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Well-preserved fossils of ancient large reptiles called pterosaurs have revealed that some species flew by flapping their wings, while others soared like vultures, according to a new study.
Pterosaurs dominated the skies during the time of the dinosaurs and met the same deadly fate 66 million years ago after an asteroid strike triggered a mass extinction event. Some of the largest pterosaur species were giants that reached the size of small airplanes and stood to heights comparable to giraffes, causing researchers to question whether pterosaurs were even capable of flight.
The newly discovered fossils preserved 3D structures within the delicate wing bones, which typically are found flattened like pancakes within rock layers.
CT scans of the fossils provided a rare glimpse inside the wing bones belonging to two species of pterosaurs, including one new to science.
The results of the research, published Friday in the Journal of Vertebrate Paleontology, shed light on a remarkable and unexpected finding: Not only could giant pterosaurs fly, but different species adapted varied flight styles.
Intact fossils offer window to the past
The fossils date back 66 million to 72 million years in the late Cretaceous Period. The team initially found the specimens in 2007 at two sites in the north and south of what’s now Jordan, buried in deposits from an ancient landmass called Afro-Arabia that once included Africa and the Arabian Peninsula.
After realizing the hollow bones still contained their original structures, the research team was eager to analyze them using high-resolution CT scans, said lead study author Dr. Kierstin Rosenbach, paleontologist and researcher within the department of Earth and environmental sciences at the University of Michigan at Ann Arbor.
Some of the fossils belonged to a giant pterosaur known as Arambourgiania philadelphiae and provided a first look at its bone structure, as well as confirmation that it had a wingspan of 32.8 feet (10 meters). The team noted a series of ridges that spiraled up and down its hollow humerus bone.
The remaining fossils were part of a pterosaur that is new to science called Inabtanin alarabia. It’s named after the large grape-colored hill, Tal Inab, where it was unearthed. The name combines the Arabic words “inab,” for grape, and “tanin,” for dragon, while “Alarabia” refers to the Arabian Peninsula.
Inabtanin alarabia is one of the most complete pterosaur fossils ever found from this region, according to the researchers. The reptile was smaller than Arambourgiania with a wingspan of 16.4 feet (5 meters).
When the researchers scanned the flight bones, they realized they were looking at a completely different structure than that of Arambourgiania.
The flight bones for Inabtanin included an interior structure of struts, or strengthening rods that helped them fly. These are not unlike those found in the wing bones of modern birds that flap their wings to fly, Rosenbach said.
In contrast, spiral ridges within Arambourgiania’s wing bones resembled the interiors of vulture wing bones, which are thought to resist the forces associated with soaring.
“The struts found in Inabtanin were cool to see, though not unusual,” Rosenbach said in a statement. “The ridges in Arambourgiania were completely unexpected, we weren’t sure what we were seeing at first.”
A diversity of pterosaur flight
The largest flying modern bird is the Andean condor, which has a wingspan of 9 feet (about 2.8 meters). But pterosaurs had massive wingspans that could reach 16.4 to 39.3 feet (5 to 12 meters).
“They represent the largest animals with the capacity to fly,” Rosenbach said of the extinct reptiles.
Discovering that pterosaurs adapted different flight styles is exciting because it provides insight into the behaviors and lifestyles of these ancient reptiles, the researchers said.
“I think they would look noticeably different if we could watch them fly side by side,” Rosenbach said. “Inabtanin would have been flapping its wings similar to modern birds, but Arambourgiania would more likely have been soaring with some wing flapping, much like a vulture or pelagic seabird.”
The fossils didn’t yield any insights into how the pterosaurs took off from the ground, but the team is using its findings to determine how these varied flight styles evolved.
“The variation in internal structure likely reflects the bone’s response to mechanical forces applied on the wings of pterosaurs,” said study coauthor Jeff Wilson Mantilla, curator and professor at the University of Michigan’s Museum of Paleontology.
The researchers can’t say for sure which style came first, although when looking at birds and bats, flapping is the most common, Rosenbach said. And even birds that soar or glide need some flapping to help them get up in the air and maintain flight.
Flight styles likely evolved due to a combination of factors, such as the pterosaurs’ environment, their body shape and size, and how they hunted for prey, the authors said.
The scientists found both fossils in areas where a large shallow sea once existed, so each species may have adapted different behaviors to forage in the same environment, Rosenbach said.
“This leads me to believe that flapping flight is the default condition, and that the behavior of soaring would perhaps evolve later if it were advantageous for the pterosaur population in a specific environment; in this case the open ocean,” she said.
An evolving look at ancient flight
Pterosaur wing bones had to cope with the strain of flight while remaining lightweight, which is why the hollow bones show different strengthening structures within their bone walls, said Michael Benton, professor of vertebrate paleontology at the University of Bristol in the United Kingdom.
“This is a nice study of the structure of the vertebrae of two large pterosaurs, one big and one huge,” said Benton, who was not involved in the research. “It’s always been a mystery how pterosaurs could be light enough and yet strong enough to fly, especially the many examples that were much larger than any known bird. This paper helps to provide the answer.”
The study authors believe their findings submit new evidence to the ongoing debate among paleontologists on whether the most massive pterosaurs could fly.
“The internal bone structure of these fossils suggest that they experienced the mechanical forces associated with flight,” Rosenbach said. “We can think of these findings as one piece in the puzzle of growing evidence that large pterosaurs maintained the ability to fly at extremely large body sizes.”
The research team is eager for the chance to see more scans of pterosaur bones and determine how the newly discovered pterosaur Inabtanin relates to the rest of the ancient reptiles.
“There is growing evidence that pterosaurs were more diverse approaching the big Cretaceous-Paleogene extinction event than we previously thought,” Rosenbach said via email, referring to the mass extinction of dinosaurs and most life on Earth. “This indicates that the extinction was catastrophic as opposed to a slow process of extinction for large reptiles.”
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