Hollow bones underlie gigantism and dinosaur flight

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An analysis coordinated by Brazilian paleontologists has just brought important clues about the origin of the peculiar anatomy of dinosaurs, pterosaurs (flying reptiles) and birds. The new study indicates that the so-called air sacs, structures crucial for flight, breathing and metabolism in these groups of animals, would have appeared more than once, independently.

The main clue comes from the skeleton of three species of Brazilian dinosaurs that are among the oldest in the world, with around 230 million years old. In these dinosaurs, whose bones were analyzed using computed tomography, there are no signs that air sacs were already present. And since they lived after their lineage split from that of pterosaurs, that means these structures likely evolved later in both groups separately.

The new research on the subject is in the specialized magazine Scientific Reports. The authors are Tito Aureliano and Fresia Ricardi-Branco, from Unicamp, Aline Ghilardi, from UFRN (Federal University of Rio Grande do Norte), and Marcelo Fernandes, from UFSCar (Federal University of São Carlos), among other researchers.

As the name implies, the air sacs are air chambers that protrude through the body of the animals that have them, also occupying, in part, hollow regions of their bones. They are present in today’s birds and also in many dinosaurs (a group that, strictly speaking, includes all birds, descendants of small carnivorous dinosaurs) and in pterosaurs.

In birds, the air sacs favor enormous respiratory efficiency because they store part of the fresh air that is inhaled when the birds breathe in. Thus, after the oxygen in the air passes the first time through the lungs of the birds, they still receive a “second dose” of the oxygen present in the air that was left in the air sacs when they exhale. Its importance, however, is not limited to this.

“If we think of birds, the air sacs greatly favored both the decrease in the density of the skeleton for flight and the intake of oxygen necessary for flight”, Aureliano explained to Sheet. “In late dinosaur lineages such as sauropods [herbívoros pescoçudos] and theropods [carnívoros]the decrease in body density was the key to the colossal sizes that many of them reached.” Finally, according to him, both the flight and the gigantism of some pterosaurs was also facilitated by these structures.

In search of signs of the presence of air sacs in the most primitive dinosaurs, the team studied the vertebrae of species that represented a good part of the group’s diversity at that time. They are all dinosaurs from the interior of present-day Rio Grande do Sul: the buriolestes it’s the pampadromeusclassified as sauropodomorphs (that is, primitive forms of the group that would give rise to the long-necked giants), and the Gnathovoraxa herrerasaurid (that is, from one of the oldest lineages of dinos, which would disappear shortly afterwards).

In short, what they saw is that the vertebrae of the three species lack the structures that would allow the air sacs to pass through them (being “invasive”, as the experts say). The internal architecture of the bones of primitive dinosaurs is dense in all cases, but that of sauropodomorphs has a more complex vascularization (presence of blood vessels). Throughout the evolution of the group, this may have served as a basis for the presence of air sacs in the future.

The consensus among scientists is that both dinosaurs and pterosaurs descended from the same close common ancestor, forming a group known as Avemetatarsalia. With the absence of air sacs in primitive dinosaurs from Rio Grande do Sul, everything indicates that the characteristic appeared after the separation of Avemetatarsalia into two different lineages. One possibility, according to Aureliano, is that both groups, each in their own way, developed this characteristic to acquire more efficient locomotion and breathing, as well as a larger size, in relation to their competitors.

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