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Science researchers catch an early bird

16.06.2006
Cretaceous Chinese fossil suggests aquatic origins for modern bird lineage

If it flapped its wings, dove like a duck, and lived in China about 110 million years ago, it must be Gansus yumenensis, one of the oldest members of the lineage leading to modern birds. Spectacular new fossil specimens of the loon-like ancient bird, reported in the June 16, 2006, issue of the journal Science, help fill in the avian family tree and suggest that today's birds may have gotten their start in aquatic environments.


Reconstruction of the Early Cretaceous (~110-115 million year old) amphibious bird Gansus yumenensis, in a lake in what is now the Changma Basin of northwestern Gansu Province, China. Despite its antiquity, Gansus is remarkably closely related to modern birds -- the most advanced Early Cretaceous bird yet discovered. Gansus also demonstrates that the ancestors of today's birds may have been semiaquatic in habit. This image relates to article that appeared in the 16 June 2006 issue of the journal Science, published by AAAS. The study, by Dr. Hai-lu You of the Chinese Academy of Geological Sciences; Dr. Matthew Lamanna of Carnegie Museum of Natural History; Dr. Jerry Harris of Dixie State College of Utah; and colleagues, was titled "A Nearly Modern Amphibious Bird from the Early Cretaceous of Northwestern China." Credit: Image courtesy of Mark A. Klingler/CMNH, via Science-AAAS.

The five well-preserved fossils of Gansus described in Science include nearly complete skeletons consisting of three-dimensional, mostly uncrushed bones. Several of the specimens are so exquisitely preserved that the carbonized remains of feathers and even webbing in the foot can be seen clearly.

Details in the hind legs and feet of Gansus indicate that the bird was probably a foot-propelled diver like today's grebes, loons, and many ducks, although they may not have been as good at diving as their modern counterparts, according to Hai-lu You of the Chinese Academy of Geological Sciences and his colleagues.

Gansus belongs to a lineage of birds called the Ornithurae, which includes all modern birds (Neornithes) and their immediate fossil ancestors. Gansus is not a member of the Neornithes, but it is the oldest known ornithuran, the Science authors report. Ornithuran birds probably arose in the Early Cretaceous, sometime between 140 and 110 million years ago.

The new fossils were discovered by You and his team near the town of Changma, about 2,000 kilometers west of Beijing, where the first Gansus fossil (a partial hind leg) was found in 1981. Most of the nearly 50 bird specimens found so far at the site appear to be Gansus, the researchers say, which would make it the oldest known fossil locality dominated by ornithuran birds.

Ornithuran fossils "are relatively rare in the Cretaceous, which is part of what makes Gansus so exciting," says co-author Matthew Lamanna of Carnegie Museum of Natural History.

Gansus was not alone in its water-loving ways; several other ancient ornithuran birds also show adaptations to an aquatic lifestyle, a fact that jumped out at the researchers as they sought out Gansus' place in the bird family tree.

"When we mapped ecology onto our evolutionary tree, a pattern became apparent that species leading up to modern birds are mostly aquatic," Lamanna says.

However, some early members of the modern bird group probably made a quick reversal to land living, according to the researchers, who note that modern terrestrial bird groups such as the ostrich and chicken family have deep roots all the way back to the Cretaceous.

The Gansus discoveries offer a rare glimpse of an important period in bird evolution, according to the Science authors. Ornithurans arrived on the evolutionary scene when another type of ancient bird, called enantiornitheans, ruled the skies. Enantiornitheans--called "opposite birds" because their shoulder joint is reversed compared to that of modern birds--disappeared along with all non-avian dinosaurs at the end of the Cretaceous, leaving no descendants.

Opposite birds dominate earlier fossil sites, such as the famous deposits in Liaoning Province (northeast of Beijing) that yielded the first "feathered dinosaur" specimens a decade ago. The Liaoning deposits are only a few million years older than those at Changma, so the Changma finds might help researchers understand how and why ornithurans rose to dominance over opposite birds.

The switch may have had something to do with environmental changes, "but it's hard to answer this question just based on bird fossils. We need more information on the paleoclimate," You says.

The Gansus fossils are preserved in rocks that were deposited in an ancient lake, a fact that accounts for the "very, very beautiful preservation" of the specimens, according to You.

"Soft tissues cannot be preserved normally, but a lake environment, compared to a river, is kind of quiet," You explains. "Extremely fine sediment can be deposited yearly and soft parts can be preserved."

To find the fossils, the paleontologists split open countless slabs of mudstone from the lake "like turning the pages of a book," Lamanna says, noting that the repetitive process is "sort of like playing the lottery."

You began work at the Changma site after finishing his doctorate at the University of Pennsylvania, where he met Lamanna and Science co-authors Jerald Harris of Dixie State College, St. George, Utah, and Peter Dodson of the University of Pennsylvania. You started his doctoral work in paleontology at Penn in 1996, the same year that the Liaoning feathered dinosaurs made big news. "So I missed all that," You jokes. However, he noticed that the rocks in the Changma region looked very similar to those at the Liaoning site, which kept him interested in the area.

Lamanna says the Changma site, with its abundance of well-preserved fossils, "might eventually be mentioned in the same breath as the feathered dinosaur quarries of Liaoning."

"Thanks to Liaoning, we have a good idea of what was going on with land ecosystems around 125 million years ago, but not as much insight into what happened shortly after that, so Changma may be very significant in that regard," he says. "It may represent not only the next chapter in bird evolution, but the next phase in the evolution of a lot of different types of organisms."

The Science paper, "A Nearly Modern, Amphibious Bird from the Early Cretaceous of Northwestern China," was authored by Hai-lu You, Shu-an Ji, Jun-chang Lu, Chong-xi Yuan, and Qiang Ji of the Chinese Academy of Geological Sciences; with Matthew C. Lamanna of Carnegie Museum of Natural History; Jerald D. Harris of Dixie State College, St. George, Utah; Luis M. Chiappe and Jingmai O’Connor of the Natural History Museum of Los Angeles County, California; Da-qing Li of the 3rd Geology & Mineral Research Exploration Academy of Gansu Province; Xing Zhang of the Provincial Museum of Gansu Province; Kenneth J. Lacovara of Drexel University; and Peter Dodson of the University of Pennsylvania.

Natasha Pinol | EurekAlert!
Further information:
http://www.aaas.org

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