Bone cavities called air sacs appeared in the ancestors of long-necked dinosaurs about 225 million years ago, according to analysis of a sample found in the southern Brazilian state of Rio Grande do Sul. A missing link has recently been found that fills the gap between the earliest dinosaurs, which ranged in size from mere centimeters to a maximum of 3 metres, and the more modern giants, which could be more than twice as long as a bus and twice as wide as a bus. appeals to the popular imagination.

Macrocollum itaquii, discovered in the Agudo region of Rio Grande do Sul state in southern Brazil and dating back 225 million years, is the oldest dinosaur known to have structures called air sacs.

These bone cavities, preserved in modern birds, enabled dinosaurs to capture more oxygen, cool their bodies and withstand the harsh conditions of their age. They also helped some become giants: Tyrannosaurus Rex and Brachiosaurus, for example. A paper about the research that led to the discovery was published in the journal Anatomical Record. Two of its authors are FAPESP-supported researchers from the State University of Campinas (UNICAMP) in the state of São Paulo.

“The air sacs reduced the density of the bones, which allowed them to grow to lengths of more than 30 meters,” said Tito Aureliano, first author of the paper. The research was carried out as part of his doctoral study at the Institute of Earth Sciences (IG-UNICAMP).

“M. itaquii was the largest dinosaur of its time, measuring about 3 meters in length. A few million years ago, the largest dinosaurs were about 1 meter long. Air cushions certainly contributed to this increase in size,” Aureliano said.

The study was one phase of the FAPESP-funded Taphonomic Landscapes project. Taphonomy is the study of how organisms decompose, fossilize, or are preserved in the paleontological record. The principal investigator of this project was Fresia Riccardi-Branco, penultimate author of the paper and IG-UNICAMP professor.

“It was one of the first dinosaurs to walk the Earth during the Triassic period,” he said. “The adaptation of the air bladder allowed it to grow and withstand the climate during this period and later during the Jurassic and Cretaceous periods. Air bladders gave dinosaurs an evolutionary advantage over other groups such as mammals and allowed them to diversify more quickly.”

In a previous study, the team showed that the oldest fossils found so far did not have air sacs; he took their absence as a sign that this trait had evolved independently at least three times.

M. itaquii was a bipedal sauropodomorph, an ancestor of giant tetrapods with a small head and a neck at least as long as its body.

Until the discovery of air sacs in M. itaquii, these vertebral spaces were known to consist of chamber or camelid tissue; the first referred to the hollow spaces observed by microtomography, and the second to spongy bone. In this case, they found “internal pneumatic chambers” that were “neither chambers nor camellias, but a new type of tissue with intermediate tissue,” according to the authors. They propose calling the new structures “protorooms” because they are “not large enough to be considered rooms, but also represent an array of camellias inside.”

“The most common hypothesis so far has been that the air sacs started as cameras and evolved into camels. Our hypothesis, based on what we observed in this example, is that this other form existed first,” Aureliano said.

The vertebrae in which air sacs were discovered also changed what was known about the evolution of these structures. Based on previously analyzed fossils, other research groups have suggested that air sacs first appeared in the abdominal region and did not appear in the cervical region until the Early Jurassic (190 million years ago), much later than when M. itaquii lived. . However, here the authors found clear signs of the presence of air sacs in the cervical and dorsal regions, without any signs of structure in the abdominal region.

“It’s as if evolution experimented several times until it reached the final system, where the air sacs go from the neck area to the tail. “It wasn’t a linear process,” Aureliano said. Source