Hollow bones of dinosaurs that made them giants evolved at least three times independently

Dinosaurs as big as buses or five-story buildings would not be possible if their bones were dense and heavy like ours. Like today’s birds, dinosaurs had hollow bones with internal structures known as air sacs that made their skeletons lighter and less dense.
These structures were apparently so advantageous that they appeared at least three times during the evolution of dinosaurs and pterosaurs (flying reptiles), according to a study supported by FAPESP and described in an article in Scientific Reports.
“Less dense bones containing more air gave dinosaurs and pterosaurs [and still give birds] more oxygen circulating in their blood as well as greater agility to hunt, run and fight or even fly. Not only did they use less energy, but they also cooled their bodies more efficiently,” said Tito Aureliano, first author of the paper. The study was part of his doctoral research at the Geosciences Institute of the State University of Campinas (IG-UNICAMP).
Aureliano analyzed the fossilized bones of three Brazilian species from the Late Triassic (about 233 million years ago), the period in which the dinosaurs appeared. All the bones were found in recent decades in Rio Grande do Sul, Brazil’s southernmost state.
Detailed knowledge of specimens belonging to different groups and dating from an early stage of their evolution provides a basis for understanding when certain traits were developed. In this case, the researchers were looking for signs of air sacs that were common in geologically more recent (and more studied) species such as tyrannosaurs or velociraptors, and are found in modern birds, as noted earlier. Air sacs are found in bones throughout the body next to the spine.
Computed tomography was used to visualize the internal structures of the fossils. Small spaces in the vertebrae were identified as openings for veins, arteries, and marrow, and attachment points for muscles and tendons were seen, but none seemed capable of serving as pneumatic chambers through which air could continuously flow.
“The Triassic was very warm and dry. What is now the Rio Grande do Sul was far from the sea in the heart of the supercontinent Pangea. In this context, more oxygen circulating in the blood would more effectively cool the body and would certainly provide a welcome advantage, so much so that it has evolved at least three times independently,” said Fresia Ricardi-Branco, penultimate author of the paper, Prof. at IG-UNICAMP and principal investigator of the project funded by FAPESP, of which the study was a part.
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The fossils analyzed were found between 2011 and 2019 by scientists from the Federal University of Santa Maria (UFSM) in an area known as Quarta Colonia near Santa Maria in Rio Grande do Sul. Some of these researchers are co-authors of the paper.
The fossils belonged to three species: Buriolestes schultzi, Pampadromaeus barberenai and Gnathovorax cabreirai. The first two were sauropodomorphs, a group of long-necked dinosaurs that became the largest animals to walk the planet. The third was the herrerasaurid, one of the first carnivorous dinosaurs. The line died out shortly after the period in which this specimen lived.
A study published in 2021 by scientists from South Africa, the United Kingdom, the United States and Canada already showed that another dinosaur lineage, the ornithians, also lacked structures that could contain air sacs. This order of dinosaurs probably appeared later, in the Jurassic (between 201 million and 145 million years ago), and included the popular Triceratops.
Data collected on ornithischians, herrerasaurids, and sauropods showed that air sacs evolved independently in each group. “We found that no common ancestor had this trait. All three groups had to have developed airbags independently,” said Aureliano.
Other groups that had air sacs were pterosaurs (including pterodactyls) and theropods (including tyrannosaurs and velociraptors, as well as extant birds). Although descended from B. schultzi and P. barberenai, hollow bones evolved later in the long-necked lineage. It is not yet known exactly when.
“The oldest dinosaurs in the world are in South America and have only been discovered in the last two decades,” Ricardi-Branco said. “More of this kind of research needs to be done to show how the dominant organisms of the time coped with a much warmer climate than ours.”
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