Mummified reptiles are revealing how breathing evolved

Apr 8th 2026|3 min read

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ALL ANIMALS breathe in oxygen and exhale carbon dioxide, but how exactly they do so differs widely. Fish, for example, take oxygen-rich water in through their mouths before pumping it past their gill tissues, where the gas exchange takes place. Amphibians perform a similar trick, using their cheeks to push water or air to their lungs. The reptiles and mammals that descend from them, however, use muscles associated with their ribs and shoulders to contract and expand the chest cavity, drawing in and expelling air accordingly.

The evolutionary leap from cheek breathing to rib-assisted breathing was a significant event that has, until now, remained mysterious. A new paper in Nature by Robert Reisz at the University of Toronto and Ethan Mooney, a PhD student at Harvard University, clarifies the picture. The researchers analyse the mummified fossil remains of an ancient reptile to conclude that the transition took place at the same time as the first animals evolved to walk on dry land at least 290m years ago.

Rib-assisted breathing would have provided many benefits to its first adopters. Cheek-breathing places strong constraints on amphibian head and neck anatomy, severely limiting the diversity of forms such animals can take on. The mechanism is also less efficient than rib-assisted breathing (it is thought that amphibians retain a permeable skin, which allows them to passively top up their oxygen reserves when they are out of water, to compensate). The shift to rib-assisted breathing, therefore, represented an evolutionary turning-point that helped reptiles rapidly diversify and, in the guise of dinosaurs, dominate the planet for over a hundred million years.

The big problem with working out when rib-assisted breathing evolved is that most of the bits involved in the process—such as lungs, muscles and cartilage—are soft, and rot away rather quickly after an animal dies. In rare environments that are either very dry, low in oxygen or saturated with chemicals that stymie the bacteria responsible for decay, however, organisms can mummify, making it possible for soft tissues to stand the test of time.

The caves near Richards Spur, Oklahoma, offer just such conditions. Most of the bacteria that consume corpses are not fond of the petroleum found in these caves, and tend to leave alone animal remains saturated in the stuff. Even less appealingly for the bacteria, 290m years ago the site would have been dry enough to mummify any animal remains. The final straw would have been the rapid accumulation of fine sediment that would have prevented oxygen from reaching them. It was just such a palaeontologist-friendly fate that two individuals of the extinct reptile genus Captorhinus endured.

Many millions of years later their bodies were found and carefully excavated. Stained black and brown by the oil and sandwiched between layers of rock, the fossils did not look like much; but Dr Reisz suspected there would be value in studying them closely. He was right. The specimens were so well preserved that CT scanning—a technology designed to make images of soft tissues—readily revealed that these animals had a flexible cartilage sternum that allowed the ribs, the rib muscles and the shoulder girdle to work together to draw air in and out of the torso. This is much the same breathing mechanism as seen in modern lizards. (Mammals are not too different either; although they have a diaphragm that increases the amount that they can inhale, they still depend upon muscles attached to their ribs.)

That the shoulder girdle was fully integrated into the breathing system of such an ancient reptile is of particular interest. This bony structure would have been vital for animals taking their first steps, supporting their weight while simultaneously providing them with the strength to walk on dry land. Dr Reisz and Mr Mooney’s finding, therefore, suggests that the evolutionary solution for breathing was intertwined with movement on dry land, supporting a long-standing hypothesis in the field. Whether movement came first and breathing came second, or vice versa, remains to be determined. ■