A recent paper in Proceedings of the National Academy of Sciences of the USA (PNAS) reports the discovery that an ancestor of mammals, a cynodont called Thrinaxodon liorhinus, had ear structures derived from redundant jaw bones that probably gave it an acute sense of hearing some 250 million years ago — around 50 million years earlier than previously believed. As nocturnal animals, a well-developed sense of hearing would have been hugely advantageous.
The research, by palaeontologists from the University of Chicago, used CT scans of the skull and jawbones of Thrinaxodon to simulate the effects of different sound pressures and frequencies on its anatomy.
Transitional fossils such as this are a major source of embarrassment to creationists because their Bronze Age mythology insists that all species were created fully formed, without ancestry, so there should never be any examples of species evolving or of existing structures being exapted over time for new functions.
Sadly for creationists, the fossil evidence paints an entirely different picture. It is a record of everything creationism predicts should not be there and everything evolution predicts will be. To most normal people, that sort of evidence should strongly suggest that creationism is wrong and that the Theory of Evolution is right.
It is rather like someone who does not believe in gravity stating that if you throw a stone into the air it will stay there and never fall back to Earth. A simple demonstration will establish the falsehood of that claim, just as the fossil record establishes the falsehood of creationist claims.
Background^ Cynodonts and the Evolution of the Mammalian Middle Ear. Cynodonts were a group of synapsid reptiles that lived from the Late Permian to the Early Jurassic and include the direct ancestors of mammals. Unlike true reptiles, cynodonts already showed many mammal-like features, including differentiated teeth, a more upright posture, a secondary palate, and increasingly complex jaw and skull anatomy. Fossils such as Thrinaxodon, Cynognathus, and later forms like Morganucodon document a clear, step-by-step transition from reptile-like synapsids to early mammals.The research is explained in an article in UChicago News by Matt Wood.
One of the most striking evolutionary changes recorded in this lineage is the origin of the mammalian middle ear. In reptiles, several small bones at the back of the lower jaw — notably the articular and quadrate — form part of the jaw joint. In mammals, these same bones are repurposed as the malleus and incus of the middle ear, joining the stapes to form the familiar three-bone hearing apparatus. This transformation did not occur suddenly; it unfolded gradually over tens of millions of years.
Fossil cynodonts preserve intermediate stages in which these jaw bones became progressively smaller, less involved in chewing, and increasingly specialised for sound transmission. Some transitional species even show a “double jaw joint,” with both the old reptilian joint and the new mammalian joint functioning simultaneously. This provides direct, physical evidence for exaptation — the evolutionary process in which structures originally evolved for one function are co-opted for a new one.
The result of this long transition was the highly sensitive mammalian middle ear, capable of detecting higher-frequency sounds far better than that of reptiles. This would have been particularly advantageous for small, nocturnal early mammals, allowing them to detect predators and prey in low-light conditions. Far from being a problem for evolutionary theory, the cynodont fossil record is one of its clearest and most elegant confirmations — and one of the most awkward facts for creationism to explain away.Creationist Claim vs Reality: The Mammalian Middle Ear
Claim:
The mammalian middle ear is “irreducibly complex” and could not have evolved because all three bones — the malleus, incus, and stapes — must be present and perfectly arranged for hearing to work.
Reality:
The fossil record preserves multiple transitional stages showing exactly how the mammalian middle ear evolved from reptile-like jaw bones. In early synapsids and cynodonts, the articular and quadrate bones formed part of the jaw joint. Over time, these bones became progressively smaller and less involved in chewing, while increasingly specialised for transmitting sound.
Claim:
There are no transitional fossils showing this transformation.
Reality:
There are many. Fossils such as Thrinaxodon, Cynognathus, Diarthrognathus, and Morganucodon preserve intermediate anatomies, including species with a functioning “double jaw joint” — one reptilian and one mammalian — operating at the same time. This is exactly what gradual evolution predicts.
Claim:
Repurposing jaw bones for hearing would destroy their original function.
Reality:
It did not. For millions of years, both functions co-existed. As the new mammalian jaw joint (between the dentary and squamosal bones) took over the role of chewing, the old jaw joint bones were freed to specialise for sound transmission. This is a textbook example of exaptation, not a paradox.
Claim:
Complex biological structures appear suddenly.
Reality:
They do not. The step-by-step transformation of jaw bones into middle ear bones is one of the best-documented transitions in the entire fossil record. It is exactly the opposite of what creationism predicts — and exactly what evolutionary theory predicts.
Fossil study rewrites timeline of evolution of hearing in mammals
UChicago paleontologists use CT scanning and simulations to show how a 250-million-year-old mammal predecessor could hear like us
One of the most important steps in the evolution of modern mammals was the development of highly sensitive hearing.
The middle ear of mammals, with an eardrum and several small bones, allows us to hear a broad range of frequencies and volumes, which was a big help to early, mostly nocturnal mammal ancestors as they tried to survive alongside dinosaurs.
