Fossils of Ancient Reptile Reveal Early Evolution of Chest-Based Breathing

The mummified remains of two Captorhinus specimens, discovered in an Oklahoma cave system, date to between 289 million and 286 million years ago. Captorhinus was a land-dwelling vertebrate less than a meter long, comparable in size to a bearded dragon.

The fossils were preserved by crude oil and mineral-rich groundwater seeping into the bodies, along with fine mud encasement, retaining rib cages, ribs, cartilage and protein remnants.

Paleontologist Robert Reisz of the University of Toronto Mississauga and colleagues reported the findings online April 8 in Nature. The preservation allowed analysis of soft tissues rarely fossilized. Reptiles evolved from amphibian-like ancestors around 320 million to 310 million years ago, developing adaptations for full-time terrestrial life, including hard-shelled eggs.

vertebrates, such as amphibians, relied on methods tied to aquatic environments for gas exchange.

Amphibians breathe underwater through their skin and use jaw movements to pump air into rudimentary lungs. Fish and some sharks pump water across gills for respiration. Early reptiles developed a breathing mechanism using chest muscles to draw air into lungs, supporting sustained land habitation.

This apparatus is shared by modern reptiles, birds and mammals, including humans. The timing of this adaptation in the reptile lineage had remained unclear due to the scarcity of soft tissue fossils.

specimen includes a partial skull, shoulder elements, some ribs and a complete forelimb, with skin nearly covering the bones and flexible cartilage fragments on the neck, shoulders and rib cage.

The second specimen shows a flexible cartilaginous sternum and pairs of ribs. Researchers used neutron computed tomography to examine the fossils without damage, revealing how these elements formed a complete, flexible breathing structure. The preserved cartilage and sternum indicate Captorhinus could pump air into its lungs using chest muscles.

This structure resembles that in contemporary land vertebrates. The findings fill a gap in understanding the transition from amphibian to reptilian respiration.

The discovery provides the oldest known cartilage and protein remnants in such fossils, offering direct evidence of the breathing apparatus.

It clarifies evolutionary adaptations that enabled reptiles to thrive on land. Future studies may explore similar soft tissue preservation in other sites to trace further respiratory developments.