Jun Chen

Costal aspiration breathing was an evolutionary innovation that was fundamental to the conquest of the terrestrial realm by amniotes (mammals, reptiles, birds and their common ancestor)^1,2,3,4,5. Extant amniotes use an integrated thoracic skeleton for costal-muscle-generated inhalation and exhalation, differing substantially from their anamniote relatives, which possess more passive cutaneous and buccal pumping forms of ventilation. This difference extends into the Palaeozoic era, but the evolutionary transformation between these modes of breathing is undocumented and largely unclear^6,7,8,9,10 in the absence of soft tissue fossils. Here we present the mummified early Permian reptile Captorhinus, which includes a covering of three-dimensional skin, native protein remnants and a complete shoulder girdle and ribcage with preserved cartilages. These are the oldest-known preserved cartilages and protein remnants in a terrestrial vertebrate. High-resolution neutron computed tomography and histology data reveal previously undescribed structures, including the cartilaginous sternum, sternal ribs, rib extensions and epicoracoids. Our skeletal reconstruction of this ancient reptile reveals the precise relationships between the ribcage and the shoulder girdle, and their pivotal role in the evolution of terrestrial breathing and locomotor regimes^11,12. This finding substantially changes expectations of soft tissue preservation in deep time to reveal the potential ancestral amniote breathing mechanism and its impact on terrestrial vertebrate evolution.