In a groundbreaking study, paleontologists from the University of Chicago have discovered that the evolution of mammalian hearing occurred much earlier than previously believed. By utilizing advanced engineering simulations, researchers have demonstrated that Thrinaxodon liorhinus, a 250-million-year-old mammal ancestor, possessed the ability to hear airborne sounds, challenging long-held assumptions about the timeline of this critical evolutionary development.
The research team employed detailed CT scans to analyze the skull and jawbones of Thrinaxodon, applying engineering methods to simulate the effects of various sound pressures and frequencies. Their findings revealed that this ancient creature likely had an eardrum capable of detecting airborne sound, nearly 50 million years before scientists had thought such an adaptation appeared in early mammals.
Revisiting a Century-Old Hypothesis
The study’s lead author, Alec Wilken, a graduate student at the University of Chicago, expressed the significance of these findings. “For almost a century, scientists have been trying to figure out how these animals could hear,” he stated. “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.”
Thrinaxodon belonged to a group of early Triassic period animals known as cynodonts, which exhibited transitional features from reptiles to mammals. These included specialized teeth, changes to the palate and diaphragm, and likely warm-bloodedness and fur. In early cynodonts, the ear bones were attached to the jawbones, a configuration that later evolved into a distinct middle ear in modern mammals.
Testing the Eardrum Theory
In the 1970s, Edgar Allin, a paleontologist at the University of Illinois Chicago, proposed that cynodonts like Thrinaxodon had a membrane across a jawbone structure that functioned as a precursor to the modern eardrum. Until now, this theory lacked definitive biomechanical testing to support it. Scientists previously believed early cynodonts relied on bone conduction or “jaw listening” to detect vibrations.
Transforming Fossils into Engineering Challenges
Modern imaging technologies, such as CT scanning, have revolutionized paleontology by providing insights unattainable from physical specimens alone. Wilken and his advisors, Zhe-Xi Luo, PhD, and Callum Ross, PhD, used a well-preserved Thrinaxodon specimen from the University of California Berkeley Museum of Paleontology, scanning it in UChicago’s PaleoCT Laboratory. The resulting 3D model offered a detailed reconstruction of its skull and jawbones, enabling the team to explore the potential functionality of an eardrum.
The researchers employed Strand7 software to perform finite element analysis, a technique that breaks down systems into smaller parts with distinct physical characteristics. This method, typically used for complex engineering problems, allowed the team to simulate how Thrinaxodon’s anatomy would respond to different sound pressures and frequencies. They utilized a library of known properties about the thickness, density, and flexibility of bones, ligaments, muscles, and skin from living animals.
“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 explained. “That hasn’t been possible before, and this software simulation showed us that vibration through sound is essentially the way this animal could hear.”
Implications for Understanding Mammalian Evolution
The results were conclusive: Thrinaxodon, with an eardrum nestled in its jawbone, could hear airborne sounds more effectively than through bone conduction. While it still relied on some jaw listening, the eardrum accounted for the majority of its hearing capabilities. This discovery not only supports Allin’s hypothesis but also reshapes our understanding of mammalian auditory evolution.
Wilken emphasized the significance of the new technology in addressing longstanding questions. “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 published in the Proceedings of the National Academy of Sciences (PNAS). It received support from the University of Chicago, the National Institutes of Health, and the National Science Foundation, with Chelsie C. G. Snipes from UChicago as an additional author.
This research not only sheds light on the evolutionary timeline of mammalian hearing but also exemplifies how modern technology can unlock secrets of the ancient past, offering a glimpse into the lives of creatures that roamed the Earth millions of years ago.
