ATHENS, Ohio -- Pterosaurs, which emerged as the first flying vertebrates during the age of dinosaurs, could grow as large as an airplane but soared through the skies with ease. New research suggests that a specialized brain and inner ear structure helped these ancient reptiles to fly and target their prey, a finding that could give scientists insight into the evolution of the brain and visual system.
Compared to modern reptiles such as alligators and lizards, pterosaurs -- commonly known as pterodactyls -- had a complicated neural system that allowed them to make deft use of massive wings and adapt to an airborne lifestyle, said Lawrence Witmer, an associate professor of anatomy in Ohio University's College of Osteopathic Medicine and lead author of the study, which will be published in the Oct. 30 issue of the journal Nature.
"These comparisons generally inform us about the rigors of flight and how animals make it work," said Witmer, whose research is funded by the National Science Foundation. Fossils of pterosaurs, which lived during the Mesozoic Era, are rare and often badly crushed. But Witmer's colleagues recently obtained nearly intact skulls of Rhamphorhynchus, a small species with a 3-foot wingspan and 4-inch-long skull that lived 150 million years ago in what is now Germany, and Anhanguera, a larger creature with a 14-foot wingspan and a 20-inch skull that lived 115 million years ago in what is now Brazil. Sankar Chatterjee of Texas Tech University and Jonathan Franzosa and Timothy Rowe of the University of Texas ran the skulls through a high-resolution CT scanner. Witmer and the Texas collaborators used the scans and sophisticated computer graphics software to reconstruct the brain cavity and inner ear canals. The scientists compared the scans to alligators and birds, which are the closest living relatives of pterosaurs.
"Pterosaurs and birds developed flight independently, but they're fairly closely related -- birds are dinosaurs and pterosaurs are close cousins of dinosaurs," Witmer said. "We can compare pterosaurs and birds to test hypotheses on how evolutionarily similar, but still quite distinct, animals adapted to life in the air."
The brains of the modern birds and the extinct flying creatures bore some similarities, but the pterosaur had a much larger flocculus, which is part of the portion of the brain that controls movement. The flocculus processes information on body, neck and head position and then relays this data to the muscles that move the eyes. This allows an animal to fix its gaze on a target, regardless of its body movements. Though other studies have noted the unusual size of the pterosaur flocculus, Witmer and his colleagues wanted to explore why it was so big and what it meant for the creature's ability to fly and hunt.
The new study suggests that the bigger neural center was necessary to process sensory data from the pterosaurs' wings, massive skin-covered structures full of sensitive muscle fibers.
"They recruited the wing as this extra sensory organ and linked it with neck, head and, ultimately, eye movement," Witmer said. "The body can change position, but the eyes stay focused on their prey."
That allowed the pterosaur to become a highly adapted, visually oriented flying predator, he said. The creatures may have lived over the ocean and hunted fish on the wing to survive.
The CT scans of the pterosaurs' inner ears also suggest that the larger of the pterosaurs in the study, Anhanguera, may have carried its head in a downward position while Rhamphorhynchus held its head more horizontally, Witmer said. When the scientists created a computer model of Anhanguera's brain, they noticed that the semicircular canals, inner ear structures that provide the sense of balance, were very large but not oriented in the skull in the same way seen in Rhamphorhynchus or other vertebrates. When the scientists aligned the inner ear canals in the correct position in the head, the new model suggested a dramatically different head posture for Anhanguera. The downward-tipped snout probably improved the animal's binocular vision and helped it move on land, the scientist said.
The finding could lead to further studies on head posture in dinosaurs, said Witmer, whose previous research has shed light on the correct facial features of dinosaurs -- from designing a new nose for Diplodocus to removing the lips on Tyrannosaurus rex.
"If you look closely enough, fossils will reveal a whole lot more than we ever thought," he said.
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