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Nocturnal dinosaurs: Small theropod’s night vision and superb hearing suggest it was a moonlight predator
The fossils of _Shuvuuia deserti_ represent a small predatory creature with exceptional night vision and hearing. Today, barn owls, bats, leopards and many other animals rely on their sharp senses to live and hunt under the dim starlight. These nocturnal specialists avoid the competition of 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 senses of the ancestors of bird dinosaurs, our team of paleontologists and paleobiologists scoured research papers and museum collections for fossils preserving the delicate structures of the eyes and ears. And we found some. Using scans of fossilized dinosaur skulls, in an article published in the journal Science on May 6, 2021, we describe the most compelling evidence to date for nocturnal dinosaurs. Two fossil species – Haplocheirus sollers 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. Shuvuuia deserti had acute hearing and low light vision that would have enabled her to hunt at night. Viktor Radermaker, CC BY-ND 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 the Tyrannosaurus rex and modern birds. But to date, the fossils of only 12 species of theropods have included the ocular structures that 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. The eye socket – and in particular the sclerical ring – of S. deserti shows an eye with a very large pupil capable of letting in large amounts of light. Mick Ellison / American Museum of Natural History, CC BY-ND Eye bones constructed for night vision Scleral ossicles are thin, rectangular bony plates that form a ring-like structure surrounding the pupils of lizards as well as birds and their ancestors – the dinosaurs. Scleral rings define the largest possible size of an animal’s pupil and can tell you how well 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. This owl skull clearly shows the large scleral ring which helps animals see in the dark. David J. Stang / WikimediaCommons, CC BY-SA Since the individual bony ossicles in these rings collapsed after these animals died over 60 million years ago, our team scans the fossils and then digitally reconstructed eyes. Of all the theropods we examined, H. sollers and S. deserti had some of the proportionately largest pupils. The pupil of S. deserti made up more than half of his eye, very similar to the night vision specialists who today live like geckos and night chasms. 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 – over 90% – that H. sollers and S. deserti are nocturnal. But these weren’t the only two theropods our team looked at. Our scan also found a few other likely nocturnal specialists – such as Megapnosaurus kayentakatae – as well as daylight specialists like Almas ukhaa. But we also found some species – like Velociraptor mongoliensis – with sight apparently suited for medium light levels. This could suggest that they were hunting at dawn or dusk. The mold in the internal ear canal of a barn owl (left) and S. deserti (right) are almost identical, suggesting that the little dinosaur had incredible hearing. Jonah Choiniere / Wits University, CC BY-ND Amazing Dinosaur Ears In nocturnal animals 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 in living mammals and birds show that the longer this channel, the wider the frequency range an animal can hear and the better it can hear very faint sounds. Our scans showed that S. deserti had an extremely elongated internal auditory 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 purposes. 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 that of S. deserti. The Life of a Nocturnal Dinosaur By studying the sensory abilities of dinosaurs, paleontologists like us not only learn which species have 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 herbivorous Protoceratops mongoliensis both lived in the same habitat and had some night vision. Paleontologists are currently not aware of the full range of animals that shared the extreme nightlife of S. deserti in the ancient deserts of Mongolia – it is rare to find fossils with the appropriate bones that allow paleontologists to study their meaning. 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. [Get the best of The Conversation, every weekend. Sign up for our weekly newsletter.]This article is republished from The Conversation, a nonprofit news site dedicated to sharing ideas from academic experts. It was written by: Lars Schmitz, Scripps College; Jonah Choiniere, University of the Witwatersrand, and Roger Benson, University of Oxford. Read more: Giant ‘toothed’ birds flew over Antarctica 40 to 50 million years agoHow many Tyrannosaurus rex have walked on Earth? Jonah Choiniere receives funding from the National Research Foundation of South Africa. Roger Benson receives funding from the European Research Council, the National Council for Environmental Research and the Leverhulme Trust. Lars Schmitz does not work, consult, own stock or receive funding from any company or organization that would benefit from this article, and has not disclosed any relevant affiliation beyond his academic appointment.