Uncovering Advanced Hearing in Early Mammal Relatives Through Fossil Analysis

One of the most significant milestones in the evolution of modern mammals was the development of highly sensitive hearing. The middle ear, which includes an eardrum and several small bones, enables mammals to detect a wide range of frequencies and volumes. This ability was particularly crucial for early mammal ancestors, which were mostly nocturnal and had to navigate a world dominated by dinosaurs.

New research from paleontologists at the University of Chicago has revealed that this advanced mode of hearing evolved much earlier than previously believed. By using detailed CT scans of the skull and jawbones of Thrinaxodon liorhinus, a 250-million-year-old precursor to mammals, the team applied engineering methods to simulate how different sound pressures and frequencies would affect its anatomy.

The findings suggest that Thrinaxodon likely had an eardrum large enough to hear airborne sounds effectively, nearly 50 million years earlier than previously thought. "For almost a century, scientists have been trying to figure out how these animals could hear," said Alec Wilken, a graduate student who led the study published in PNAS. "These ideas have captivated the imagination of paleontologists who work in mammal evolution, but until now we haven't had very strong biomechanical tests."

"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 that showed features transitioning from reptiles to mammals. These included specialized teeth, changes to the palate and diaphragm to improve breathing and metabolism, and possibly warm-bloodedness and fur.

In early cynodonts like 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, a key development in the evolution of modern mammals.

Fifty years ago, Edgar Allin, a paleontologist at the University of Illinois Chicago, proposed that cynodonts like Thrinaxodon had a membrane suspended across a hooked structure on the jawbone that served as a precursor to the modern eardrum. Before this, scientists believed that early cynodonts heard through bone conduction or "jaw listening," where they placed their mandibles on the ground to pick up vibrations.

While the eardrum idea was intriguing, there was no way to definitively test whether such a structure could function for airborne sound detection.

Turning Fossils into an Engineering Problem

Modern imaging tools like CT scanning have transformed paleontology, enabling scientists to extract vast amounts of information that couldn't be obtained through traditional methods. Wilken and his advisors, Zhe-Xi Luo and Callum Ross, used 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 provided a detailed reconstruction of its skull and jawbones, allowing the team to analyze the dimensions, shapes, angles, and curves necessary to determine how a potential eardrum might function.

They then used a software tool called Strand7 to perform finite element analysis, an approach commonly used in engineering to predict stresses on structures like bridges, aircraft, and buildings. The team simulated how Thrinaxodon’s anatomy would respond to different sound pressures and frequencies, using data on the thickness, density, and flexibility of bones, ligaments, muscles, and skin from living animals.

The results showed that Thrinaxodon could hear airborne sounds effectively through an eardrum tucked into a crook on its jawbone, more efficiently than through bone conduction. The size and shape of its eardrum would have produced the right vibrations to move the ear bones and stimulate auditory nerves, detecting sound frequencies.

While Thrinaxodon still 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 emphasized that 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."

More information: Wilken, Alec T. et al, Biomechanics of the mandibular middle ear of the cynodont Thrinaxodon and the evolution of mammal hearing, Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2516082122. doi.org/10.1073/pnas.2516082122