A 250-million-year-old fossil reveals the origins of mammal hearing

New findings from paleontologists at the University of Chicago suggest that this advanced form of hearing appeared far earlier than scientists once believed. Using detailed CT scans of the skull and jaw of Thrinaxodon liorhinus, a mammal ancestor that lived about 250 million years ago, the researchers applied engineering-based simulations to test how sound would have traveled through its anatomy. Their results indicate that Thrinaxodon probably had an eardrum large enough to detect airborne sound efficiently, pushing the origin of this trait back by nearly 50 million years.

"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 recently 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."

Revisiting a Longstanding Idea About Early Mammal Hearing

Thrinaxodon belonged to a group called cynodonts, animals from the early Triassic period that show a mix of reptile and mammal traits. These included specialized teeth, changes in the palate and diaphragm that supported more efficient breathing and metabolism, and likely features such as warm-bloodedness and fur. In early cynodonts, including Thrinaxodon, the ear bones (malleus, incus, stapes) were still connected to the jaw. Much later in evolution, these bones separated to form the distinct middle ear seen in modern mammals, a shift considered critical to improved hearing.

About 50 years ago, paleontologist Edgar Allin of the University of Illinois Chicago proposed that cynodonts like Thrinaxodon may have had a membrane stretched across a hooked part of the jawbone, serving as an early version of the mammalian eardrum. At the time, most researchers thought these animals primarily detected sound through bone conduction, or through so-called "jaw listening," by placing their lower jaws against the ground to sense vibrations. Allin's idea was intriguing, but there was no practical way to test whether such a membrane could actually transmit airborne sound.

Turning Ancient Fossils Into Digital Test Subjects

Advances in imaging technology have transformed paleontology, allowing scientists to extract detailed information from fossils without damaging them. Wilken and his colleagues, Zhe-Xi Luo, PhD, and Callum Ross, PhD, both Professors of Organismal Biology and Anatomy, scanned a well-studied Thrinaxodon specimen from the University of California Berkeley Museum of Paleontology at UChicago's PaleoCT Laboratory. The scans produced a high-resolution 3D model of the skull and jaw, capturing precise shapes, angles, and dimensions needed to evaluate how a potential eardrum might work.

The team then used engineering software called Strand7 to run a finite element analysis. This method divides a complex structure into many small components, each with specific physical properties. It is commonly used to study how bridges bear weight, how aircraft handle stress, or how heat moves through engines. In this case, the researchers simulated how Thrinaxodon's skull and jaw would respond to different sound pressures and frequencies, drawing on known data about the thickness, density, and flexibility of bones, ligaments, muscles, and skin in living animals.

Evidence for Early Airborne Hearing

The simulations produced a clear result. An eardrum positioned within a curved section of the jawbone would have allowed Thrinaxodon to hear airborne sounds far more effectively than relying on bone conduction alone. The modeled size and shape of the membrane generated vibrations strong enough to move the ear bones, stimulate auditory nerves, and detect a range of sound frequencies. Although jaw-based vibration sensing likely still played a role, the eardrum would have handled most of the animal's 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 modern tools finally made it possible to test a decades-old question. "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, titled "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 also an author.