Paleontologists in Argentina recently found the first unambiguous evidence that a group of theropod dinosaurs had hollow bones capable of holding air sacs – an ability that helps birds fly, according to a study published in PLOS One.

The discovery adds to a growing body of research that has revealed that all theropods and sauropods may have had bones with air sacs.

“This is one feature that they inherited [from a common ancestor],” says Guillermo Windholz, a paleontologist at Argentina’s National Scientific and Technical Research Council (CONICET) and the National University of Rio Negro. “This is the reason why actual birds fly — it’s a really crazy feature.”

Dinosaurs with Hollow Bones

Alvarezsaurids were a group of theropod dinosaurs that lived mostly in Argentina in the Late Cretaceous roughly 83 million years to 86 million years ago. They weren’t huge — the smallest of them were roughly chicken-sized while the largest ones found were about 10 feet to 13 feet long from snout to tail.

It’s unclear what these creatures ate, though based on their teeth, they were carnivorous, Windholz says.

Birds today have hollow bones, which give space for soft tissue air sacs that are connected to the lungs. These features help lighten the bodies of birds — important especially for flight.

Pterosaurs and some dinosaurs also have hollow bones, though it’s not entirely clear that these features allowed space for air sacs. One study published in 2022 found that some of the oldest known avemetatarsalians — the ancestors of birds, pterosaurs, and dinosaurs — did not have traces of air sacs. Neither did ornithischians, one of the three major groups of dinosaurs alongside theropods and sauropods. But all saurischians so far examined did have these features.

Read More: As Meat-Eating Hunters With Strong Jaws, Theropods Ruled the Mesozoic

Examining Hollow Bones with CT Scans

Most dinosaur bones are merely examined on the outside — determining whether they are hollow would require cracking them open. As such, while some are believed to be hollow, many dinosaur bones haven’t been examined thoroughly enough to prove for sure that the cavities inside vertebrae are somehow connected to the lungs. This means it’s difficult to determine whether these cavities contain air sacs similar to birds today.

But technology like CT scans have allowed for a more thorough examination of the insides of fossils.

In the recent study, Windholz and his colleagues did just that, and scanned 11 vertebrae from various Bonapartenykus ultimus specimens.

The CT scans revealed that the B. ultimus vertebrae were indeed hollow — not exactly a surprise since most other theropods and sauropods examined had hollow bones. But not all vertebrae are pneumatic — some theropods have tail vertebrae that didn’t fill with air, Windholz says.

But the scans allowed Windholz and his colleagues to see that these hollow cavities inside the vertebrate were connected to the outside through cortical openings called foramina. Essentially, these are passages in which air might have traveled from the dinosaur’s lungs to the inside of vertebrae and back.

Why Did Dinosaurs Have Hollow Bones?

Windholz’ recent study has helped improve paleontologists’ knowledge of the use of these hollow cavities in theropods — it has confirmed that at least in alvarezsaurids, the cavities were used for air sacs.

It’s more difficult to determine why these creatures needed hollow bones and air sacs in their vertebrae. For fast-moving theropods, hollow bones may have helped to increase their speed by lightening their load.

But if this is true, then why do large sauropods also have hollow bones? These large creatures, which presumably walked around on four solid legs, wouldn’t have been trying to take flight, after all. Some scientists have speculated that the hollow bones may have helped them lighten their superlative mass as they moved around, but there isn’t necessarily a consensus about this, Windholz says.

The 2022 study suggested that sauropods, pterosaurs, and theropods may have all evolved air sacs in their bones independently from one another. But this may be because the vertebrae from a more recent common ancestor from the avemetatarsalians, that does have evidence of air sacs, has yet to be examined or discovered.

If there is a common ancestor, it’s also possible that hollow bones were a relic from the ancestors of these dinosaurs, Windholz says. What may have been useful in saurischians, for example, may have been essentially useless — a Mesozoic appendix — in sauropods and theropods.

At least until birds and their direct ancestors came along and used these air-filled bones to take flight.

Read More: About 120 Million Years Ago, a Theropod With Giant Claws May Have Ruled Australia

Article Sources

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Joshua Rapp Learn is an award-winning D.C.-based science writer. An expat Albertan, he contributes to a number of science publications like National Geographic, The New York Times, The Guardian, New Scientist, Hakai, and others.