Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile

Armoured dinosaurs are well known for their evolution of specialized tail
weapons— paired tail spikes in stegosaurs and heavy tail clubs in advanced
ankylosaurs1. Armoured dinosaurs from southern Gondwana are rare and
enigmatic, but probably include the earliest branches of Ankylosauria2–4.
Here we describe a mostly complete, semi-articulated skeleton of a small
(approximately 2 m) armoured dinosaur from the late Cretaceous period of
Magallanes in southernmost Chile, a region that is biogeographically
related to West Antarctica5. Stegouros elengassen gen. et sp. nov. evolved
a large tail weapon unlike any dinosaur: a flat, frond-like structure
formed by seven pairs of laterally projecting osteoderms encasing the
distal half of the tail. Stegouros shows ankylosaurian cranial characters,
but a largely ancestral postcranial skeleton, with some stegosaur-like
characters. Phylogenetic analyses placed Stegouros in Ankylosauria;
specifically, it is related to Kunbarrasaurus from Australia6 and
Antarctopelta from Antarctica7, forming a clade of Gondwanan ankylosaurs
that split earliest from all other ankylosaurs. The large osteoderms and
specialized tail vertebrae in Antarctopelta suggest that 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.