New research demonstrates that the brain of Thescelosaurus neglectus, a small plant-eating neornithischian dinosaur that lived just before the end-Cretaceous mass extinction event, was relatively small compared to most other neornithischians, suggesting cognitive capabilities within the range of living reptiles. Other traits include a narrow hearing range, with limited ability to distinguish high frequencies, paired with an unusually well-developed sense of smell (olfaction) and vestibular sensitivity.
Thescelosaurus neglectus lived in what is now North America just before the end-Cretaceous mass extinction, around 66 million years ago.
This dinosaur was a small (3.6 m, or 11.8 feet, in length) but heavy (340 kg) herbivore.
In the new research, paleontologists David Button and Lindsay Zanno used a CT scanner to reconstruct soft tissues in the skull of Thescelosaurus neglectus — such as the brain and inner ear — that were lost to the fossilization process.
Comparing these sensory structures to other dinosaurs and their living relatives allowed the researchers to determine the relative size of the dinosaur’s brain, as well as what her senses of smell, hearing, and balance were like.
“The irony is that paleontologists generally think of these animals as pretty boring,” said Dr. Zanno, a paleontologist at the North Carolina Museum of Natural Sciences, and co-author of the work.
“When we first looked at our results we thought, yeah, this animal is plain as toast. But then we took a big step back and realized there was something unique about the combination of the dinosaur’s sensory strengths and weaknesses.”
The researchers determined that Thescelosaurus neglectus’ hearing range was limited.
The dinosaur could only hear about 15% of the frequencies humans can detect, and between 4 and 7% of what dogs and cats can hear.
In particular, Thescelosaurus neglectus was bad at hearing high-pitched sounds.
“We found that Thescelosaurus neglectus heard low frequency sounds best, and that the range of frequencies it could hear overlaps with T. rex,” Dr. Zanno said.
“This doesn’t tell us they were adapted to hearing T. rex vocalize, but it certainly didn’t hurt them to know when a major predator was tooling about in the area.”
“More interesting to us was the fact that these particular deficiencies are often associated with animals that spend time underground.”
Thescelosaurus neglectus balanced its poor hearing with an excellent sense of smell.
“We found that the olfactory bulbs — the regions of the brain that process smell — were very well developed in Thescelosaurus neglectus,” said Dr. Button, a paleontologist at the University of Bristol.
“They were relatively larger than those of any other dinosaur we know of so far, and similar to those of living alligators, which can smell a drop of blood from miles away.”
“Thescelosaurus neglectus may have used its similarly powerful sense of smell to instead find buried plant foods like roots and tubers.”
“It also had an unusually well-developed sense of balance, helping it to pinpoint its body position in 3D space, another trait often found in burrowing animals.”
Thescelosaurus neglectus’ poor cognitive and hearing abilities, coupled with powerful arms and legs and overdeveloped senses of smell and balance, are all features characteristic of animals that spend time underground and/or engage in digging behaviors today.
“While we can’t say definitively that these animals lived part of their lives underground, we know that their ancestors did,” Dr. Button said.
“This fact, together with their unique combination of sensory abilities, strongly suggests Thescelosaurus neglectus engaged in similar behaviors.”
The results were published this week in the journal Scientific Reports.
D.J. Button & L.E. Zanno. 2023. Neuroanatomy of the Late Cretaceous Thescelosaurus neglectus (Neornithischia: Thescelosauridae) reveals novel ecological specialisations within Dinosauria. Sci Rep 13, 19224; doi: 10.1038/s41598-023-45658-3