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Unique Bone Structure Helped Long-Necked Pterosaurs Fly

Bicycle wheel-like spokes connected the vertebrae’s central column to its outer surface, offering serious strength

The largest pterosaurs had wingspans like small aircraft and longer necks than giraffes. (Illustration by Davide Bonadonna)
smithsonianmag.com

The azhdarchid group of pterosaurs were some of the largest animals ever to fly. The giant reptiles lived between 66 million and 225 million years ago, and their unusual proportions—big heads at the end of long, inflexible necks—have long puzzled scientists.

Now, research published on Wednesday in the journal iScience offers a detailed look inside of the neck vertebrae of one pterosaur. To achieve the strength necessary to hold up its head and carry prey, while keeping the bone lightweight enough for flight, the vertebra has spokes like a bicycle wheel that connect a center column to the outer surface of the bone. The spokes follow a helical pattern, like a spiral staircase leading through the inside of the bone.

“We just could not believe it,” says University of Illinois, Urbana-Champaign paleontologist Cariad Williams, to Rodrigo Pérez Ortega at Science magazine. “We have never seen anything like it before. … It was really impressive.”

The research focused on one example of pterosaur vertebra that measured 6.2 inches long. While studying the surface of the specimen, the researchers had an unexpected opportunity to use a CT scanner to take measurements.

“It seemed churlish to turn the offer down,” says University of Portsmouth paleontologist David Martill, lead author of the study, in a statement.

The CT scan revealed the spokes, which are called trabeculae, inside the bone. Like bicycle wheel spokes, trabeculae that were close together often overlapped.

A mathematical model of the vertebrae showed how effective the spokes are at strengthening the bone without adding much weight. An azhdarchid pterosaur has nine vertebrae in its neck, an animal with a four-foot-long neck would have vertebrae of about 5.5 inches long each.

For a vertebra of that size, just 50 spokes would allow the pterosaur to lift 90 percent more weight than if it had no spokes at all. That means it could carry away prey that weighed up to 24 pounds.

“They were using less energy to optimize their strength in their neck to be able to lift the prey,” says Williams to Becky Ferreira at the New York Times.

The research begins to answer questions raised by previous research, which suggested that pterosaurs had very little flexibility in their necks, the Williams and Martill write for the Conversation. Modern long-necked birds, like herons, usually bend their necks and tuck their heads close to their bodies while they fly.

But pterosaurs would have kept their necks outstretched. Therefore, their necks needed to support the weight of the head not only against gravity but also against the strong winds that would hinder their flight. At the same time, their bones became thinner and filled with air sacs to make them lightweight.

Those lightweight, thin bones makes it difficult to find well-preserved specimens today. The vertebra used in the new study was found at the Kem Kem fossil bed in Morocco. About 100 million years ago, the fossil bed held a river that was home to Cretaceous sharks and carnivorous dinosaurs, alongside pterosaurs.

The study offers a “nice confirmation” of the mechanical structure of azhdarchid vertebrae, says Queen Mary University of London paleontologist David Hone, who was not involved in the study, to the New York Times. But the paper does not address whether the spoke structure is unique to azhdarchids or shared among many pterosaurs.

“It’s a very neat finding that there is this weird arrangement of struts and that this is about the minimum possible to strengthen the bone,” says Hone to the Times. “But it’s also not much of a surprise as we know azhdarchids had incredibly reduced bones and were extraordinarily light for their size.”

Paleontologist Alexander Kellner, the director of the National Museum at the Federal University of Rio de Janeiro, tells Science magazine, “I’m not very impressed. I think you should have more specimens to really make a claim.”

The researchers plan to take more measurements of additional vertebrae when they find more well-preserved specimens. Paleontologist and co-author Nizar Ibrahim of the University of Portsmouth tells Science magazine the team has several locations in mind to search after the Covid-19 pandemic.

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