For decades, the fossils of armoured dinosaurs – the thyreophorans – were virtually non-existent on southern continents. But over the last two years, that has changed. And done so exponentially.
The latest dinosaur discovery is a testament to this fact.
On August 11th, paleontologists led by Sebastian Apesteguia and Facundo Riguetti revealed the discovery of a primitive armoured dinosaur from the Late Cretaceous of Argentina. Named Jakapil kaniukura, this small herbivore represents a bizarre (and somewhat confusing) glimpse into the evolution of armoured dinosaurs in the Southern Continents.
The remains of Jakapil are unlike those of other thyreophorans. The most noteworthy anomaly was its bipedal stance. The two main thyreophoran lineages, the ankylosaurids and stegosaurids, were both quadrupedal. The only other thyreophorans known to have been bipedal were base members of the lineage that evolved before the ankylosaur-stegosaur split, dinosaurs like Scutellosaurus and Scelidosaurus from the Early Jurassic Period.
Bipedalism isn’t the only strange trait exhibited by Jakapil. Its jaw resembles those of horned dinosaurs and early ornithischians more than it does other armoured dinosaurs. Its leaf-shaped teeth are found in later ankylosaurids, but not in dinosaurs like Scutellosaurus and Scelidosaurus. The shoulder blade, the most complete fossil bone, is similar to some stegosaurids. Jakapil appears to have traits found across ornithischian dinosaurs, making accurate assessments of its relations challenging.
Paleontologists have tentatively identified Jakapil as a basal thyreophoran[i]. Under this assessment, it’s strange features can be explained by the division of the Northern and Southern continents into two land masses, Laurasia (North America, Asia, Europe) and Gondwana (South America, Africa, Australia, Antarctica, India, and Madagascar). Jakapil’s isolation from its Laurasian cousins allowed it to evolve separately, thus retaining some of the basal traits of the lineage.
At first glance, it seems logical to slot Jakapil into the parankylosaurid family, a group of primitive ankylosaurids found across Southern continents. The parankylosaurids, the newest family of dinosaurs to be identified (coined in late 2021), currently consist of three species: the Chilean Stegouros, the Antarctic Antarctopelta, and the Australian Kunbarrasaurus[ii]. The unique tail of Stegouros helped demonstrate that these were primitive and bizarre thyreophorans. Why not slot Jakapil alongside them instead of a lineage previously believed to have been long-extinct?
As the researchers discuss, Jakapil and the parankylosaurids aren’t anatomically similar. Though they were a strange order, parankylosaurids were still ankylosaurids. Consequently, parankylosaurids possessed many traits unique to ankylosaurids. Broad skulls, depressed vertebrae, quadrupedal behaviours, and more are found in parankylosaurids, but not Jakapil[iii]. This points to it being part of a separate lineage of thyreophorans native to the southern hemisphere that lived alongside the parankylosaurids.
This is rather shocking, given the rarity of southern thyreophorans. Years ago, the notion of a single lineage would have shocked paleontologists; to suggest there were two would have been blasphemous. It is important to note that a more complete specimen of Jakapil could place it as a parankylosaurid. If this were the case, the difference between Jakapil and dinosaurs like Stegouros points toward high diversification and adaptive radiation within the parankylosaurids, making them a fascinating lineage to examine further.
An alternative proposal worth mentioning is that Jakapil isn’t a thyreophoran at all. Dr. Susannah Maidment at the Natural History Museum London has argued that Jakapil may be a ceratopsian (horned dinosaur) instead. Dr. Maidment argues that Jakapil has little in common with the thyreophorans besides armour and instead shares dental structure with the ceratopsians. She argues that the rest of the skeleton is too fragmentary to make conclusions, making any estimate of its overall physique misguided[iv].
Her arguments are fair, but they create some new problems.
First, Jakapil would immediately become the first ceratopsian from any southern continent. How did they get there? Ceratopsians originated in Asia and only spread to North America around 100 million years ago. So, how did one get to South America without an obvious land bridge within three million years? The answer: it probably couldn’t.
Second, the armour of Jakapil has never been observed in ceratopsians before. If Jakapil was a ceratopsian, it would mean that osteoderms – the bony projections that form armour – evolved separately over four times within dinosaurs. Not an impossible notion, but one that seems unlikely.
To put it bluntly, this theory is probably a long shot.
What it does highlight is the sheer bizarreness of Jakapil. It has traits of many different dinosaur lineages, but which is it part of? Is it a stem-armoured dinosaur that somehow survived for 100 million years after the other known members of its lineage? If so, will more of these primitive thyreophorans be discovered soon? Is it a southern species that morphed drastically from its closest relatives? Is it another lineage of dinosaurs that somehow made their way south?
Currently, the answer is unclear, though I am excited to see how this plays out. In any scenario, any addition to the dinosaur family tree is welcome news, especially when it’s mysterious.
That’s why we all love Spinosaurus, right?
Thank you for reading today’s article! If you’re interested in Stegouros and other southern ankylosaurids, then I recommend you reading “When is a Stegosaur not a Stegosaur,” here at Max’s Blogosaurus!
I do not take credit for any images found in this article.
Header image courtesy of Yoofilos on twitter, found here
Jakapil with size comparison courtesy of Joschua Knüppe, found here
Jakapil fossils courtesy of Riguetti et al
Pair of Jakapil courtesy of PaleoGDY on twitter, found here
Stegouros courtesy of DevinQuigleyArt on twitter, found here
Jakapil headshot courtesy of Emily Stepp on twitter, found here
[i] Riguetti, Facundo J., et al. “A New Cretaceous Thyreophoran from Patagonia Supports a South American Lineage of Armoured Dinosaurs.” Scientific Reports, vol. 12, no. 1, 2022, https://doi.org/10.1038/s41598-022-15535-6.
[iii] Riguetti, Facundo J., et al. “A New Cretaceous Thyreophoran from Patagonia Supports a South American Lineage of Armoured Dinosaurs.” Scientific Reports, vol. 12, no. 1, 2022, https://doi.org/10.1038/s41598-022-15535-6.
[iv] Maidment, Susannah (@Tweetisaurus). “For the enthusiasts out there, here are extracts from my review that explain why I don’t think Jakapil is a thyreophoran:” August 11, 2022, 2:28 PM, https://twitter.com/Tweetisaurus/status/1557796087758725120