Paleontologists have examined an almost complete skeleton of Macrocollum itaquii, an unaysaurid sauropodomorph dinosaur that lived in what is now Brazil during the Late Triassic epoch.
Life restoration of Macrocollum itaquii. Image credit: Nobu Tamura, http://spinops.blogspot.com / Alan Frijns / Sci.News.
Sauropods were an emblematic clade of long-necked dinosaurs that colonized all continents during the Jurassic and Cretaceous.
Some lineages attained giant sizes, evolving into the largest animals to ever inhabit terrestrial landscapes.
One of the features that allowed the body enlargement was a structural decrease of density caused by the presence of a bird-like air sac system.
“Air sacs made their bones less dense, allowing them to grow to more than 30 m (98 feet) in length,” said Dr. Tito Aureliano, a paleontologist at the University of Campinas.
In the new research, Dr. Aureliano and colleagues analyzed Macrocollum itaquii, the oldest and largest long-necked sauropodomorph dinosaur of the Lower Norian Candelária Sequence of South Brazil.
“Macrocollum itaquii was the largest dinosaur of its time, with a length of about 3 m (9.8 feet),” Dr. Aureliano said.
“A few million years before then, the largest dinosaurs were about 1 m (3.3 feet) long. Air sacs certainly facilitated this increase in size.”
“Until air sacs were discovered in Macrocollum itaquii, these vertebral cavities were known to consist of either camerate or camellate tissue, the former referring to hollow spaces observed by microtomography, and the latter to spongy bone.”
“In this case, we found internal pneumatic chambers, which are neither camerate nor camellate, but a new type of tissue with an intermediate texture.”
“We propose to call the new structures protocamerate, as they are not large enough to be considered camerae, but also present a camellate array internally.”
Skeletal reconstruction of the unaysaurid sauropodomorph Macrocollum itaquii showing vertebral elements along the spine and putative reconstruction of the air sac systems involved. Image credit: Aureliano et al., doi: 10.1002/ar.25209.
“The most widely held hypothesis until now was that the air sacs began as camerae and evolved into camellae,” Dr. Aureliano said.
“Our proposal, based on what we observed in this specimen, is that this other form existed first of all.”
The vertebrae in which the air sacs were found also change what was known about the evolution of these structures.
“Based on the fossils analyzed previously, other research groups proposed 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, a long time after the period in which Macrocollum itaquii was alive,” Dr. Aureliano said.
“However, we found clear evidence of air sacs in the cervical and dorsal regions, with no sign of the structures in the abdominal region.”
“It’s as if evolution had conducted different experiments until it arrived at the definitive system, in which air sacs run from the cervical region to the tail. It wasn’t a linear process.”
The study was published in the journal Anatomical Record.
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Tito Aureliano et al. The origin of an invasive air sac system in sauropodomorph dinosaurs. Anatomical Record, published online March 27, 2023; doi: 10.1002/ar.25209
