Scientists have used CAT scans, a staple of modern hospitals, to peer inside the heads of extinct reptiles known as pterosaurs, the first flying vertebrates to emerge during the age of the dinosaurs. The findings, published today in the journal Nature, indicate that a specialized brain and inner ear structure allowed the creatures to fly and hunt down prey successfully.
Lawrence M. Witmer of Ohio University and his colleagues analyzed fossil skulls from two species of pterosaurs: Rhamphorhynchus, which lived 150 million years ago in what is now Germany, and Anhanguera, which dates to 115 million years ago and was discovered in what is now Brazil. Because making casts of the remains is extremely difficult due to their delicate nature, the researchers turned to computerized axial tomography. The CAT scanner they used is many times more powerful than machines used for medical purposes and allowed them to develop three-dimensional computer images of the brain cavities and ear canals of the two pterosaurs. The team then compared the results to images obtained from alligators and birds, the creatures' closest living relatives. "We can compare pterosaurs and birds to test hypotheses on how evolutionarily similar, but still quite distinct, animals adapted to life in the air," Witmer explains.
The scientists noticed a number of differences between the extinct aerial reptiles and their extant cousins. For one, the semicircular canals, inner ear structures that aid in balance, were very large in Anhanguera. In addition, their orientation suggests that Anhanguera hunted with its snout pointed downwards, unlike Rhamphorhynchus and other vertebrates who hunt with their heads oriented more horizontally.
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The size of a brain region known as the flocculus, which is involved in controlling movement, is particularly noteworthy. In both Rhamphorhynchus, a small species with a three-foot wingspan and a four-inch skull, and the much larger Anhanguera, which had a 14-foot wingspan and 20-inch skull, the flocculi occupied about 7.5 percent of the total brain mass. Birds, in contrast, utilize only about 2 percent of their brains for this region, and mammals use even less than that. (The image above shows Anhanguera, (foreground) and Rhamphorhynchus. The floculli appear in green.) The scientists suggest that this enlargement was necessary to process the abundant amounts of information gathered by the pterosaurs wing membranes. "They recruited the wing as this extra sensory organ and linked it with neck, head and, ultimately, eye movement," Witmer comments. "The body can change position, but the eyes stay focused on their prey." This adaptation made the creatures agile flyers. "Equipped with their 'smart' wings, pterosaurs would have had excellent flight control," writes David M. Unwin of Humboldt University in Berlin in an accompanying commentary. "Despite their antiquity, they could even have outperformed modern birds and bats."