The missing link between the earliest dinosaurs, which ranged in size from a few centimeters to no more than three meters, and the more recent giants, which could be more than twice as long as a bus and captivate the public’s imagination, has just been discovered.
The oldest dinosaur with structures known as air sacs that has been studied so far is Macrocollum itaquii, which was buried 225 million years ago in what is now Agudo, a town in the state of Rio Grande do Sul in South Brazil.
Dinosaurs were able to take in more oxygen, keep their bodies cool, and withstand the harsh conditions of their time thanks to these bone cavities, which are still present in birds today. They also helped a few people become giants: Tyrannosaurus rex and Brachiosaurus, for instance.
The Anatomical Record publishes an article on the study that led to the discovery. The State University of Campinas (UNICAMP) in the state of So Paulo is the location of two of its authors.
“During the Triassic period, this was one of the first dinosaurs to walk the Earth. The air sac adaption allowed it to grow and survive in this and later Jurassic and Cretaceous periods. Air sacs provided dinosaurs with an evolutionary edge over other animals, such as mammals, allowing them to diversify more quickly.”
Tito Aureliano, first author of the article.
According to Tito Aureliano, the article’s first author, “air sacs made their bones less dense, allowing them to grow to more than 30 meters in length.” As part of his Ph.D. research at the Institute of Geosciences (IG-UNICAMP), the study was carried out.
“M. itaquii was the largest dinosaur of its era, measuring approximately 3 meters in length. Before that, the largest dinosaurs weighed about 1 meter and lived a few million years ago. Aureliano added, “Air sacs unquestionably facilitated this increase in size.”
The project “Taphonomic Landscapes” included the study as one of its stages. The study of how living things decompose into fossils or are preserved in the paleontological record is called taphonomy.
Fresia Ricardi-Branco, the penultimate author of the article and a professor at IG-UNICAMP, was the project’s principal investigator.
She stated, “This was one of the first dinosaurs to walk the Earth during the Triassic period.” It was able to grow and survive the climate during this time period, as well as later, in the Jurassic and Cretaceous, thanks to the air sac adaptation. Dinosaurs were able to diversify more quickly and had an evolutionary advantage over other groups like mammals thanks to their air sacs.
The group believed that the absence of air sacs in the earliest fossils discovered thus far indicated that this trait had evolved at least three times independently. In a previous study, they demonstrated this.
M. itaquii was a sauropodomorph, a biped, and an ancestor of the giant quadrupeds. It had a neck at least as long as its trunk and a small head.
Nonlinear evolution Prior to the discovery of air sacs in M. itaquii, it was known that these vertebral cavities were made of camerate or camellate tissue. The former was thought to be hollow spaces seen on microtomography, and the latter was thought to be spongy bone. “Internal pneumatic chambers,” which are “neither camerate nor camellate, but a new type of tissue with an intermediate texture,” were discovered in this instance, according to the authors. Because they “are not large enough to be considered camerae but also present a camellate array internally,” they propose to name the new structures “protocamerate.”
“Until now, the most widely accepted theory was that the air sacs started out as camerae and developed into camellae. According to Aureliano, “Our proposal, based on what we observed in this specimen, is that this other form existed first.”
The air sacs that were discovered in the vertebrae also alter what is known about how these structures evolved. Other research groups proposed, based on the previously analyzed fossils, that air sacs first appeared in the abdominal area and did not appear in the cervical area until the early Jurassic (190 million years ago), a long time after M. itaquii lived. The authors, on the other hand, discovered clear evidence of air sacs in the cervical and dorsal regions but not in the abdominal area.
“It’s as if evolution had tried different things before it found the perfect system, where air sacs run from the neck to the tail. Aureliano stated, “It wasn’t a linear process.”
More information: Tito Aureliano et al, The origin of an invasive air sac system in sauropodomorph dinosaurs, The Anatomical Record (2023). DOI: 10.1002/ar.25209
