A. Leppe, Joao F. Botelho, José Palma-Liberona, Carolina Simon-Gutstein, Roy A. Fernández, Héctor Ortiz, Verónica Milla, Bárbara Aravena, Leslie M. E. Manríquez, Jhonatan Alarcón-Muñoz, Juan Pablo Pino, Cristine Trevisan, Héctor Mansilla, Luis Felipe Hinojosa, Vicente Muñoz-Walther and David Rubilar-Rogers published in the journal Nature.

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