New research by palaeontologists shows the development of highly sensitive hearing evolved much earlier – nearly 50 million years – than previously thought.
The University of Chicago scientists said one of the most important steps in the evolution of modern mammals was the development of highly sensitive hearing. They said the middle ear of mammals, with an eardrum and several small bones, allowed hearing of a broad range of frequencies and volumes.
This was a big help to early, mostly nocturnal mammal ancestors as they tried to survive alongside dinosaurs, the scientists said.
They worked with CT scans of the skull and jawbones of a 250-million-year-old mammal predecessor, Thrinaxodon liorhinus and used engineering methods to simulate the effects of different sound pressures and frequencies on its anatomy.
Their models showed the creature likely had an eardrum large enough to hear airborne sound effectively, nearly 50 million years before scientists previously thought this had evolved in early mammals.
“For almost a century, scientists have been trying to figure out how these animals could hear,” said Mr Alec Wilken, a graduate student who led the study.
“These ideas have captivated the imagination of palaeontologists 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.”
The study, published in PNAS on 8 December 2025, tested a 50-year-old hypothesis.
The scientists said Thrinaxodon was a cynodont, a group of animals from the early Triassic period with features starting to transition from reptiles to mammals including specialised teeth, changes to the palate and diaphragm to improve breathing and metabolism.
“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,” the scientists said.
They said that 50 years ago at the University of Illinois palaeontologist Mr Edgar Allin 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 early cynodonts heard through bone conduction, or via “jaw listening” where they set their mandibles on the ground to pick up vibrations, they added. While the eardrum idea was fascinating, there was no way to definitively test if such a structure could work to hear airborne sounds.
Wilken and his advisors, Professor Zhe-Xi Luo and Professor Callum Ross, scanned a Thrinaxodon specimen from the University of California Berkeley Museum of Paleontology.
The 3D model gave them a detailed reconstruction of its skull and jawbones, with all dimensions, shapes, angles and curves they needed to determine how a potential eardrum might function.
They used a software tool to simulate how its anatomy would respond to different sound pressures and frequencies, using known properties about the thickness, density, and flexibility of bones, ligaments, muscles, and skin from living animals.
They found Thrinaxodon, with an eardrum tucked into a crook on its jawbone, could hear that way more effectively than through bone conduction. They said 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 would have relied on some jaw listening, the eardrum was already responsible for most of its hearing.
