Stellasaurus: Beast of the Week

This week we'll be checking out a newly described ceratopsian dinosaur.  Say hello to Stellasaurus anchellae!  Stellasaurus lived in what is now Montana, USA, during the late Cretaceous period, about 75.2 million years ago.  From beak to tail it measured about 20 feet long and would have eaten plants when alive.  The genus name, Stellasaurus, translates to "Star Lizard" due to its flamboyant horn ornamentation and in honor of the late rockstar, David Bowie, and his song, "Starman".

My life reconstruction of Stellasaurus done in watercolors.  Ziggy Stardust-style facial markings and body stripes may have been an adaptation to communicate with members of its own species.  (Nature has produced weirder things.)

Stellasaurus is characterized by having a massive nose horn that curved backwards, towards the tail end of the animal, much like the horn of a modern rhinoceros.  Stellasaurus' nose horn was also slightly laterally compressed, like a blade, rather than having a more rounded cross-section, which is unusual for ceratopsians.  Finally, this dinosaur sported two long, upwards-facing horns on either side of its frill.  The evolutionary purpose of these horns was likely for some sort of intraspecies communication, likely display and/or combat with rivals.  Because there is just so much variation in headgear amongst ceratopsian species, it is unlikely they were purely for defense against predators.  Stellasaurus was a member of the centrosaurine group of ceratopsians, which are characterized by their deeper snouts and proportionally smaller neck frills.

Right and left views of Stellasaurus' massive rhinoseros-like nose horn. (image from Wilson's 2020 paper linked below)

There are a lot of ceratopsian taxa on the fossil record.  New members of this wildly successful group of dinosaurs are being published every year.  That being said, Stellsasaurus is a particularly important find because it appears to help show an actual evolutionary line in action.  We know evolution is an ongoing process of older forms of organisms changing over time into newer forms in response to environmental changes.  That being said, it isn't common to be able to say with certainty one taxa was a direct ancestor or descendent of another in the fossil record.  However, looking at the frill horns of Stellasaurus, and then comparing them to the frill horns of other ceratopsians that lived in the same place as it from different times, paleontologists were able to see a likely linear transition between at least five different kinds of ceratopsians!

Left and right sides of Stellasaurus' frill and frill horns.  Note how it has two growing from each side.  (Image from Wilson's 2020 paper linked below.)

A close relative of Stellasaurus, Styracosaurus, which lived slightly earlier than Stellasaurus, had similar frill horns, but they were more numerous, with three on each side instead of Stellasaurus' two.  Einiosaurus, which lived a bit after Stellasarus, only had one of these frill horns on each side.  The even younger Achelousaurus, also had two horns on its frill, but they angled slightly outwards.  Finally, the youngest piece to the puzzle, Pachyrhinosaurus, also had these horns, but they were even more dramatically curved outwards.  The only thing that may not seem to match up is the difference in nose ornamentation between Stellasaurus' backwards curving horn, and Einiosaurus' extremely forward-facing horn, but keep in mind there were millions of generations of dinosaurs between these individuals, and therefore many more transitional forms that existed there.  Maybe we'll find yet another ceratopsian to fit into this line to make it even more detailed!

Stellasaurus provides a crucial puzzle piece in showing a possible direct transitional line among several different taxa of centrosaurine ceratopsians.

That is all for this week.  As always please leave your thoughts below!

References

Wilson, John P.; Ryan, Michael J.; Evans, David C. (2020). "A new, transitional centrosaurine ceratopsid from the Upper Cretaceous Two Medicine Formation of Montana and the evolution of the 'Styracosaurus-line' dinosaurs". Royal Society Open Publishing. 7 (4).