Recent expeditions in a remote area of China have unearthed unusually well-preserved fossils of an ancient bird that lived between 105 million and 115 million years ago. The fossils of the surprisingly modern-looking bird suggest that today's birds may have originated from an aquatic ancestor.
A nearly complete fossil skeleton of Gansus yumenensis is one of several that reveals the 110-million-year-old bird's anatomy is strikingly similar to that of modern birds. The fossils also indicate G. yumenensis was equipped for life in an aquatic habitat. Photo is by Hai-lu You, courtesy of the Chinese Academy of Geological Sciences.
In 1981, paleontologists searching for fossil fish in northwestern China's Gansu province accidentally discovered the fossilized foot of a bird they named Gansus yumenensis. However, the foot revealed only minimal information about G. yumenensis, and questions about the animal remained unanswered until a team of researchers, led by Hai-lu You of the Chinese Academy of Geological Sciences, reinvestigated the Gansu area in 2004 and 2005 and found the new batch of G. yumenensis fossils.
In the June 16 issue of Science, the team describes fossil remains of
five G. yumenensis individuals that form almost a complete skeleton,
minus the skull and the first few neck vertebrae. Most of the bones are still
in their original, 3-D form, unlike many fossils which get crushed over time.
The exceptional condition of the remains gives scientists an unprecedented view
into the life of the extinct bird. "We finally have a glimpse, with these
new fossils, of what Gansus yumenensis looked like," says Julia
Clarke, an avian paleontologist at North Carolina State University in Raleigh
who is not associated with the expeditions.
G. yumenensis, as it turns out, looks quite modern. "With a few exceptions, you could put any of its bones next to those of a modern bird and you would be hard pressed to see major differences," says Jerald Harris of Dixie State College in Utah, who was part of the expedition team. Like today's birds, G. yumenensis has a thin, flexible wishbone and a large crest running down the middle of its breastbone. By comparison, more primitive birds, which were less sophisticated in their ability to fly than modern birds, have thick, heavy wishbones and only small crests on their breastbones. G. yumenensis also has long arms that bear asymmetrical feathers. These features indicate that "Gansus could fly in the same way modern birds can," Harris says.
The similarity of G. yumenensis to modern birds came as a surprise: Previously, such advanced anatomical features had been seen only in birds that lived during and after the late Cretaceous period within the last 100 million years. "In short, no one expected to find a bird this modern in rocks this old," Harris says, referring to the shale formation where the team made its discovery. The shale dates to the early Cretaceous period, which occurred between 100 million and 145 million years ago. As a result of the finding, G. yumenensis now has the distinction of being "the oldest-known bird that is really, really modern in its anatomy," he says.
As such, G. yumenensis may help fill in gaps in the branch of the evolutionary tree that gave rise to today's birds. Specifically, scientists are looking to G. yumenensis for present-day evidence of whether modern birds originated in aquatic or terrestrial habitats.
An artist's reconstruction shows Gansus yumenensis in a lake in China's Gansu province. The bird probably had the ability to dive and swim underwater, much like modern loons or grebes. Illustration by Mark A. Klingler, courtesy of the Carnegie Museum of Natural History.
"The most primitive living birds for example, ostriches, rheas, and chickens tend to be terrestrial" and therefore would seem to hint that today's birds got their start on land, says Matthew Lamanna, a research team member from the Carnegie Museum of Natural History in Pennsylvania. Yet two other features of G. yumenensis webbed feet and a large crest on its lower leg bone that would have provided a strong anchor for leg muscles suggest the bird was aquatic and had the ability to dive and swim underwater like loons or grebes. According to You's research team, this supports the idea that modern birds actually originated in water habitats.
Nonetheless, the G. yumenensis find does not definitively rule out the idea of a land-based ancestor for modern birds. According to Sylvia Hope, a specialist in the early fossil record of modern birds at the California Academy of Sciences in San Francisco, the rarity of terrestrial birds in the fossil record of the early Cretaceous period "may be due to the known poor potential for preservation of delicate bird skeletons in terrestrial habitats." Still, she says, the new G. yumenensis specimens represent a significant finding.
A future priority for You's team is to better pinpoint the age of the G. yumenensis
remains, currently estimated to be between 105 million and 115 million years
old. Determining the age of G. yumenensis with respect to other fossil birds,
Harris says, "particularly others on the branch leading to modern birds,
will give us a good handle on how quickly many of the anatomical features we
associate with modern birds evolved."
Chinese Academy of Sciences
"Penguins endure extinction event," Geotimes, June 2006
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