Engineering simulations rewrite the timeline of the evolution of hearing in mammals

One of the most important steps in the evolution of modern mammals was the development of highly sensitive hearing. The middle ear of mammals, with an eardrum and several small bones, allows us to hear a broad range of frequencies and volumes, which was a big help to early, mostly nocturnal mammal ancestors as they tried to survive alongside dinosaurs. 

New research by paleontologists from the University of Chicago shows that this modern mode of hearing evolved much earlier than previously thought. Working with detailed CT scans of the skull and jawbones of Thrinaxodon liorhinus, a 250-million-year-old mammal predecessor, they used engineering methods to simulate the effects of different sound pressures and frequencies on its anatomy. Their models show that the creature likely had an eardrum large enough to hear airborne sound effectively, nearly 50 million years before scientists previously thought this evolved in early mammals. 

“For almost a century, scientists have been trying to figure out how these animals could hear. These ideas have captivated the imagination of paleontologists who work in mammal evolution, but until now we haven’t had very strong biomechanical tests,” said Alec Wilken, a graduate student who led the study, which was published this week in PNAS. “Now, with our advances in computational biomechanics, we can start to say smart things about what the anatomy means for how this animal could hear.” 

Testing a 50-year-old hypothesis 

Thrinaxodon was a cynodont, a group of animals from the early Triassic period with features beginning to transition from reptiles to mammals, like specialized teeth, changes to the palate and diaphragm to improve breathing and metabolism, and probably warm-bloodedness and fur. In early cynodonts, including Thrinaxodon, the ear bones (malleus, incus, stapes) were attached to their jawbones; later, these bones separated from the jaw to form a distinct middle ear, considered a key development in the evolution of modern mammals. 

Fifty years ago, Edgar Allin, a paleontologist at the University of Illinois Chicago, first speculated that cynodonts like Thrinaxodon had a membrane suspended across a hooked structure on the jawbone that was a precursor to the modern eardrum. Until then, scientists who studied mammal evolution mostly believed that early cynodonts heard through bone conduction, or via so-called “jaw listening” where they set their mandibles on the ground to pick up vibrations. While the eardrum idea was fascinating, there was no way to definitively test if such a structure could work to hear airborne sounds.  

Turning fossils into an engineering problem 

Modern imaging tools like CT scanning have revolutionized the field of paleontology, allowing scientists to unlock a wealth of information that wouldn’t have been possible through studying physical specimens alone. Wilken and his advisors, Zhe-Xi Luo, PhD, and Callum Ross, PhD, both Professors of Organismal Biology and Anatomy, took a well-known Thrinaxodon specimen from the University of California Berkeley Museum of Paleontology and scanned it in UChicago’s PaleoCT Laboratory. The resulting 3D model gave them a highly detailed reconstruction of its skull and jawbones, with all the dimensions, shapes, angles and curves they needed to determine how a potential eardrum might function. 

Next, they used a software tool called Strand7 to perform finite element analysis, an approach that breaks down a system into smaller parts with different physical characteristics. Such tools are usually used for complex engineering problems, like predicting stresses on bridges, aircraft, and buildings, or analyzing heat distribution in engines. The team used the software to simulate how the anatomy of Thrinaxodon would respond to different sound pressures and frequencies, using a library of known properties about the thickness, density, and flexibility of bones, ligaments, muscles, and skin from living animals. 

The results were loud and clear: Thrinaxodon, with an eardrum tucked into a crook on its jawbone, could definitely hear that way much more effectively than through bone conduction. The size and shape of its eardrum would have produced the right vibrations to move the ear bones and generate enough pressure to stimulate its auditory nerves and detect sound frequencies. While it still would have relied on some jaw listening, the eardrum was already responsible for most of its hearing. 

“Once we have the CT model from the fossil, we can take material properties from extant animals and make it as if our Thrinaxodon came alive,” Luo said. “That hasn’t been possible before, and this software simulation showed us that vibration through sound is essentially the way this animal could hear.” 

Wilken said the new technology allowed them to answer an old question by turning it into an engineering problem. “That’s why this is such a cool problem to study,” he said. “We took a high concept problem—that is, ‘how do ear bones wiggle in a 250-million-year-old fossil?’--and tested a simple hypothesis using these sophisticated tools. And it turns out in Thrinaxodon, the eardrum does just fine all by itself.” 

The study, “Biomechanics of the mandibular middle ear of the cynodont Thrinaxodon and the evolution of mammal hearing,” was supported by UChicago, the National Institutes of Health, and the National Science Foundation. Chelsie C. G. Snipes from UChicago was an additional author. 

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