Ancient specimens retained the rib cage and cartilage to pump air to the lungs
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The mummified remains of a land vertebrate help reveal how early ancient reptiles began breathing with their entire chests – the style of breathing used by modern reptiles, birds and mammals.
The two new specimens of a small lizard-like reptile called Captorhinus were found in a cave system in Oklahoma. Captorhinus was less than a meter long, perhaps the size of a bearded dragon. The remains, dating between 289 and 286 million years ago, were carefully embalmed by the slow seepage of crude oil and mineral-rich groundwater into their bodies, while being delicately encased and preserved in fine mud.
Consequently, the fossils contain not only preserved rib cages and ribs, but also the oldest known cartilage and protein remainsUniversity of Toronto Mississauga paleontologist Robert Reisz and colleagues report online April 8 in Nature.
Reptiles evolved from amphibian-like ancestors around 320 to 310 million years ago – not only dipping a toe on the shore, but survive on dry land full timethanks to evolutionary adaptations such as hard shells protecting their eggs.
Then there is breathing. Ancient animal breathing methods were linked to water: amphibians, for example, can breathe underwater thanks to gas exchange on the moist, porous surface of their skin; fish and sharks breathe by pumping water rhythmically over their gills. Early amphibians had rudimentary lungs, but used other methods to pump air into them, such as raising and lowering their jaws.
At some point, early reptiles developed a new respiratory apparatus that used chest muscles to pump air into the lungs, allowing the animals to stay on land full time. It is the same type of device used today by their descendants, including humans. But it is not known exactly when this evolutionary adaptation occurred in the reptile lineage, particularly because the soft tissue fossilization that could provide insight into this transition is extremely rare.
Breathe
For the first time, scientists have recovered soft cartilaginous tissue (in yellow, against the background of a whole skeleton) from a 289 million year old mummified fossil. Captorhinus. These tissues helped reveal that the creature was able to breathe by pumping its pectoral muscles – the style of breathing used today by modern reptiles, birds and mammals.
The team used a technique called neutron tomography to examine the fossilized specimens without disturbing them. One of the two new fossils consists of a partial skull, a shoulder and some ribs, as well as a complete forelimb. These elements were still almost covered by skin, and there were also 3D fragments of flexible cartilage clinging to the neck, shoulders, and ribcage. The second specimen revealed that the animal had a flexible cartilaginous sternum as well as 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 complete, flexible breathing apparatus — one that looks awfully familiar, says Elizabeth Brainerd, a biologist at Brown University who was not connected to the study. “We know that the ribcage and shoulder girdle work together to breathe in modern lizards,” says Brainerd. “This fossil shows that the same respiratory mechanism was possible in this ancient reptile.”
