289-Million-Year-Old Reptile Fossil Reveals Origins of Mammal Breathing

A mummified reptile from the early Permian period has provided the oldest known evidence of costal aspiration, the breathing mechanism that allowed early amniotes to conquer terrestrial environments and serves as the ancestral foundation for how mammals, birds and reptiles breathe.

The fossil, identified as a Captorhinus, dates back approximately 289 million years. According to research published April 8, 2026, in Nature, the specimen was discovered in an Oklahoma cave system and exhibits an extraordinary level of soft-tissue preservation, including skin, protein remnants, and a complete ribcage with preserved cartilages.

The Evolution of Terrestrial Breathing

Costal aspiration is an evolutionary innovation where an integrated thoracic skeleton is used to generate inhalation and exhalation through muscle movement. This differs significantly from the ventilation methods used by anamniotes, such as early amphibians, which relied on more passive forms of breathing.

Before the evolution of this system, early land-dwelling vertebrates utilized cutaneous respiration, where gas exchange occurred across moist, porous skin, or buccal pumping, which involved rhythmically raising and lowering the jaw to pump air into rudimentary lungs.

The discovery of the mummified Captorhinus reveals that a rib-assisted breathing system was already in place 289 million years ago. This provides researchers with a clear look at the transition from water-tied breathing methods to the muscle-powered thoracic ventilation used by modern terrestrial vertebrates.

Anatomical Discoveries and Imaging

The preservation of the Captorhinus is among the oldest known examples of soft tissue and protein remnants in a terrestrial vertebrate. Researchers utilized high-resolution neutron computed tomography and histology to identify structures that are typically lost during fossilization.

The analysis revealed several previously undescribed structures, including:

• A cartilaginous sternum
• Sternal ribs
• Rib extensions
• Epicoracoids

These elements allow scientists to reconstruct the precise relationship between the shoulder girdle and the ribcage, clarifying how these structures functioned together to support both breathing and locomotion on land.

Rare Conditions for Mummification

The preservation of skin and cartilage over hundreds of millions of years is highly unusual, as these tissues typically collapse or break apart. The Captorhinus was preserved due to a specific combination of environmental factors within the Oklahoma cave system.

The animal was encased in fine clays and subjected to the slow seep of crude oil and hyper-mineralized groundwater. These elements acted as a form of natural embalming, protecting the protein remnants and cartilaginous framework from decay.

The specimen was small, measuring less than a meter in length, which researchers compare to the size of a modern bearded dragon.

Scientific Significance

The findings challenge previous paleontological assumptions regarding the preservation of soft tissues in deep-time fossils. By documenting the cartilage framework of the respiratory system, the study provides empirical evidence for the origins of the amniote breathing mechanism.

This is an exciting discovery in paleontology with great evolutionary significance

Robert Reisz, University of Toronto Mississauga

Amniotes evolved from amphibian-like ancestors between 320 million and 310 million years ago. While adaptations like hard-shelled eggs allowed them to survive on dry land full-time, the development of the costal aspiration system was fundamental to their continued expansion and evolutionary success in terrestrial realms.