In a groundbreaking study, paleontologists from the University of Chicago have revealed that the evolution of sensitive hearing in mammals occurred much earlier than previously believed. By employing advanced engineering simulations, researchers discovered that Thrinaxodon liorhinus, a 250-million-year-old ancestor of modern mammals, likely possessed the ability to hear airborne sounds effectively, challenging long-held assumptions about the timeline of mammalian auditory evolution.
The study, published in the Proceedings of the National Academy of Sciences (PNAS), utilized detailed CT scans of Thrinaxodon’s skull and jawbones to simulate the effects of various sound pressures and frequencies on its anatomy. The findings suggest that this ancient creature had an eardrum large enough to detect airborne sounds nearly 50 million years earlier than scientists had previously thought.
“For almost a century, scientists have been trying to figure out how these animals could hear,” said Alec Wilken, the graduate student who led the study. “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 early Triassic period animals that exhibited transitional features from reptiles to mammals. These included specialized teeth, changes to the palate and diaphragm for improved breathing and metabolism, and possibly warm-bloodedness and fur. In early cynodonts, the ear bones were attached to their jawbones, a configuration that later evolved into the distinct middle ear of modern mammals.
Fifty years ago, Edgar Allin, a paleontologist at the University of Illinois Chicago, speculated that cynodonts like Thrinaxodon had a membrane across a hooked jawbone structure, serving as a precursor to the modern eardrum. This theory challenged the prevailing belief that early cynodonts relied on bone conduction or “jaw listening” to hear. However, until now, there was no definitive way to test whether such a structure could function to hear airborne sounds.
Turning Fossils into an Engineering Problem
The advent of modern imaging tools like CT scanning has revolutionized paleontology, enabling scientists to extract information that physical specimens alone could not provide. Wilken and his advisors, Professors Zhe-Xi Luo and Callum Ross, utilized a Thrinaxodon specimen from the University of California Berkeley Museum of Paleontology, scanning it in UChicago’s PaleoCT Laboratory. The resulting 3D model provided a highly detailed reconstruction of its skull and jawbones, essential for determining the potential function of an eardrum.
Using the software tool Strand7, the team performed finite element analysis, a method typically used for complex engineering challenges such as predicting stresses on bridges and aircraft. This approach allowed them to simulate how Thrinaxodon’s anatomy would respond to different sound pressures and frequencies, using known properties of bones, ligaments, muscles, and skin from living animals.
The results were conclusive: Thrinaxodon, with an eardrum positioned on its jawbone, could hear more effectively through airborne sound than through bone conduction. The eardrum’s size and shape would have produced vibrations sufficient to move the ear bones and stimulate auditory nerves, indicating that it was primarily responsible for 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,” said Luo. “This software simulation showed us that vibration through sound is essentially the way this animal could hear.”
Implications and Future Directions
The study not only challenges previous timelines of mammalian hearing evolution but also demonstrates the potential of engineering techniques in paleontological research. By transforming an ancient biological question into an engineering problem, the researchers were able to provide new insights into the evolutionary history of mammals.
“That’s why this is such a cool problem to study,” Wilken explained. “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 the University of Chicago, the National Institutes of Health, and the National Science Foundation, with Chelsie C. G. Snipes as an additional author.
As the field of paleontology continues to integrate advanced technologies, further research may uncover even more about the evolutionary adaptations that have shaped the sensory capabilities of modern mammals. This study marks a significant step in understanding the complex history of mammalian hearing and opens new avenues for exploring the intricate connections between anatomy and function in ancient species.
