Small theropod’s night vision and superb hearing suggest it was a moonlight predator
Today, barn owls, bats, leopards and many other animals rely on their acute senses to live and hunt under the dim starlight. These night specialists avoid competition during daylight hours, hunting their prey under the cloak of darkness, often using a combination of night vision and acute hearing.
But was there nightlife 100 million years ago? In a world without owls and leopards, did dinosaurs work at night? If so, what senses did they use to find food and avoid predators in the dark? To better understand the meanings of the ancestors of bird dinosaurs, our team of paleontologists and paleobiologists scoured research papers and museum collections for fossils that preserved the delicate structures of the eyes and ears. And we found some.
Using scans of fossilized dinosaur skulls, in a paper published in the journal Science on May 6, 2021, we describe the most convincing evidence to date for nocturnal dinosaurs. Two fossil species – Haplocheirus soils and Shuvuuia deserti – probably had very good night vision. But our work also shows that S. deserti also had incredibly sensitive hearing similar to that of modern day owls. This is the first time that these two traits have been discovered in the same fossil, suggesting that this small desert dinosaur that lived in ancient Mongolia was likely a specialized night hunter of insects and small mammals.
In search of theropods
While studying the fossilized eye bones, one of us, Lars Schmitz, had already discovered that some small predatory dinosaurs may have hunted at night. Most of these potentially nocturnal hunters were theropods, the group of three-toed dinosaurs that includes Tyrannosaurus rex and modern birds. But to date, the fossils for only 12 species of theropods included the eye structures which can inform paleontologists about night vision.
Our team identified four other theropod species with clues for their sense of vision – for a total of 16. We then searched for fossils that preserve the structures of the inner ear and found 17 species. Interestingly, for four species we were able to get measurements for the eyes and ears.
Eye bones designed for night vision
The scleral ossicles are thin rectangular bony plates that form a ring-shaped structure surrounding the pupils lizards as well as birds and their ancestors – dinosaurs. Scleral rings define the largest possible size of an animal’s pupil and can tell you how great that animal can see at night. The larger the pupil relative to the size of the eye, the better a dinosaur can see in the dark.
Since the individual bony ossicles of these rings collapsed after the death of these animals over 60 million years ago, our team performed scans of the fossils and then digitally reconstructed the eyes. Of all the theropods we have examined, H. sollers and S. deserti was among the proportionately taller students.
S. desertiThe pupil’s pupil made up more than half of his eye, very similar to the night vision specialists who today live as geckos and nightjars. Our team then compared the fossils to 55 living species of lizards and 367 species of birds with known patterns of day or night activity. According to the statistical analyzes carried out by our team, there is a very high chance – greater than 90% – that H. sollers and S. deserti were nocturnal.
But these weren’t the only two theropods our team looked at. Our analysis also found a few other probable nocturnal specialists – such as Megapnosaurus kayentakatae – as well as daylight specialists such as Almas ukhaa. But we also found species – like Velociraptor mongoliensis – with a view apparently suitable for medium light levels. This could suggest that they were hunting at dawn or dusk.
Amazing dinosaur ears
In the nocturnal animals of today, hearing can be as important as keen eyesight. To determine how well these extinct dinosaurs could hear, we scanned the skulls of 17 fossil theropods to decipher the structure of their internal ears, then compared our scans to the ears of modern animals.
All vertebrates have a tube-shaped duct called a cochlea deep in their inner ear. Studies on living mammals and birds show that the longer this channel, wider range of frequencies an animal can hear and the better they can hear very low sounds.
Our scans have shown that S. deserti had an extremely elongated internal ear canal for its size – also similar to that of the living barn owl and proportionately much longer than all of the 88 other living bird species we analyzed for comparison. Based on our measurements, among dinosaurs, we found that predators generally had better hearing than herbivores. Several predators – including V. mongoliensis – also had moderately elongated inner ears, but none rivaled S. deserti‘s.
The life of a nocturnal dinosaur
By studying the sensory abilities of dinosaurs, paleontologists like us not only learn which species roamed at night, but can also begin to infer how these dinosaurs lived and shared their resources.
S. deserti had extreme night vision and sensitive hearing, and this little dinosaur probably used his incredible senses to hunt for prey at night. He could probably hear and follow the rustle from a distance before visually detecting his prey and digging it out of the ground with his short, single-claw arms. In the dry, desert habitats of millions of years ago, it might have been an evolutionary benefit to be active in the cooler nighttime temperatures.
But according to our analysis, S. deserti was not the only dinosaur active at night. Other dinosaurs like V. mongoliensis and the plant eater Protoceratops mongoliensis both lived in the same habitat and had some night vision.
Paleontologists currently don’t know the full range of animals that shared S. desertiThe extreme nightlife lifestyle in the ancient deserts of Mongolia – it’s rare to find fossils with intact right bones that allow paleontologists to study their senses. However, the presence of a specialized night gatherer makes it clear that, just like today, some dinosaurs avoided the dangers and competition of daylight and wandered under the stars.
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