Scientists have made an exciting discovery that bridges the gap between the earliest small dinosaurs and the gigantic creatures that captured our imagination. The missing link, known as Macrocollum itaquii, was found in Agudo, a town in Rio Grande do Sul state, South Brazil, and is the oldest dinosaur studied so far to possess air sacs. These air sacs, similar to those found in modern birds, allowed dinosaurs to obtain more oxygen, regulate body temperature, and thrive in challenging environments. The presence of air sacs also played a role in the evolution of giant dinosaurs like Tyrannosaurus rex and Brachiosaurus.
The groundbreaking study, published in The Anatomical Record journal, involved researchers from the State University of Campinas (UNICAMP) in São Paulo state. Tito Aureliano, the first author of the article and a Ph.D. researcher at the Institute of Geosciences (IG-UNICAMP), explained that the air sacs made dinosaur bones less dense, enabling them to grow to astonishing lengths of over 30 meters. Meanwhile, Macrocollum itaquii, which lived approximately 225 million years ago, measured about 3 meters in length and represented the largest dinosaur of its time. This species exemplified how air sacs facilitated size increase in dinosaurs.
The study was part of the larger project “Taphonomic landscapes,” which focused on the study of fossilization and preservation processes in paleontology. Fresia Ricardi-Branco, the principal investigator of the project and a professor at IG-UNICAMP, highlighted that Macrocollum itaquii was one of the earliest dinosaurs that roamed the Earth during the Triassic period. The adaptation of air sacs allowed them to thrive in the climate of that period as well as in the subsequent Jurassic and Cretaceous periods. Ricardi-Branco emphasized that air sacs provided dinosaurs with an evolutionary advantage over mammals and facilitated their rapid diversification.
In a previous study, the same research group demonstrated that the earliest known fossils lacked air sacs, indicating that this trait evolved independently at least three times. Macrocollum itaquii, a bipedal sauropodomorph, represented an ancestor of the massive quadrupedal dinosaurs characterized by small heads and necks at least as long as their trunks. This discovery sheds new light on the evolutionary mechanisms that allowed dinosaurs to grow and dominate the prehistoric world.
The recent discovery of air sacs in Macrocollum itaquii has brought forth new insights regarding the composition and evolution of these structures. Previously, researchers had identified two types of tissue in vertebral cavities: camerate, characterized by hollow spaces observed through microtomography, and camellate, referring to spongy bone. However, the authors of the study on M. itaquii propose a new category called “protocamerate” for the internal pneumatic chambers found in this specimen. These structures exhibit an intermediate texture, distinct from both camerate and camellate tissues.
According to Tito Aureliano, the first author of the article, the prevailing hypothesis suggested that air sacs initially developed as camerae and later evolved into camellae. However, the discovery of protocamerate in M. itaquii challenges this notion, indicating that this different form of tissue might have preceded the other types. Therefore, the authors propose the term “protocamerate” to describe these structures, as they are smaller than camerae but still exhibit an internal camellate arrangement.
Moreover, the presence of air sacs in the vertebrae of M. itaquii challenges previous notions about their evolutionary timeline. Previous analyses of fossils led some research groups to propose that air sacs first appeared in the abdominal region and only later emerged in the cervical region during the early Jurassic period, which was long after the time of M. itaquii. However, the authors of the study found clear evidence of air sacs in the cervical and dorsal regions of M. itaquii, with no indications of these structures in the abdominal region.
Tito Aureliano suggests that the evolution of air sacs might have followed a non-linear path, akin to different experiments conducted by evolution. The definitive system, where air sacs extend from the cervical region to the tail, might have been the result of various evolutionary iterations. This finding challenges the notion of a linear progression and suggests a more complex and diverse evolutionary history for air sacs.
In summary, the discovery of protocamerate in M. itaquii introduces a new type of tissue in the vertebral cavities, distinct from camerate and camellate tissues. The presence of air sacs in the cervical and dorsal regions of M. itaquii, rather than the previously assumed abdominal region, challenges existing hypotheses about the evolutionary timeline of these structures. These findings indicate that the evolution of air sacs might have involved multiple experimental paths before reaching the definitive system observed in later dinosaurs.