Friday, December 3, 2021

[Paleontology • 2021] Stegouros elengassen • Bizarre Tail Weaponry in A Transitional Ankylosaur (Thyreophora: Parankylosauria clade nov.) from Subantarctic Chile

Stegouros elengassen 
Soto-Acuña, Vargas, Kaluza, Leppe, Botelho, Palma-Liberona, Gutstein, Fernández, Ortiz, Milla, Aravena, Manríquez, Alarcón-Muñoz, Pino, Trevisan, Mansilla, Hinojosa, Muñoz-Walther & Rubilar-Rogers, 2021

Illustration: Luis Pérez López 

Armoured dinosaurs are well known for forms that evolved specialized tail weapons: paired tail spikes in stegosaurs, and heavy tail clubs in advanced ankylosaurs. Armoured dinosaurs from southern Gondwana are rare and enigmatic, but likely include the earliest branches of Ankylosauria. Here, we describe a mostly complete, semiarticulated skeleton of a small (about 2m) armoured dinosaur from the late Cretaceous of Magallanes in southernmost Chile, a region biogeographically related to West Antarctica. Stegouros elengassen gen. et sp. nov. evolved a large tail weapon unlike any dinosaur: A flat, frond-like structure formed by 7 pairs of laterally projecting osteoderms encasing the distal half of the tail. Stegouros shows ankylosaurian cranial characters, but a largely primitive postcranial skeleton, with some stegosaur-like characters. Phylogenetic analyses placed Stegouros in Ankylosauria, and specifically related to Kunbarrasaurus from Australia and Antarctopelta from Antarctica, forming a clade of Gondwanan ankylosaurs that split earliest from all other ankylosaurs. Large osteoderms and specialized tail vertebrae in Antarctopelta suggest it had a tail weapon similar to Stegouros. We propose a new clade, the Parankylosauria, to include the first ancestor of Stegouros but not Ankylosaurus, and all descendants of that ancestor.

KEYWORDS: dinosaurs, tail weapons, anylosaurus, stegosaurus

Dinosauria Owen, 1842 
Ornithischia Seeley, 1887 
Thyreophora Nopcsa 1928 
Ankylosauria Osborn, 1923 

Parankylosauria clade nov. 

Stegouros elengassen gen. et sp. nov. 

Etimology— Stegouros, after the Greek stego (roof) and the Greek uros (tail) in reference to the covered tailelengassen, after an armoured beast in the mythology of the local Aónik’enk people.

Locality and horizon—Río de las Chinas Valley, Estancia Cerro Guido, Magallanes Region, Chilean Patagonia (51°S). Lower section of Dorotea Formation (upper Campanian – lower Maastrichtian), between 71.7 ± 1.2 Ma and 74.9 ± 2.1 Ma10,11 (Supplementary Information section 2, Supplementary Fig. 2).

Armoured dinosaurs are the only amniotes to have evolved three different specialized tail weapons in Stegosauria, Stegouros and Ankylosaurinae.

Sergio Soto-Acuña, Alexander Vargas, Jonatan Kaluza, Marcelo Leppe, Joao Botelho, José Palma-Liberona, Carolina Gutstein, Roy Fernández, Hector Ortiz, Verónica Milla, Bárbara Aravena, Leslie Manríquez, Jhonatan Alarcón-Muñoz, Juan Pino, Christine Trevisan, Héctor Mansilla, Luis Hinojosa, Vicente Muñoz-Walther and David Rubilar-Rogers. 2021. Bizarre Tail Weaponry in A Transitional Ankylosaur from Subantarctic Chile. Nature. DOI: 10.1038/s41586-021-04147-1