Inner ear and eye structures shed light on dinosaur parental care and nighttime hunting

May 6, 2021

Photograph of fossilized Shuvuuia deserti skeleton. (Mick Ellison/AMNH)

Two separate studies that investigated the inner ear and eyes of more than 200 species of dinosaurs and their living relatives offer hints about how important hunting, flight and communication behaviors might have evolved.

One study identified several distinct ear-shape patterns that were strongly linked to the way in which a species walked or flew. The researchers also found evidence that the hearing organ, or cochlea, became more sensitive at an early point in reptilian evolution, perhaps allowing dinosaur parents to hear the chirps of their offspring.

For the other report, researchers observed that several dinosaurs had ear and eye adaptations similar to those of modern-day owls, suggesting that these species were nocturnal hunters. 

Both studies were published Thursday in Science.

The components of the inner ear are specialized to serve very precise functions, which means they can offer a unique window into the behavior of extinct animals, says Bhart-Anjan Bhullar, an assistant professor of earth and planetary sciences at Yale University and last author of the first study.

The upper part of the inner ear, which includes the arches of the semicircular canals, detects motion information vital for an animal's sense of balance. The lower part consists of the cochlea. 

Bhullar and his colleagues examined the areas of the skull that housed these structures in a wide variety of reptiles that included modern birds, lizards, turtles and crocodilians. Among the numerous extinct species the researchers considered were the primitive, lizard-like reptile known as prolacerta, velociraptor and other two-legged dinosaurs, and the early bird archaeopteryx.

"The shape variation of the vestibular system was well-explained by differences in the way in which animals move around, and the cochlear system was well-explained by the way in which animals hear," Bhullar said.

He and his team observed that the semicircular canals were shaped very differently in four-legged reptiles, bipedal reptiles and species with limited flight skills, and more advanced fliers such as songbirds and most raptors. The canals grow increasingly pronounced across these three groups, to the point where, in the most aerially skilled birds, the front semicircular canal becomes large enough to wrap around its neighbors, Bhullar says. 

"It's a very complicated structure; it seems to be maximizing the amount of canal that one has," he said. "That's just a very sophisticated-looking instrument compared to the other two, but it's interesting that there's a reversion from that to the unsophisticated version in several different lines of birds."

Among these were "reluctant" fliers such as chickens that take to the air in short bursts and vultures and other birds that soar high above the ground and don't need to maneuver much, as well as the flightless penguins. Intriguingly, birds in this category had inner ears that were similar to those of archaeopteryx and a group of little dinosaurs called troodontids that resemble gracefully built velociraptors.

This group could shed light on how ancient reptiles developed the sensory capabilities necessary to climb trees and take wing. 

"Those [are] especially important, that cluster of very birdlike dinosaurs, in understanding what happened in the last stages toward flight," Bhullar said. "The inner ear shapes are really being driven by the needs of those animals to detect their changes in orientation; it really is being driven by maneuverability."

He and his colleagues saw another trend emerge when they investigated the lower part of the inner ear. Modern birds and crocodilians had an elongated cochlea, a feature that is associated with more sensitive hearing, particularly at high frequencies. Lizards and turtles — which are less closely related to dinosaurs — did not.

When the researchers examined extinct reptiles, they found that the cochlea seemed to have lengthened just once in their evolutionary history, reaching its midpoint in two reptiles known as euparkeria and parasuchus. The structure was distinctively elongated in all the fossils that followed, including those of archosaurs — the group encompassing dinosaurs, modern birds and crocodiles.

"We argue that archosaur ancestors as early as the Triassic had the ability to hear high-pitched sounds, and that the best explanation for that is that they were actively caring for their offspring — something that birds and crocodilians still do," said Eva Hoffman, a Ph.D. student in comparative biology at the American Museum of Natural History and co-author of the study.

Baby birds and crocodilians use high-pitched chirps to catch their parents' attention — as do many mammals, which also have elongated cochleae. It's possible that gaining sensitive hearing not only allowed these animals' reptilian ancestors to better care for their broods, but also helped pave the way for the development of more sophisticated calls and songs, Bhullar says. 

The new paper "provides strong evidence that all dinosaurs vocalized — their inner ears were designed to hear high-pitched baby squeals," said Michael Lee, a professor of evolutionary biology at Flinders University and the South Australian Museum who was not involved in the research. 

However, he notes, many researchers think that parental care was notably lacking in the long-necked sauropods, a group of dinosaurs that includes Brachiosaurus. 

