If paleontologists had a wishlist, it would almost certainly include an overview of two particular phenomena: how dinosaurs interact with each other, and how they began to fly.
The problem is, using fossils to infer such behavior is a tricky business. But a new study from Yale offers a promising entry point – the inner ear of an ancient reptile.
According to the study, the shape of the inner ear offers reliable signs of whether an animal hovered gracefully in the air, flew only intermittently, walked on the ground, or occasionally went swimming. In some cases, the inner ear even indicates whether a species has parented by listening to the high-pitched cries of its babies.
“Of all the structures that can be reconstructed from fossils, the inner ear is perhaps the one that most resembles a mechanical device,” said Yale paleontologist Bhart-Anjan Bhullar, lead author of the new study, published in the journal Science.
“It is entirely dedicated to a particular set of functions. If you are able to reconstruct its shape, you can reasonably draw conclusions about the actual behavior of extinct animals in an almost unprecedented way,” said Bhullar, assistant professor. of Earth and Planetary Sciences and Assistant Curator at the Yale Peabody Museum of Natural History.
Working with colleagues at the American Museum of Natural History, Bhullar and first author Michael Hanson of Yale compiled an inner ear data matrix for 128 species, including modern animals such as birds and crocodiles. , as well as dinosaurs such as Hesperornis, Velociraptor, and the pterosaur Anhanguera.
Hesperornis, an 85 million year old bird-like species that had both teeth and a beak, was the inspiration for the research. The Yale Peabody Museum of Natural History has the world’s only three-dimensional fossil that preserves an inner ear of Hesperornis.
“I was aware of the literature associating cochlear dimensions with hearing ability and semicircular canal structure with locomotion in reptiles and birds, so I became curious as to how Hesperornis would fit into the image, ”said Hanson, a graduate student at Yale.
Hanson and Bhullar analyzed Hesperornis’ inner ear with CT scanning technology to determine its three-dimensional shape.
Then the researchers conducted the same analysis with a variety of other fossils – and current species – to determine if the inner ear provided strong behavioral clues. In many cases, researchers have created 3D models from crushed or partially crushed skull fossils.
After collating the data, the researchers found groups of species with similar inner ear traits. The clusters, they said, correspond to the similar ways that species move and perceive the world.
Several clusters were the result of the structure of the upper part of the inner ear, called the vestibular system. This, says Bhullar, is “the three-dimensional structure that informs you about the maneuverability of the animal. The shape of the vestibular system is a window into understanding moving bodies.”
A vestibular group corresponded to “sophisticated” aviators, species with a high level of aerial maneuverability. This included birds of prey and many songbirds.
Another group centered around “simple” aviators like modern poultry, which fly in fast, straight gusts, and soaring seabirds and vultures. Most importantly, the inner ears of the bird-like dinosaurs called troodontids, pterosaurs, Hesperornis and Archeopteryx “dino-bird” are part of this group.
The researchers also identified a group of species that had a similar lengthening of the lower part of the inner ear – the cochlear system – which has to do with hearing range. This group included a fairly large group of species, including all modern birds and crocodiles, which together form a group called archosaurs, the “dominant reptiles.”
Bhullar said the data suggests that the transformation of the cochlear form in ancestral reptiles coincided with the development of acute location, danger and hatching calls in juveniles.
This implies that the adults used their new function of the inner ear to raise their young, the researchers said.
“All archosaurs sing to each other and have very complex vocal repertoires,” Bhullar said. “We can reasonably infer that the common ancestors of crocodiles and birds also sang. But what we didn’t know was when this happened in the evolutionary lineage that led them. We discovered a cochlea of transition into the archosaur stalk Euparkeria, suggesting that The ancestors of the archosaurs began to sing when they were small, fast predators, much like reptilian foxes. “
The co-authors of the study are Mark Norell and Eva Hoffman of the American Museum of Natural History.
Yale’s Department of Earth and Planetary Sciences, the Yale Institute for Biospheric Studies, the American Museum of Natural History, and the National Science Foundation funded the research.