Keeping Your Ears to the Ground: Was Thescelosaurus a Burrowing Dinosaur?

Life in North America during the Late Cretaceous Period was a dangerous endeavour. Throughout the continent, the most infamous predatory dinosaur to ever roam the earth – Tyrannosaurus rex – dominated terrestrial ecosystems. They weren’t the only predators, as many others – including Dromaeosaurs, crocodiles, and Azhdarchid pterosaurs – offered their fair share of danger. The herbivores were nothing to scoff at either, as dinosaurs like Triceratops and Ankylosaurus could pack a punch for any animals unlucky enough to bother them.

Life for the smaller animals sharing these ecosystems required adaptation. How they adapted varied between organisms, but recent discoveries have suggested that one dinosaur opted for a unique strategy in the face of danger.

Enter Thescelosaurus, a dinosaur famous for being… ordinary. Ranging between 2-4 meters in length, Thescelosaurus could hardly intimidate other dinosaurs with its size and lacks any form of intriguing ornamentation to make it noteworthy. The dreariness of Thescelosaurus is added to by its herbivorous diet, which often leads to a perception of inherent dullness from the paleontological community. It’s a matter of fact that a predator bias exists in the field; why else would the Allosaur cousin Sinraptor, a dinosaur proclaimed as vanilla by paleontologist Thomas Holtz, be featured in multiple documentaries while poor Thescelosaurus gets none?

However, our perception of Thescelosaurus may be about to change. On November 6^th, 2023, Paleontologists David Button and Lindsay Zanno published a new study examining Thescelosaurus’ brain anatomy using the specimen NCSM 15728, colloquially known as Willo. Analysis of Willo’s brain has painted an unflattering picture, as its brain size relative to body mass places Thescelosaurus amongst some famously unintelligent dinosaurs (such as Triceratops and the Ankylosaur Euoplocephalus) and closer to modern reptiles[i]. If anything, this discovery has only added a literal element to Thescelosaurus’ dull reputation!

The low brain-body ratio was not the only discovery made. The brain’s olfactory tract and lobes were well developed, suggesting an advanced sense of smell on par with animals like Alligators and rodents. Portions of the inner ear, known as the anterior semicircular canal, were elongated, suggesting that Thescelosaurus had a good sense of balance. Further analysis of its inner ear indicates that the hearing range of Thescelosaurus was quite limited and well-suited for deciphering low-frequency sounds.

All these traits are fine and all, but what do they mean? After assembling their data, Button and Zanno found that the combination of adaptations exhibited in Thescelosaurus is often present in fossorial animals – animals that burrow underground. While the notion that a 4-meter-long dinosaur could burrow seems unorthodox, it is far from unprecedented.

For starters, one close relative of Thescelosaurus – known as Oryctodromeus – is known to have practised this behaviour. In 2007, paleontologists working in Montana discovered the remains of three Oryctodromeus nestled within their ancient burrow, inferred by the different sediment types within the burrow that resulted from the den being flooded and buried by sand[ii]. While some of the traits that aided in Oryctodromeus’ ability to burrow are absent in Thescelosaurus, namely the reinforcement of the pelvis, many other adaptations (such as a broad shoulder blade) are shared between the two species.

Next, the large size of Thescelosaurus does not preclude it from being able to burrow. In the present, Polar Bears – animals comparable in weight to Thescelosaurus – create dens in the Arctic ice and snow throughout the winter to shelter themselves and their young. Even more astounding are the burrows dug by extinct Ground Sloths scattered throughout South America, which can reach dimensions of 2×4 meters and are routinely carved through solid rock[iii]. Though the exact species that carved these tunnels is unknown, it likely dwarfed Thescelosaurus if the size of its claw marks is any indication!

