Modern mammals have unique hearing abilities, able to detect a wide range of loudness and frequencies using features of the middle ear, such as the eardrum and several small bones.

A new study by paleontologists at the University of Chicago reveals that these physical traits started appearing nearly 50 million years earlier than we thought.

They found evidence of this in 250-million-year-old fossils of mammalian ancestors. Thrinaxodon riolinus. use computed tomography scan They created a 3D model of the animal’s skull and jaw so they could simulate how it moves. Trinaxodon‘s anatomy may have responded to different sound pressures and frequencies by using engineering software to observe how the bones “wiggled” in response.

Related: Our ears are still trying to rotate to hear better, research finds

Trinaxodon lived between Early Triassicbefore the first dinosaur. it is Cynodont – A relative of early mammals – Their body is somewhere between a lizard and a fox.

Some of its genes follow the same blueprint that modern mammals have today, and this new study suggests that the structure of its hearing is also one it shares with us.

Early cynodonts had ear bones (malleus, incus, and stapes) attached to the jaw. In later species, these small pieces were eventually separated from the jaw, apparently forming the mammalian middle ear.

Before the advent of the middle ear and its associated “tympanic membrane” hearing ability, animals relied on bone-conducted sound, with nerves transmitting signals from jawbone vibrations to the brain.

Paleontologists have speculated for decades that Trinaxodon It may be the “missing link” in the evolution of mammalian hearing. 1975, University of Wisconsin anatomist Edgar Allyn proposed that Trinaxodon An early form of the tympanic membrane may have extended across the still-attached hook-like bone structure protruding from the jaw.

But at the time, Allyn didn’t have the technology to prove his suspicions. So researchers in the new study used engineering software to revisit the question.

“For almost a century, scientists have been trying to figure out how these animals are able to hear. These ideas have captivated the imaginations of paleontologists who study mammalian evolution, but until now very strong biomechanical tests have not been conducted.” say Alec Wilken, evolutionary scientist at the University of Chicago.

“We tackled a highly conceptual problem: ‘How does a 250-million-year-old fossil ear bone shake?’ – and used these sophisticated tools to test a simple hypothesis.”

The 3D model allowed the team to examine the animal’s skull and jawbone in unprecedented detail, including the curvature of the jawbone, where an early eardrum may have been stretched.

They then simulated what would happen using tools familiar to engineers for testing vibration stresses in infrastructure such as airplanes and bridges. Trinaxodon‘s skull and jaw will be affected by different sounds.

Of course, there’s more to a living head than bones, so the scientists also used known parameters from living animals to fill in the gaps about what types of soft tissue might be involved as well.

“Once we have a CT model from a fossil, we can take the material properties from an extant animal and create it as if it were our own animal. Trinaxodon I came back to life.” say Zhe-Xi Luo, Wilken’s advisor. “This was not possible before, and this software simulation shows us that sound-induced vibrations are essentially what this animal hears.”

Taken together, the results suggest that Trinaxodon‘s eardrum would have worked very well even if the bones of the middle ear had not been separated. And that would be a significant advance in bone conduction, potentially marking a transition point for mammals to rely on tympanic hearing.

Wilken and his team believe that with this built-in equipment, Trinaxodon They could have achieved an audible range of 38 hertz to 1,243 hertz (for reference, healthy young people can hear their surroundings) 20-20,000 hertz), they were most sensitive to sounds at 1,000 hertz when the sound pressure was 28 decibels (a sound level between a whisper and a normal conversation).

this would have been helpful Trinaxodon They may have played a role in finding prey, avoiding predators, and even reproduction.

This study Proceedings of the National Academy of Sciences.