The discovery of a fossil from Oklahoma sheds light on a critical stage in the evolution of breathing among the earliest land-dwelling vertebrates. A tiny, mummified reptile that died approximately 289 million years ago reveals the oldest known breathing mechanism using rib muscles in amniotes, the group that includes all reptiles, birds, mammals, and their common ancestors.

The study, published in the journal Nature, describes the extraordinary preservation of the earliest known rib-based breathing system in a specimen of Captorhinus aguti, a small reptile from the early Permian period. The fossil, just a few centimeters long, preserves not only bones but also three-dimensional skin, calcified cartilage, and protein residues that predate any other known finding by nearly 100 million years.

“Captorhinus is an interesting lizard-like reptile that is crucial for understanding the early evolution of amniotes,” said Ethan Mooney, who co-led the research as a student at the University of Toronto, in the laboratory of Professor Robert R. Reisz.

Mooney, now a doctoral candidate in the Department of Organismic and Evolutionary Biology at Harvard University, works with paleontologist Professor Stephanie Pierce, continuing his research on early reptiles.

A Unique Fossil from the Permian Period

Species of Captorhinus, ranging in size from a few centimeters to several feet, were among the first reptiles to try life on land. They were found in the Richards Spur caves in Oklahoma, a site exceptionally rich in fossils from the late Paleozoic era, known for the variety of its land-dwelling vertebrates.

The unique conditions of the site, hydrocarbons from oil seeps and oxygen-free mud, allowed preservation not only of bones but also of skin and cartilage, creating a three-dimensional fossil in a death pose, with a forelimb folded under the body.

Discovery Through Cutting-Edge Technology

Using neutron computed tomography (nCT) at a specialized facility in Australia, the researchers were able to examine the fossil without destroying it. “I started seeing structures wrapped around the bones… there was a beautiful skin envelope around the torso, with an accordion-like texture,” Mooney noted. The pattern resembled the scales of modern skinks, small reptiles that live underground.

The First Reconstruction of Breathing in an Early Reptile

The team studied three Captorhinus specimens from Richards Spur and identified a cartilaginous sternum, costal cartilages, and connections to the shoulder girdle. For the first time, it was possible to reconstruct the complete breathing mechanism of an early amniote.

Before this system appeared, amphibians breathed through their skin and via a buccal and throat pump, a method insufficient for a more active lifestyle. Breathing through rib contraction, expanding and compressing the chest with the intercostal muscles, was a key evolutionary development, allowing more oxygen in and carbon dioxide out.

“We propose that the system found in Captorhinus represents the ancestral form of rib-assisted breathing that exists today in reptiles, birds, and mammals,” explained Reisz.

Evolution and Protein Preservation

The use of chest muscles was a defining innovation for the conquest of land by the earliest ancestors of modern reptiles and mammals. “It was a game changer that allowed these animals to adopt a much more active lifestyle,” Mooney noted.

Even more impressive was the discovery of protein residues in bones, cartilage, and skin, detected via synchrotron infrared spectroscopy. These organic molecules are the oldest ever found in Paleozoic-era fossils, predating the previous oldest known sample by nearly 100 million years.

“The discovery of protein residues is extraordinary and dramatically extends the boundaries of our understanding of soft tissue preservation in fossils,” Mooney emphasized.

The fossils are now housed at the Royal Ontario Museum in Toronto, where they are available for future research. Mooney continues at Harvard to explore the evolutionary secrets of early reptiles, contributing to our understanding of how these ancient organisms shaped the world we know today.