The mummified remains of of a land-dwelling vertebrate are helping to reveal how early ancient reptiles began to take a breath with their whole chests — the breathing style used by modern reptiles, birds and mammals.
The two new specimens of a small, lizardlike reptile called Captorhinus were found in an Oklahoma cave system. Captorhinus was less than a meter long, perhaps the size of a bearded dragon. The remains, dating to between 289 million and 286 million years old, were carefully embalmed by the slow seep of crude oil and mineral-rich groundwater into their bodies, while they were also being gently encased and preserved in fine mud.
As a result, the fossils contain not only preserved rib cages and ribs but also the oldest-known cartilage and remnants of protein, paleontologist Robert Reisz of the University of Toronto Mississauga and colleagues report online April 8 in Nature.
Reptiles evolved from amphibian-like ancestors sometime around 320 million to 310 million years ago — not just dipping a toe onto the shore, but surviving on dry land full-time, thanks to evolutionary adaptations such as hard shells protecting their eggs.
Then there’s the breathing. Earlier animal breathing methods were tied to water: Amphibians, for example, can breathe underwater via gas exchange across the moist, porous surface of their skin; some fish and sharks breathe by rhythmically pumping water across their gills. Early amphibians did have rudimentary lungs, but used other methods to pump air into them, such as raising and lowering the jaw.
At some point, early reptiles evolved a novel breathing apparatus that used chest muscles to pump air into the lungs, enabling the animals to stay full-time on land. It’s the same type of apparatus used by their descendants — including humans — today. But when this evolutionary adaptation occurred in the reptile lineage hasn’t been known, not least because the fossilization of soft tissues that could provide insight into this transition is extremely rare.
The team used a technique called neutron computed tomography to peer into the fossilized specimens without disturbing them. One of the two new fossils consists of a partial skull, shoulder and some of the ribs, along with one complete forelimb. These elements were still nearly covered by skin, and there were also 3-D fragments of flexible cartilage clinging to the neck and shoulders and rib cage. The second specimen revealed the animal had a flexible cartilaginous sternum, along with pairs of ribs.
What’s really exciting about these fossils is that it’s possible to see how the pieces fit together to form a whole, flexible breathing apparatus — one that looks awfully familiar, says Elizabeth Brainerd, a biologist at Brown University who was not connected with the study. “We know that the rib cage and shoulder girdle work together for breathing in modern lizards,” Brainerd says. “This fossil shows that the same breathing mechanism was possible in this ancient reptile.”
