When is a Stegosaur not a Stegosaur?

Basic common-sense lesson #479:

Never give dinosaurs similar names when they aren’t related to each other.

That is advice I wish paleontologists adhere to. Yet this is not always the case, as paleontologists seem to like messing around and will give two very different dinosaurs similar names. Take for instance Barosaurus and Barrosasaurus, dinosaurs that are from different continents, lived over 70 million years apart, and were discovered 100+ years apart, yet still possess extremely similar names. It’s almost like paleontologists want to troll those who keep up with the field.

The bizarre trend continued on December 1^st, when paleontologists announced the discovery of Stegouros elengassen. To clarify, Stegouros is not a spelling mistake for Stegosaurus – it is a new genus of dinosaur. Stegouros isn’t closely related to Stegosaurus either, as it was an ankylosaur (armoured) dinosaur. While ankylosaurs may be the closest relatives to the stegosaurs on the dinosaur family tree, it still doesn’t explain why paleontologists gave such a similar name.

So, why did paleontologists name an ankylosaur after Stegosaurus? Well, it has to do with anatomy. While Stegouros does possess some key traits of ankylosaurs, there are a lot of strange features mixed in as well. For one thing, the limb bones are thin, a trait that is found amongst stegosaurs but not the ankylosaurs, whose legs are famous for their stockiness.

More importantly, the tail club of Stegouros is unique amongst ankylosaurs. Nicknamed macuahuitl after the weapons used by the Aztecs, the club is made of fused bone like other ankylosaurs, yet Stegouros has seven pairs of bone whereas most ankylosaurs only have one. The club is jagged and sharp, while its relatives have blunted and often circular clubs. Once again, this trait is more similar to the sharp thagomizers (tail spikes) of Stegosaurus than other ankylosaurs. It isn’t quite like Stegosaurus either, as the bone is fused in pairs while thagomizers occurred in unfused pairs on each side of the tail.

The distinct tail of Stegouros seems to have a little bit of both ankylosaur and stegosaur traits. This raises an interesting question: was it a transitionary species between the two? The answer is no, as ankylosaurs did not evolve directly from stegosaurs. Instead, the two lineages had a common ancestor at the start of the Jurassic Period, sometime shortly after 200 million years ago. Stegouros lived during the late Cretaceous, some 75 million years ago, and thus cannot be the missing link between stegosaurs and ankylosaurs.

This doesn’t mean that Stegouros is insignificant, however. The strange combination of ankylosaur and stegosaur traits suggest that while Stegouros was an ankylosaur, its lineage diverged shortly after the split between stegosaurs and ankylosaurs. Early divergence from later relatives like Ankylosaurus or Zuul would explain why Stegouros possessed primitive features despite living at roughly the same time as its relatives.

If Stegouros is part of a new, primitive ankylosaur lineage, then why has it not been identified before? The fossils of Stegouros hail from southern Chile, making it the first South American ankylosaur. ankylosaurs are rare in southern continents, with only four other species – Kunbarrasaurus and Minmi in Australia, Spicomellus in Morocco, and Antarctopelta in Antarctica – having been identified. With scarce and incomplete remains, the relation of southern ankylosaurs to their northern counterparts has been unclear. With any luck, expanded research in places like South America and Africa will yield more ankylosaurs, giving further support to the notion of a distinct lineage.

It’s likely that the separation between northern and southern ankylosaur populations is what led to the distinct traits of Stegouros and its close relatives. When two populations are separated, they grow independent adaptations to best suit their environments. The separation of populations is what led Darwin to the theory of evolution and may have been what caused many other important evolutionary events. For ankylosaurs, the southern populations may have retained the primitive traits of their descendants before the ankylosaur-stegosaur split, while the northern populations became more advanced.

If Stegouros is part of a new ankylosaur lineage, then great! Any new group of dinosaurs is always exciting and makes understanding the complexity of these organisms even more interesting. With the discovery of strange new members like Stegouros and the spiky Spicomellus, ankylosaurs have become one of the most diverse dinosaur families known to science. I mean, what other dinosaur family has members that look like Aztec weapons, porcupines, tanks, and turtles?

This doesn’t mean I have to like the name Stegouros. Strongly dislike may capture my emotions best. When I was searching images for this article, Google gave me the classic “did you mean to search: Stegosaurus?” When I told my friends about the article, they promptly said it would be “a pain because the urge would be to say Stegosaurus.” Thanks, MB.

And to Stegouros, thank you for being the most confusing yet somehow important dinosaur known to science.

Except for Spinosaurus, of course.

Works Cited:

I do not take credit for any images found in this article.

Header image courtesy of Adam Rufino, whose works can be found at his Tumblr here

Waterfront Stegouros courtesy of Luis Perez Lopez, found here

Model of tail club courtesy of Felipe PoGa, found here

Drinking Stegouros courtesy of Joschua Knuppe, found at his twitter here

Evolution of armoured dinosaurs and their tail weaponry courtesy of Sergio Soto-Acuña et al.

Brusatte, Stephen. The Rise and Fall of the Dinosaurs: A New History of a Lost World. Gale, a Cengage Company, 2019.

Greshko, Michael. “This Bizarre Armored Dinosaur Had a Uniquely Bladed Tail Weapon.” Science, National Geographic, 1 Dec. 2021, https://www.nationalgeographic.com/science/article/this-bizarre-armored-dinosaur-had-a-uniquely-bladed-tail-weapon.

Soto-Acuña, Sergio, et al. “Bizarre Tail Weaponry in a Transitional Ankylosaur from Subantarctic Chile.” Nature, 2021, https://doi.org/10.1038/s41586-021-04147-1.

Thompson, Richard S., et al. “Phylogeny of the Ankylosaurian Dinosaurs (Ornithischia: Thyreophora).” Journal of Systematic Palaeontology, vol. 10, no. 2, 2011, pp. 301–312., https://doi.org/10.1080/14772019.2011.569091.