With an impressive array of feathers, the 147-million-year-old Archaeopteryx is clearly dressed like a bird. But almost all of its skeleton, from its teeth to its long, bony tail, resembles that of a carnivorous dinosaur. Now, the first look inside the head of this classic transitional fossil reveals a fundamentally birdlike brain, well suited for flying.
Paleontologist Angela Milner of the Natural History Museum in London, U.K., and colleagues inspected the brain of the so-called London specimen of Archaeopteryx, one of seven known fossils of the magpie-sized creature. Although the brain itself isn't preserved, during life the brain pressed against the skull, leaving an impression of its lobes.
Working with paleontologist Tim Rowe and his imaging team at the University of Texas, Austin, the researchers scanned the 20-millimeter-long braincase with an industrial computerized tomography (CT) scanner, which has a higher resolution than medical CT scanners. They assembled the images by computer into a three-dimensional reconstruction of the brain (Science, 9 June 2000, p. 1728).Archaeopteryx's brain was much like that of modern birds, the group reports this week in Nature. For starters, it's big relative to body mass. With a volume of about 1.6 milliliters, its brain was three times larger than those of living reptiles. But it wasn't full-fledged: Modern birds, for their body size, have brains that are 33% to 500% larger than Archaeopteryx's.Birdlike features of the anatomy include enlarged cerebral lobes, compared with its reptile relatives. In living birds, these lobes process sensory information from the inner ear and muscles. "It's the command and control center for flight," Milner explains. That center was likely kept busy: Feeding into it were the optic lobes, each enlarged almost to the size of the cerebellum and located on the sides of brain--just as they are in birds and pterosaurs.The new view of the anatomy will help explain the evolutionary transition from dinosaurs to birds and the evolution of flight, says Lawrence Witmer, a paleontologist at Ohio University College of Osteopathic Medicine in Athens. "Archaeopteryx was agile, quick, and jerky in its movements," says Witmer, who likens the extent of its acrobatics more to those of a chicken than to those of a falcon or swallow. Even though Archaeopteryx lacked some of the skeletal features needed to fly like an eagle, it appears to have evolved all the brains for it.