Ankylosaurs are commonly known for their broad, heavily armoured backs and for their rounded tail clubs. These dinosaurs are well documented in the fossil record of the Northern Hemisphere (the ancient supercontinent of Laurasia), where they became extremely diverse. However, ankylosaur fossils are rarely found in the Southern Hemisphere (the ancient supercontinent of Gondwana); the fossil record includes only two named species — from Antarctica and Australia — as well as isolated bones and teeth from South America and Africa1. This imbalance hinders research into the group’s early evolution, because ankylosaurs from Gondwana are thought to include early branches, which might inform our understanding of the origins of these dinosaurs. Moreover, the Antarctic ankylosaur — Antarctopelta, the first dinosaur to be discovered there — is highly enigmatic. It was preserved in a coastal environment and only about 15% of the skeleton is available. However, because some parts are so unusual, it was even proposed to be a mix of an ankylosaur and the remains of marine reptiles2.
The Magallanes region in Chile comprises the southernmost tip of South America. This region was much closer to Antarctica 80–66 million years ago, towards the end of the Cretaceous period, than it is now, and shows evidence of intercontinental biotic exchange3. The Cretaceous is documented in the Magallanes region by the Dorotea Formation in the Las Chinas Valley, Chile, in which abundant fossils of land plants and vertebrates are preserved.
We expected that remains of ankylosaurs could be found here, given the contemporaneous presence of Antarctopelta in the nearby Antarctic Peninsula. Our most optimistic expectations were surpassed, however, by the discovery of the nearly complete skeleton (Fig. 1a) of a new species — Stegouros elengassen — that is fully articulated from the waist down. We discovered the skeleton in what might have been a death trap such as quicksand. Notably, this animal had half of its tail (toward the tip) encased in a large, flattened structure of fused dermal bones with laterally directed spikes (Fig. 1b): a unique tail weapon, comparable to a macuahuitl — the war club used by the ancient Aztecs. Much of the skeletal anatomy of Stegouros is ancestral, with few of the specializations that are normally found in ankylosaurs, indicating that it originated from an ancient lineage. In light of the information obtained from Stegouros, we re-examined and re-interpreted the remains of the enigmatic Antarctopelta. Comparison of Antarctopelta’s skeletal elements with those of Stegouros enables us to conclusively reject the notion of a mix with marine reptiles; moreover, we propose that the large, previously unclear dermal bones of Antarctopelta probably belonged to a comparable macuahuitl — something that would have been impossible to infer without the fossil of Stegouros.
Because of the relatedness of Stegouros and Antarctopelta — and perhaps also the Australian Kunbarrasaurus — we propose the existence of an ancient lineage of southern ankylosaurs, the Parankylosauria (meaning ‘at the side of Ankylosauria’). The Parankylosauria would have split early from the lineage that became dominant in the north, which we have named the Euankylosauria (‘true’ Ankylosauria). Future discoveries should confirm whether most Parankylosauria had a macuahuitl and whether they tended to be more lightly armoured, more slender and smaller than the Euankylosauria (Stegouros and Kunbarrasaurus are only about 2 metres and 2.5 m long, respectively, compared with Antarctopelta’s length of roughly 4 m). The fossil record of armoured dinosaurs in the Southern Hemisphere is still limited, but remains are increasingly being reported1,4. Hopefully, it is only a matter of time before other well-preserved specimens provide a more complete picture of the ankylosaurs’ diversity. — Alexander O. Vargas and Sergio Soto-Acuña are at the University of Chile, Santiago, Chile.
Behind the paper
The discovery of Stegouros was enabled by more than a decade of studies and exploration of the Las Chinas Valley led by Marcelo Leppe. Stegouros was extracted mostly from a block that had a few exposed bones: its surprising anatomy was gradually uncovered as it was prepared in the laboratory. The pelvis was stegosaur-like, as were other traits, and for a while we thought it might be a very unusual stegosaur (which normally have upright armour plates and a tail weapon of paired spikes). However, when Stegouros was included in phylogenetic analyses, these traits were no longer unequivocally stegosaurian. Stegouros is an ankylosaur, but its lineage probably split off shortly after the last ancestor shared with Stegosauria, which could explain the mix with stegosaur-like traits. The study of Stegouros was led by Chilean scientists with Chilean funding, and is a milestone for vertebrate palaeontology in this country, where the field is new. — A.O.V.
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