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Dinosaur-Era Bird Could Fly, Brain Study Says

John Pickrell in England
for National Geographic News
August 4, 2004
 
The earliest known bird was discovered in a Bavarian quarry in 1861. Ever since, scientists have disagreed as to whether Archaeopteryx was fully capable of flight.

Exquisitely preserved fossils reveal that the winged, feathered animal had numerous modern birdlike features, but much of its primitive reptilian skeleton betrays a close kinship to meat-eating dinosaurs.

Now a study into the shape of Archaeopteryx's brain says that the animal already possessed many of the prerequisites for flight, such as great vision and a good sense of balance—traits all birds share today.

The analysis, which will be detailed tomorrow in the science journal Nature, provides some of the best evidence yet that Archaeopteryx spent much of its time on the wing.



"If you fly, you need a very sophisticated coordination-and-control command center," said study co-author Angela Milner of London's Natural History Museum. "We can now show that the brain and sensory systems of Archaeopteryx were fully equipped for flight."

Dinosaur? Bird? Both?

Archaeopteryx was discovered shortly after the publication of Charles Darwin's Origin of Species (1859). In the book, Darwin described his theory of evolution by natural selection.

Considered a bird, 147-million-year-old Archaeopteryx nevertheless seems to be a halfway house between birds and dinosaurs. Since Victorian times, it has been "taken as one of the icons of evolution in action," Milner said.

Like a bird, Archaeopteryx had feathered wings. But it also had a cumbersome bony tail and lacked the large breastbone (and therefore the wing-powering mass of muscle on its chest) that are characteristic avian traits, Milner said.

Some experts have argued that the animal used its wings to hop and scrabble about in trees rather than for powered flight.

However, "the majority of scientists now accept that it could get airborne," said fossil-anatomy expert Larry Witmer of Ohio University's College of Osteopathic Medicine. Witmer is the author of an accompanying commentary that will also be published in tomorrow's Nature.

"The question is, once it was airborne, was it just a glider, a weakly flapping flyer, or a strong flyer?" he said.

To answer this question, Milner joined Patricio Dominguez Alonso of Universidad Complutense de Madrid in Spain, and other colleagues. They applied modern imaging techniques to the first discovered Archaeopteryx fossil. One of just six in existence, the specimen is housed in London's Natural History Museum.

The team used x-ray technology called computed tomography. In this technique a computer combines a series of flat cross-sectional images to create a three-dimensional model of a body structure. The process allowed Milner and her colleagues to look inside the animal's tiny braincase and make a 3-D reconstruction of its brain and inner ear.

Earlier, Witmer and others had performed a similar study on fossils of the flying reptiles called pterodactyls, which are not related to Archaeopteryx. Published last year in Nature, the pterodactyl study showed that these reptiles had many of the same brain features as modern birds. The report suggested that certain brain features are minimum requirements for the development of flight.

Birdbrained

The new analysis by Milner and her colleagues shows for the first time that Archaeopteryx's brain had many of the features that birds use to hone their flying abilities today.

Archaeopteryx's brain was smaller, in proportion to its body size, than the average bird brain today. The ancient creature's brain, though, was around three times as big as the brains of comparably sized reptiles of the same time period.

Furthermore, the way Archaeopteryx's brain was organized was also very birdlike, according to the study. The cerebral hemispheres and other parts of the brain involved with vision and movement were relatively large. And the size and shape of Archaeopteryx's inner ear hint that the animal had a keen sense of balance and spatial awareness.

"Though less sophisticated than modern birds, Archaeopteryx appears to have had all the neurosensory mechanisms necessary for flight," Milner said. "It probably wasn't an endurance flyer but was certainly capable of proper, powered, flapping flight."

Finding that the brain is so sophisticated in this species is surprising. Though Archaeopteryx is the oldest bird known from the fossil record, the discovery suggests that flight must have begun long before and therefore much further back in time than expected, Milner said.

"Ground-dwelling animals live for the most part in two-dimensional space, but flying animals live very much in three dimensions," Ohio University's Witmer said. "So the ability to sense your position in space and use that information to make constant adjustments needs to be very well developed."

These pressures lead to the development of certain parts of the brain in both birds and pterodactyls, he said.

Analyzing the brains of some meat-eating dinosaurs for similarities to Archaeopteryx's brain may allow researchers to test a highly controversial theory that some dinosaurs are the "secondarily flightless" descendents of Archaeopteryx, Witmer said.

Some researchers have suggested that Archaeopteryx could in fact be the ancestor of a group of dinosaurs that includes velociraptors, made famous by Jurassic Park. Those carnivores have many similarities to Archaeopteryx but appear later in the fossil record.

The study by Milner and her co-authors will now "allow us to tease apart the transition between birds and dinosaurs in a whole new way," Witmer added.

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