Published May 6, 2021
7 min read
It’s a golden age for paleontology: In recent years, scientists have gathered all kinds of clues about the way dinosaurs looked and lived, from awe-inspiring fossil reconstructions to preserved footprints and bite marks on bones. Now, paleontologists are showing that some of the most tantalizing indications of how these extinct animals behaved are enclosed inside their skulls.
A pair of studies published today in the journal Science details a technique using x-ray imaging to study the preserved inner ears and eye sockets of dinosaurs and other prehistoric reptiles. These scans are allowing paleontologists to learn about aspects of dinosaurs’ lives that might otherwise have been lost to time.
“Inner ear shape has always been linked to the lifestyle and behavior of an animal,” says University of Edinburgh paleontologist Julia Schwab, who was not involved with the research. For instance, human inner ears allow us to hear sounds within a specific range of frequencies, from a leaf falling on a sidewalk to a thunderclap, and the inner ear shape is linked to our bipedal species’ sense of balance.
Dinosaur skulls evolved to be thick and protect the brain and associated structures, like the tubular canals of the inner ear, keeping those precious clues intact for tens of millions of years. But those protective bones make it difficult to see the structures encased within. So in one of the studies, led by Yale University graduate student Michael Hanson and his advisor Bhart-Anjan Bhullar, the team created a set of scans from 124 archosaurs—a group that includes dinosaurs, other ancient reptiles, crocodilians, and living birds—spanning 252 million years ago through today.
The results offered more detail than the paleontologists hoped. By identifying patterns in the structures of the animals’ inner ears and eyes, the researchers were able to glean new information about what the dinosaurs could see, and what kind of movement their inner ears were tuned for. This provides another way of tracking the evolution of flight in dinosaurs and, by extension, their modern descendants: birds.
What’s more, the results from both studies offer exceedingly rare clues to what dinosaurs might have sounded like. Dinosaur vocalization is notoriously difficult to reconstruct. The sound-producing organs of their bodies generally decay soon after death, and relatively few species had bony features related to sound. But the anatomy of a dinosaur’s inner ear offers some insight into what the animals could hear, and therefore what sounds they might have made.
“Honestly, I never thought that we’d be taking a crack at dinosaur noises,” Bhullar says.
A skull hardwired for flight
For their research, Bhullar and his team examined scans from a wide array of species, including theropods such as Velociraptor and a stubby-armed animal called Shuvuuia; non-dinosaur reptiles, such as pterosaurs; extinct toothed birds, such as Hesperornis; and living birds and crocodiles for comparison.
When the paleontologists looked at scans of sickle-clawed dinosaurs called troodontids that thrived during the Cretaceous period 145 to 66 million years ago, they found that these dinosaurs had similar inner ears to early, flying birds from the preceding Jurassic period, which began 201 million years ago. That was something of a surprise, given most known troodontids were terrestrial dinosaurs that didn’t fly.
But the similarities in the inner ear reveal an evolutionary trait necessary for airborne creatures, raising new questions about how flight evolved.
Bhullar hypothesizes that troodontids, which were about the size of turkeys, inherited ears suited to flight from a more ancient common ancestor with birds—perhaps a flying dinosaur, similar to feathered species such as Anchiornis, that lived 165 million years ago. And an inner ear adapted to the complex movements of flight, helping animals balance while in the air, could have had other uses on the ground.
“I do think that even non-flighted dinosaurs that were closely related to birds were moving around in complex ways,” Bhullar says, such as climbing trees or running up inclines. In dinosaurs closely related to birds, these behaviors may have helped the inner ear develop in such a way that allowed for flight—an activity that requires complex movements and limb control.
Nighttime stalkers
Not all bird-like dinosaurs moved like their avian relations, however. Some dinosaurs, the researchers found, moved and likely hunted in ways that run counter to paleontological expectations.
The turkey-size dinosaur Shuvuuia, for example, has long been a mystery to paleontologists. Known for its short arms tipped with large, single claws and toothless or nearly toothless jaws, this genus belongs to a group of bipedal theropods called alvarezsaurs. Bhullar and colleagues were surprised to find that Shuvuuia has an inner ear similar to that in four-legged animals with relatively simple locomotion.
The second Science study may offer insight into Shuvuuia’s odd inner ear. This study looked at both the inner ears and the eyes of dinosaurs to gain insight into the behavior of the extinct animals.
“Both studies complement each other,” says study author and Los Angeles County Museum of Natural History biologist Lars Schmitz—and together they indicate that Shuvuuia truly was an odd dinosaur.
Shuvuuia had long inner ear canals, broadening the range of the dinosaur’s hearing. Schmitz and colleagues propose this dinosaur had excellent hearing, comparable to the auditory ability of modern barn owls. Such precise hearing, combined with Shuvuuia’s large eyes, suggest this dinosaur was active at night.
Precisely what Shuvuuia was hunting is unclear—perhaps small mammals, or social insects like ants. But Schmitz notes that there are many reasons a dinosaur might have evolved to prefer the darker hours. “Body size, foraging style, climate, competition,” all matter, Schmitz notes.
Dinosaur song
The new analyses also led to a greater understanding of how these animals may have communicated with one another. The researchers found that ancestors and early relatives of dinosaurs evolved a longer region of the inner ear called the cochlea, which is associated with hearing high-frequency sounds.
The most likely reason, the paleontologists propose, is that this adaptation allowed adult animals to hear the squeaks and chirps of their hatchlings, similar to the attentive parenting of modern-day alligators and crocodiles. The singing birds of today therefore may trace their vocal abilities to the squeaks that tiny, scaly reptiles made as they hatched over 200 million years ago.
“We gently suggest that modern bird song, in all its mellifluous glory, is a retention in adults of juvenile high-pitched chirps,” Bhullar says.
This wealth of information about dinosaur behavior, gleaned by peering into fossilized skulls, represents the rapidly progressing technologies used to study the prehistoric past.
“I think the availability of modern imaging and rendering techniques is a big factor,” Schmitz says, adding that discoveries about the sensory systems of modern animals can also help paleontologists better examine and understand the anatomy and behavior of long-extinct species, which means even as living animals inform what we know about dinosaurs, the dinosaurs are changing how we see the creatures around us.