A 289-million-year-old mummified reptile has revealed the oldest known amniote breathing apparatus, giving researchers an unusually complete view of how rib-powered ventilation emerged in vertebrates adapting to life on land. The fossil, identified as Captorhinus aguti, preserves not only an articulated ribcage and shoulder girdle but also three-dimensional skin, calcified cartilage, and protein remnants, making it one of the most informative soft-tissue fossils yet recovered from a terrestrial vertebrate of this age.
The study addresses a longstanding gap in vertebrate evolution. Modern amniotes rely on costal aspiration breathing, but the evolutionary shift from earlier buccal pumping and cutaneous respiration has remained unclear because fossils rarely preserve the soft tissues needed to reconstruct it.
To investigate that transition, the team turned to exceptionally preserved fossils from Richards Spur, Oklahoma, combining high-resolution neutron computed tomography with histology and geochemical analyses. The scans revealed previously undescribed structures including a cartilaginous sternum, sternal ribs, intermediate ribs, cervical rib extensions, and epicoracoid cartilages. Together, these features showed how the ribcage and shoulder girdle formed an integrated thoracic skeleton similar to that used by living reptiles during costal breathing.
The preservation itself was equally unusual. Alongside a nearly complete covering of three-dimensional skin, histology and spectroscopic analyses distinguished ossified bone, calcified cartilage, and preserved soft tissues. Synchrotron infrared spectroscopy detected amide-bearing protein signatures in bone, cartilage, and skin, which the authors describe as the oldest known protein remnants in a terrestrial vertebrate and nearly 100 million years older than previously reported fossil biomolecules.
“The protein remnant finding is exceptional,” said Ethan Mooney, co-corresponding author, in the team’s press release. “It dramatically pushes our understanding of what is possible in terms of soft tissue preservation in the fossil record.”
On the basis of the articulated thorax, the authors argue that Captorhinus was capable of costal aspiration breathing and may approximate the ancestral amniote condition. They suggest that this rib-based system was a major step in terrestrial adaptation, allowing early amniotes to sustain more active lifestyles and likely contributing to their spread and diversification on land.
“We propose that the system found in Captorhinus represents the ancestral condition for the kind of rib assisted respiration present in living reptiles, birds, and mammals,” said co-corresponding author Robert Reisz.
The authors suggest that similarly preserved fossils could now help test how broadly this thoracic arrangement was shared among early amniotes and refine reconstructions of the evolutionary transition to rib-powered breathing.
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