Some other traits of Thescelosaurus could point towards it being a burrower. Its robust body, in addition to its inner ear anatomy, suggests it was relatively slow for a dinosaur its size. Instead, its stout hindlimbs and long forelimbs – which could touch the ground while in a bipedal stance[iv] – could indicate an ability to dig proficiently. They may have even sported bony spurs on their forelimbs to aid their digging (though I should note that this discovery has not yet been published in the scientific literature). Even if it wasn’t a burrower, Thescelosaurus could have used its powerful limbs and advanced sense of smell to locate and dig up underground roots and tubers for an easy meal.

Given that Thescelosaurus lived in ecosystems dominated by Tyrannosaurus rex, it makes perfect sense that Thescelosaurus resorted to burrowing. Dr. Zanno has noted how the low-frequency hearing ability of Thescelosaurus matches that of T. rex[v], which could have allowed for early detection of the massive predator. After hearing the coming danger, Thescelosaurus could have retreated to the safety of a burrow beyond the reach of T. rex and allowed the smaller herbivore to escape unscathed. 

All anatomical and circumstantial evidence points to Thescelosaurus being a burrower, but it is important to note that something is missing: fossil burrows themselves. Without this material, we cannot definitively assert that Thescelosaurus did burrow. Thescelosaurus may have inherited the various traits that paleontologists associate with this behaviour from an ancestor in the vein of Oryctodromeus but lost the behaviour itself. At present, further evidence is required for paleontologists to confirm if Thescelosaurus’ adaptations were vestigial or still being used.

Regardless, the possibility of burrowing behaviours has pulled Thescelosaurus out of the realm of dinosaur obscurity. No longer is Thescelosaurus just another dull genus; now, it is an intellectually dull genus capable of behaving like a groundhog. In an ecosystem dominated by giant dinosaurs, Thescelosaurus’ ability to get underground and out of the way may have proved essential for escaping predation or raising their young.

If nothing else, this discovery highlights the importance of imaging technology in paleontology. If we can tell that a dinosaur burrowed based on its brain structure, who knows what paleontologists will discover next?

Thank you for reading today’s article! While it is the most famous, Button and Zanno’s study has not been the discovery related to Thescelosaurus in recent years, as one fossil from the infamous Tanis Fossil Site of North Dakota may preserve a mummified Thescelosaurus leg! If you want to know more, read about it here at Max’s Blogosaurus!

I do not take credit for any images found in this article. All images come courtesy of the individuals noted in each image caption or below.

Header image courtesy of Anthony Hutchings from his twitter here.

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[i] Button, David J., and Lindsay E. Zanno. “Neuroanatomy of the Late Cretaceous Thescelosaurus Neglectus (Neornithischia: Thescelosauridae) Reveals Novel Ecological Specialisations Within Dinosauria.” Scientific Reports, vol. 13, no. 1, Springer Science and Business Media LLC, Nov. 2023. Crossref, https://doi.org/10.1038/s41598-023-45658-3.

[ii] Varricchio, David J., et al. “First Trace and Body Fossil Evidence of a Burrowing, Denning Dinosaur.” Proceedings of the Royal Society B: Biological Sciences, vol. 274, no. 1616, The Royal Society, Mar. 2007, pp. 1361–68. Crossref, https://doi.org/10.1098/rspb.2006.0443

[iii] Lopes, Renato Pereira, et al. “Megaichnus Igen. Nov.: Giant Paleoburrows Attributed to Extinct Cenozoic Mammals From South America.” Ichnos, vol. 24, no. 2, Informa UK Limited, Sept. 2016, pp. 133–45. Crossref, https://doi.org/10.1080/10420940.2016.1223654.

[iv] Senter, Philip, and Jared Mackey. “Forelimb Motion and Orientation in the Ornithischian Dinosaurs Styracosaurus and Thescelosaurus, and Its Implications for Locomotion and Other Behavior.” Palaeontologia Electronica, Coquina Press, 2023. Crossref, https://doi.org/10.26879/1289.

[v] De Lazaro, Enrico. “One of the Last Surviving Non-Avian Dinosaurs Lived in Burrows, Had Super Senses: Study.” Sci.News: Breaking Science News, 8 Nov. 2023, http://www.sci.news/paleontology/thescelosaurus-neglectus-12431.html.