Bird Brain? Cranial scan of fossil hints at flight capability

A detailed X-ray scan of the fossilized braincase of an Archaeopteryx shows that several features of the ancient feathered creature’s brain and inner ear were highly developed and similar to those of modern birds.

HEADY STUFF. Digital reconstruction of the 20-millimeter-long braincase of Archaeopteryx shows bulges (arrow) that housed the brain’s vision-processing system. P. Domínguez Alonso/Complutense Univ. Madrid

Many scientists consider Archaeopteryx, which lived about 147 million years ago, to be the world’s oldest known bird. The creature had many characteristics of dinosaurs, such as a full set of teeth and a long, bony tail, but it also had wings and feathers. Only seven fossils of the species have been recovered, says Timothy B. Rowe, a paleontologist at the University of Texas at Austin.

Rowe and his colleagues used a computerized tomography (CT) scanner to map the portion of the skull that houses the brain of the Archaeopteryx specimen from the Natural History Museum of London. In the resulting stack of more than 1,300 images representing the 20-millimeter-long braincase, the researchers could discern details as small as 20 micrometers across.

Examining bulges as well as fine markings on the skull’s inner surface, Angela C. Milner, a paleontologist at the museum, and her coworkers reconstructed the overall anatomy of the ancient creature’s brain. The regions associated with vision make up almost one-third of the brain’s volume. Other well-developed lobes include those responsible for hearing and for muscle coordination, Milner notes. The researchers estimate the brain’s total volume to be 1.6 cubic centimeters, or about a third of a teaspoon. Milner, Rowe, and their teammates report their CT analyses in the Aug. 5 Nature.

An adult Archaeopteryx, about the size of today’s magpies and grackles, probably weighed just under half a kilogram, says Milner. The creature’s ratio of brain volume to body mass ranks well above that typical of modern reptiles but somewhat below those of today’s birds. For instance, birds with the same body mass as Archaeopteryx have brains that range from 30 percent larger to five times as large. However, the ancient species’ brain is about three times the volume of brains of modern reptiles of similar mass.

The skull scan also revealed the detailed structure of Archaeopteryx‘s inner ear. The arrangement of canals there more closely resembles that of modern birds than of reptiles, says Milner. Also, the length of the cochlea—the tubular duct in the ear where sounds are converted into nerve impulses—is similar to that of modern birds but longer than that of reptiles. These characteristics suggest that Archaeopteryx had keen senses of hearing, balance, and spatial perception.

Such attributes would be critical for flying, says Lawrence M. Witmer, a paleontologist at Ohio University in Athens. Archaeopteryx‘s highly developed brain would have served as the on-board computer necessary to manipulate the creature’s wings and feathered flight surfaces. Pterosaurs, a lineage of flying reptiles that ruled the skies between 235 million and 65 million years ago and is unrelated to Archaeopteryx, had similar enhancements of the brain and inner ear, says Witmer.

Results of the new research are important for several reasons, Witmer contends. For one thing, paleontologists can compare Archaeopteryx‘s anatomy with that of the reptiles that preceded it and look for patterns of brain and inner ear evolution. Also, scientists can look at braincase structures in more-recent flightless feathered dinosaurs, such as Caudipteryx (SN: 8/19/00, p. 119: Available to subscribers at Feathered fossil still stirs debate), to see whether those creatures might be descendants of early birds.

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