Fossil reveals evolutionary trace of dinosaur gigantism – 04/20/2023 – Science

The missing link between the oldest dinosaurs, whose size varied from a few centimeters to a maximum of three meters in length, and the most recent giants, which could be larger than two buses and became popular in the popular imagination, has just been found.

O macrocollum itaquiiburied 225 million years ago in what is now the municipality of Agudo, in Rio Grande do Sul, is the oldest dinosaur studied to date that has structures known as air sacs.

These hollow spaces in the bones, still present in today’s birds, helped dinosaurs both to obtain more oxygen, better cool the body and withstand the harsh conditions of their time, and to become giants, like the tyrannosaurus rex and the Brachiosaurusfor example.

The discovery was published in the journal Anatomical Record, in an article whose main authors are researchers from Unicamp (State University of Campinas) supported by FAPESP.

“The air sacs made the bones less dense, allowing dinosaurs to exceed 30 meters in length”, explains Tito Aureliano, first author of the study conducted during his doctorate at Unicamp’s Institute of Geosciences (IG).

“O macrocollum, in turn, was the largest of its time, measuring around three meters in length, whereas a few million years earlier the largest dinosaurs were around one meter long. The air sacs certainly facilitated this increase in size.”

The work is part of the “Taphonomic Landscapes” project, financed by Fapesp and coordinated by Fresia Ricardi Branco, a professor at IG-Unicamp who also signs the article.

“This was one of the first dinosaurs to walk on Earth, in the Triassic period. This adaptation made it possible for them to grow and withstand the climate of both that period and those that came later, the Jurassic and Cretaceous. The air sacs were an evolutionary advantage over others groups, such as mammals, allowing dinosaurs to diversify more quickly”, says the researcher.

In a previous study, the group had shown that the oldest fossils ever found lacked evidence of air sacs, suggesting that this feature evolved at least three times independently.

the biped macrocollum is a sauropodomorph, ancestor of the giant quadrupedal and long-necked dinosaurs.

non-linear evolution

Until the discovery of air sacs in the macrocollum, these empty spaces within the vertebrae were known to have two possible shapes. The chambers are large spaces inside the bones, quite evident when seen by microtomography, a technology used in the study that allows knowing the inside of the fossils without damaging them.

The camels, in turn, are much smaller, however, much more branched, generating, in practice, the same effect as the chambers. What the researchers observed in the macrocollum it was an intermediate structure, neither too big nor too small, although branched. They proposed the name of protochambers to this new form of bone architecture shaped by air sacs.

“The most accepted hypothesis until then was that the air sacs started as chambers and evolved into camels. What we are proposing, based on what we saw in this specimen, is that, before all, there was this other form”, says Aureliano.

The vertebrae where the air sacs were found also change what was previously known about the evolution of these structures. Based on the fossils examined so far, other research groups had proposed that the hollow spaces appeared in the cervical region only well after the period in which the human lived. macrocollum, already at the beginning of the Jurassic (190 million years ago). According to this hypothesis, the air sacs would initially have occurred only in the abdominal region.

However, the macrocollum surprised the scientists precisely because there was clear evidence of air sacs in the cervical and dorsal regions, with no signs of structures in the abdominal region.

“It is as if evolution had carried out different experiments until reaching the definitive system, in which the air sacs went from the cervical region to the tail. It was not a linear process”, concludes Aureliano.