A closeup of a dinosaur bone reveals the tiny fibers of the pneumosteum, a bony tissue that marks the former presence of air sacs. Photo by Lambertz et al./Biol. Lett
Jan. 3 (UPI) -- The highly efficient breathing mechanism known as the lung-air sac system, found among early birds and dinosaurs, has long intrigued paleontologists and evolutionary biologists. But tracing its evolutionary history has proven difficult, until now.
Scientists have developed a way to identify the presence of the lung-air sac system and its precedents on the surface of bones.
Air sacs are tiny, bellows-like bumps on bone that inflate and deflate, triggering the flow of gas in a separate chamber, such as lungs. But air sacs don't just extend out from bones, they can also invade inward.
When bones are colonized by air sacs, it's known as pneumatization. The development of pneumatized bones enabled birds and dinosaurs, like sauropods, to develop lighter bones, allowing for flight and gigantism.
Scientists believe this development preceded the emergence of the lung-air sac system, but air sacs in bones can't survive fossilization, making it difficult to trace their evolution.
However, new analysis of pneumatized bones suggests the presence of air sacs leaves behind an observable structural signature.
"Bones that are in contact with air sacs exhibit a unique structure composed of very fine and densely packed fibers," Filippo Bertozzo, a researcher at the University of Bonn, said in a news release. "After it turned out that this was true both in modern birds and extinct dinosaurs, we proposed to name this special kind of bony tissue 'pneumosteum.'"
Follow-up studies revealed the presence of the unique bony tissue on the cervical vertebrae of gigantic sauropods.
"Such cavities had already previously been hypothesized as potential locations of air sacs, but only our microscopic analysis now provides convincing arguments for this," said Markus Lambertz, a researcher at Bonn's Institute for Zoology.
Scientists detailed their analysis in a new paper published this week in the journal Biology Letters.
While other features, like muscles, can leave behind fibrous marks, the signature left by air sacs is unique. Scientists say the discovery will help them better understand when and how dinosaurs and birds developed the unique structures -- structures that helped the extant species grow to giant sizes, as well as adopt highly-efficient respiratory systems and evolve the ability to fly.
"This project once again highlights the importance of the interdisciplinary collaboration between zoologists and paleontologists for elucidating evolutionary history," Lambertz said.