A team of archaeologists and paleontologists from Argentina, the U.S. and China has unearthed the first known example of a fossilized Alvarezsauridae skeleton with evidence of air pockets in its bones. In their paper published in the open-access journal PLOS ONE, the group describes where they found the fossil, its condition, and their surprise at finding evidence of pneumaticity in most of its bones.
Over many years, scientists have found evidence of pneumaticity (air pockets in bones) in archosaurs, including pterosaurs and saurischian dinosaurs—but never in Alvarezsauridae, which were a family of long-legged dinosaurs. In modern times, birds are the only form of archosaurs to have pneumaticity.
The research team was working at a dig site at a middle Campanian-lower Maastrichtian Allen Formation located in Río Negro Province, in Argentina, when they unearthed what were clearly several Alvarezsauridae fossils of various sizes. They were dated to approximately 70 million years ago.
After bringing the fossils back to lab for study, which included conducting CT scans at a local hospital, the researchers found examples of air pockets throughout its axial skeleton—some in unexpected places, such as halfway down the length of its tail. They also found uneven patterns of air pockets in different fossils, suggesting that there was no clear means of progression, a possible sign that their purpose may have been associated with blood pathways.
Prior research has shown that, in addition to reducing weight, air pockets in the bones of birds help with maintaining body temperature and assisting with respiratory activities. Scientists have therefore hypothesized that dinosaurs developed air pockets in their bones for similar reasons. Reducing weight in some of the larger dinosaurs, they note, could have reduced energy requirements and allowed the creatures to move more quickly.
They suggest their finding of pneumaticity in Alvarezsauridae skeletons hints at the possibility that it also evolved in other dinosaurs. They state future studies may uncover more examples.
More information: G.J. Windholz et al, First unambiguous record of pneumaticity in the axial skeleton of alvarezsaurians (Theropoda: Coelurosauria), PLOS ONE (2025). DOI: 10.1371/journal.pone.0320121
Journal information: PLoS ONE
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