"This study only looked at primitive sauropods," Lee said. "I wonder if the big advanced sauropod dinosaurs ... have the same ear structure."

For the other new study, researchers sought anatomical evidence that some dinosaurs were active at night. 

"Mammals do a lot of their business in the dark," said Roger Benson, a professor of paleobiology at the University of Oxford and last author of the study. "That's very different to birds; very few birds are nocturnal."

Birds that do hunt at night, such as owls and frogmouths, rely on their senses of both sight and hearing to find prey.

"The barn owl has the best hearing among birds [and] it has eyes with super-wide apertures specialized for gathering light," Benson said. "Even in the absence of light, if it knows the environment it can locate and capture prey just based on hearing alone."

To find out whether any dinosaurs had similar talents, he and his colleagues examined the skulls of modern birds such as owls, woodpeckers, cormorants and sparrows, as well as early birds and bipedal dinosaurs, including several tyrannosaurs and velociraptor. The researchers were interested in both the length of the cochlea and in a feature called the scleral ring. This ring of bones is found under the surface of the eyeball in reptiles. 

"It goes around the pupil, so by looking at the scleral ring we can tell the size of the pupil," Benson said.

Only three of the dinosaurs the team examined had skulls that were adapted for a nocturnal lifestyle. Haplocheirus and Shuvuuia belonged to a group known as alvarezsauroids. 

"They're probably animals that would be much more comfortable in the dark than they would in the light," Benson said.

Haplocheirus preceded Shuvuuia by about 90 million years, and the third dinosaur, known as Megapnosaurus, was older still.

"We find evidence for nocturnal specialization even among some of the oldest dinosaurs, even though it was very rare," Benson said.

Shuvuuia deserti, which lived toward the end of the dinosaurs' reign during the late Cretaceous, was particularly impressive. In addition to scleral rings that suggest large pupils ideal for low-light vision, this dinosaur had an "incredibly long" cochlea rivaling that possessed by the barn owl, Benson says. 

"That tells us that Shuvuuia is really doing something that's putting it to its limits in terms of sensory adaptations of vision and hearing together," Benson said. "It's exciting because it shows that combinations of sensory adaptations you see in animals today evolved [independently] in the distant past among dinosaurs."

Shuvuuia probably was similar in weight to a house cat, with very long legs, a long neck with "a small chicken-sized head" on the end, and stubby arms with one large claw ideal for digging, Benson said. His team's findings suggest that these "really weird" animals were well-suited to forage for actively moving prey such as insects and small mammals in the darkness.

"It really underscores the 24/7 nature of dinosaur ecosystems," Benson said. "When the sun went down, you'd get new sets of predators coming out."

He and his team only studied a handful of known dinosaur species, so it's unclear how widespread night-owl behavior might have been.

"Given the huge diversity of dinosaurs, it is reasonable to expect that some of them would be nocturnal, but until now this has been very difficult to prove," Lee said. "What is convincing about [this] study is that two independent sources of evidence — eye proportions and inner ear anatomy — both lead to the same conclusion: The morphologically weird alvarezsauroids were also behaviorally divergent, being nocturnal."

The study, "The early origin of a birdlike inner ear and the evolution of dinosaurian movement and vocalization," published May 6 in Science, was authored by Michael Hanson and Bhart-Anjan S. Bhullar, Yale University and Peabody Museum of Natural History; and Eva A. Hoffman and Mark A. Norell, American Museum of Natural History.

The study, "Evolution of vision and hearing modalities in theropod dinosaurs," published May 6 in Science, was authored by Jonah N. Choiniere and David P. Ford, University of the Witwatersrand; James M. Neenan, University of the Witwatersrand and Oxford University Museum of Natural History; Kimberley E. J. Chapelle, University of the Witwatersrand and American Museum of Natural History; Roger B. J. Benson, University of the Witwatersrand and University of Oxford; Lars Schmitz, Claremont McKenna, Scripps and Pitzer Colleges and Los Angeles County Museum of Natural History; Amy M. Balanoff, American Museum of Natural History and Johns Hopkins University; Mark A. Norell, American Museum of Natural History; Justin S. Sipla, University of Iowa; Justin A. Georgi, Midwestern University; Stig A. Walsh, National Museums Scotland and University of Edinburgh; Xing Xu, Chinese Academy of Sciences and Chinese Academy of Sciences Center for Excellence in Life and Paleoenvironment; and James M. Clark, The George Washington University.

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