Paleontologists have unearthed the fossilized remains of two different large-bodied azhdarchoid pterosaur species — the new species Inabtanin alarabia and the previously known species Arambourgiania philadelphiae — in the Upper Cretaceous horizons of Jordan. The fossils have enabled the researchers to hypothesize that not only could the largest pterosaurs take to the air, but their flight styles could differ too.
Inabtanin alarabia (top) flew by flapping their wings while Arambourgiania philadelphiae (bottom) soared like vultures. Image credit: Terryl Whitlatch.
Pterosaurs are a group of extinct flying reptiles that existed from the Late Triassic to the end-Cretaceous mass extinction event.
They evolved powered flight approximately 50 million years before flight feathers appeared in the fossil record, and some achieved wingspans twice that of the largest flying birds (12 m vs. 6 m).
“Our team was extremely surprised to find three-dimensionally preserved pterosaur bones, this is a very rare occurrence,” said University of Michigan paleontologist Kierstin Rosenbach.
“Since pterosaur bones are hollow, they are very fragile and are more likely to be found flattened like a pancake, if they are preserved at all.”
Dr. Rosenbach and colleagues discovered two specimens of azhdarchoid pterosaurs in Jordan: one is referrable to the giant species Arambourgiania philadelphiae (ca. 10 m wingspan) and the second to a new, smaller species named Inabtanin alarabia (ca. 5 m wingspan).
They used high-resolution computed tomography (CT) scans to then analyze the internal structure of the wing bones.
“With 3D preservation being so rare, we do not have a lot of information about what pterosaur bones look like on the inside, so I wanted to CT scan them,” Dr. Rosenbach said.
“It was entirely possible that nothing was preserved inside, or that CT scanners were not sensitive enough to differentiate fossil bone tissue from the surrounding matrix.”
“Luckily, though, what we uncovered was remarkable, via exciting internal structures not only preserved, but visible in the CT scanner.”
The new specimen of Arambourgiania philadelphiae confirms its 10-m wingspan and provides the first details of the reptile’s bone structure.
The CT images revealed that the interior of its humerus, which is hollow, contains a series of ridges that spiral up and down the bone. This resembles structures in the interior of wing bones of vultures.
The spiral ridges are hypothesized to resist the torsional loadings associated with soaring (sustained powered flight that requires launch and maintenance flapping).
The specimen of Inabtanin alarabia is one of the most complete pterosaurs ever recovered from Afro-Arabia.
The CT scans revealed the structure of its flight bones was completely different from that of Arambourgiania philadelphiae.
The interior of the flight bones were crisscrossed by arrangement with struts that match those found in the wing bones of modern flapping birds.
This indicates it was adapted to resist bending loads associated with flapping flight, and so it is likely that Inabtanin flew this way — although this does not preclude occasional use of other flight styles too.
“The struts found in Inabtanin alarabia were cool to see, though not unusual,” Dr. Rosenbach said.
“The ridges in Arambourgiania philadelphiae were completely unexpected, we weren’t sure what we were seeing at first.”
“Being able to see the full 3D model of Arambourgiania philadelphiae’s humerus lined with helical ridges was just so exciting.”
The discovery of diverse flight styles in differently-sized pterosaurs is exciting, because it opens a window into how these animals lived.
It also poses interesting questions, like to what extent flight style is correlated with body size and which flight style is more common among pterosaurs.
“There is such limited information on the internal bone structure of pterosaurs across time, it is difficult to say with certainty which flight style came first,” Dr. Rosenbach said.
“If we look to other flying vertebrate groups, birds and bats, we can see that flapping is by far the most common flight behavior.”
“Even birds that soar or glide require some flapping to get in the air and maintain flight.”
“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.”
A paper on the findings was published in the Journal of Vertebrate Paleontology.
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Kierstin L. Rosenbach et al. New pterosaur remains from the Late Cretaceous of Afro-Arabia provide insight into flight capacity of large pterosaurs. Journal of Vertebrate Paleontology, published online September 5, 2024; doi: 10.1080/02724634.2024.2385068
Source : Breaking Science News