New research by paleontologists from the University of Chicago shows that this modern mode of hearing evolved much earlier than previously thought. Working with detailed CT scans of the skull and jawbones of Thrinaxodon liorhinus, a 250-million-year-old mammal predecessor, they used engineering methods to simulate the effects of different sound pressures and frequencies on its anatomy.
Their models show the creature likely had an eardrum large enough to hear airborne sound effectively, nearly 50 million years before scientists previously thought this evolved in early mammals.
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. 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.
Alec T. Wilken, lead author
Department of Organismal Biology and Anatomy
The University of Chicago
Chicago, IL, USA.
Testing a 50-year-old hypothesis
Fossil specimen of the Thrinaxodon skull and jaw used for the study.Photo by Matt Wood.
Thrinaxodon was a cynodont, a group of animals from the early Triassic period with features beginning to transition from reptiles to mammals. They had specialized teeth, changes to the palate and diaphragm to improve breathing and metabolism, and probably warm-bloodedness and fur.
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.
Simulations showed that sound waves applied to the eardrum of "Thrinaxodon" (top) would have enabled it to hear much more effectively than through bone conduction alone (bottom).Infographic courtesy of April I. Neander, Alec Wilken
Fifty years ago, Edgar Allin, a paleontologist at the University of Illinois Chicago, first 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 that early cynodonts heard through bone conduction, or via so-called “jaw listening” where they set their mandibles on the ground to pick up vibrations.
While the eardrum idea was fascinating, there was no way to definitively test if such a structure could work to hear airborne sounds.
Turning fossils into an engineering problem
Modern imaging tools like CT scanning have revolutionized the field of paleontology, allowing scientists to unlock a wealth of information that wouldn’t have been possible through studying physical specimens alone.
Wilken and his advisors, Zhe-Xi Luo and Callum Ross, both professors of organismal biology and anatomy, took a well-known Thrinaxodon specimen from the Museum of Paleontology at the University of California, Berkeley, and scanned it in UChicago’s PaleoCT Laboratory. The resulting 3D model gave them a highly detailed reconstruction of its skull and jawbones, with all the dimensions, shapes, angles and curves they needed to determine how a potential eardrum might function.
Next, they used a software tool called Strand7 to perform finite element analysis, an approach that breaks down a system into smaller parts with different physical characteristics. Such tools are usually used for complex engineering problems, like predicting stresses on bridges, aircraft and buildings, or analyzing heat distribution in engines. The team used the software to simulate how the anatomy of Thrinaxodon would respond to different sound pressures and frequencies, using a library of known properties about the thickness, density and flexibility of bones, ligaments, muscles and skin from living animals.
The results were loud and clear: Thrinaxodon, with an eardrum tucked into a crook on its jawbone, could definitely hear that way much more effectively than through bone conduction. 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 still would have relied on some jaw listening, the eardrum was already responsible for most of its 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. That hasn’t been possible before, and this software simulation showed us that vibration through sound is essentially the way this animal could hear.
Professor Zhe-Xi Luo, corresponding author.
Department of Organismal Biology and Anatomy
The University of Chicago
Chicago, IL, USA.
Wilken said the new technology allowed them to answer an old question by turning it into an engineering problem.That’s why this is such a cool problem to study. 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.
Alec T. Wilken.
Publication:
For creationists, this discovery is yet another reminder of how badly their Bronze Age mythology fails when confronted with real-world evidence. The evolutionary origin of the mammalian middle ear is no longer a theoretical reconstruction inferred from comparative anatomy; it is a physical, fossil-documented transition preserved in stone. The fact that Thrinaxodon already shows mammal-like hearing structures 250 million years ago simply pushes that transition even further back in time and fills in yet another gap that creationists like to pretend does not exist.
It also underlines a point that creationists have been trying to evade for decades: evolution does not require sudden leaps or the magical appearance of fully formed organs. What it requires is exactly what the fossil record shows — incremental modifications of existing structures, shaped by selection, and repurposed for new functions as circumstances change. Jaw bones that once transmitted bite forces gradually became exquisitely tuned instruments for transmitting sound. That is not a problem for evolutionary theory; it is one of its strongest empirical confirmations.
Worse still for Intelligent Design advocates, the researchers show no hesitation whatsoever in interpreting what they found within the framework of evolutionary biology. There is no hint of mystery, no appeal to unknown designers, and no suggestion that natural processes are inadequate to explain what is observed. Instead, the anatomy of Thrinaxodon fits neatly into a well-established evolutionary sequence that has been mapped out for decades and is now being refined in ever greater detail as new fossils and new technologies come to light.
So once again, we are left with a familiar contrast. Evolutionary biology makes clear, testable predictions about what we should find in the fossil record — and those predictions keep being confirmed. Creationism, by contrast, predicts that none of this should exist at all. When one worldview consistently matches the evidence and the other consistently fails, there is no honest ambiguity about which one is right.
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