Dinosaurs: Unveiling the Evolution of Bone-Based Breathing

Somewhere in Earth’s past, some branches on the tree of life adopted a body plan that revolutionized breathing and cooling down. This development played a crucial role in the evolution of some of the largest dinosaurs to ever roam the planet. It was so successful that it persisted in three different groups of extinct species and continues to exist in the living descendants of dinosaurs.

But how do scientists study the breathing capabilities of these ancient creatures when lungs don’t usually fossilize? The answer lies within their bones.

In a series of groundbreaking papers published in late 2022 and early 2023, paleontologists examined the fossil microstructure of some of the earliest known dinosaurs to uncover the evolution of their respiratory system.

Breathing efficiency

In species like birds, the respiratory system includes air sacs within bones throughout the body. Unlike mammals, who inhale and exhale separately, these air sacs enable unidirectional breathing, allowing for efficient oxygen intake and heat extraction. This system, known as postcranial skeletal pneumaticity, is exclusive to birds today. While crocodilians also have unidirectional breathing, their bones lack air sac cavities. A study from 2012 confirmed the absence of postcranial skeletal pneumaticity in crocodile-line archosaurs.

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“There are no anatomical signs of any true pneumatic features that would have been related to invasive air sacs prior to the evolution of pterosaurs and saurischian dinosaurs,” explained Tito Aureliano, a paleontologist at Brazil’s Universidade Federal do Rio Grande do Norte.

Aureliano, the lead author of the recent papers, highlighted the advantages of an invasive air sac system. Unlike mammals, who can become breathless or overheated during intense physical activity or heat exposure, creatures with air sacs can avoid these issues.

So when did these air sacs first appear? They were already present in Cretaceous theropods, pterosaurs, and sauropods, which lived approximately 145 to 66 million years ago. To trace their origin, the research team delved further back into the Triassic period (about 252 to 201 million years ago). They studied two types of sauropodomorphs and a carnivorous dinosaur named Gnathovorax.

“The big question was,” Aureliano explained, “did the common ancestor of these three groups in the Triassic already have air sacs, or did the air sacs evolve independently three times?”

Multiple origins

If fossil bone tissue showed traces similar to those found in modern birds, it would indicate the presence of this respiratory adaptation. However, none of the early species exhibited these traces, suggesting that postcranial skeletal pneumaticity had not yet evolved. This means it couldn’t have been present in the common ancestor of dinosaurs and other major groups where it was also found.

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This discovery surprised co-author Aline Ghilardi, a paleontologist and tenured assistant professor. She noted that since the earliest dinosaurs lacked invasive pneumatic structures, the air sacs must have evolved later. This implies that pterosaurs also independently evolved this respiratory system.

Furthermore, the fossils indicated that the evolution of air sacs had already begun during the Triassic period. The sauropodomorphs and herrerasaurid studied lived approximately 233 million years ago. However, Pampadromaeus, which existed slightly later than Buriolestes, showed a significant change in its skeleton. It had a new type of tissue that may have been a precursor to air sac evolution.

“The whole vasculature was different [in Pampadromaeus],” Aureliano explained. “It was less dense and had fractals inside, small chambers for blood and fat tissues.” This unique structure may have paved the way for future invasive air sacs to occupy the space created by those fractals, as air sacs function similarly to fractals extending from the lungs.