Gansus yumenesis, which lived some 105 to 115 million years ago during the Early Cretaceous period, took modern birds through a watery path out of the dinosaur lineage.

The report was co-authored by Peter Dodson of the University of Pennsylvania and his former students Hai-lu You of the Chinese Academy of Geological Sciences, Jerald Harris of Dixie State College of Utah and Matthew Lamanna of Carnegie Natural History Museum in Pittsburgh.

"Gansus is very close to a modern bird and helps fill in the big gap between clearly non-modern birds and the explosion of early birds that marked the Cretaceous period, the final era of the Dinosaur Age," said Peter Dodson, professor of anatomy at Penn's School of Veterinary Medicine and professor in Penn's Department of Earth and Environmental Sciences.

"Gansus is the oldest example of the nearly modern birds that branched off of the trunk of the family tree that began with the famous proto-bird Archaeopteryx."

Gansus yumenensis takes its name from the Gansu region, where it was found, and the nearby city of Yumen. According to Dodson, Gansus is something of a lost species, originally described from a fossil leg found in 1983, but since largely ignored by science.

The five specimens described by Dodson and his colleagues had many of the anatomical traits of modern birds, including feathers, bone structure and webbed feet, although every specimen lacked a skull.

"It appears that the early ancestors of modern birds lived lifestyles that today we would stereotype as being duck-like, heron-like, stork-like, loon-like, etc.," said Jerald Harris, director of paleontology at Dixie Sate College of Utah.

"Gansus likely behaved much like its modern relatives, probably eating fish, insects and the occasional plan. We won't have a definitive dietary answer until we find a skull."

The skeletons, headless as they are, offer plenty of evidence for a life on the water. Its upper body structure offers evidence that Gansus could take flight from the water, like a modern duck, and the webbed feet and bony knees are clear signs that Gansus swam.

"Webbed feet is an adaptation that has evolved repeatedly in widely separate groups of animals, such as sea turtles, whales and manatees, and would only hinder climbing or landing in trees," Harris said.

"The big bony crest that sticks off the knee-end of their lower leg bones are similar to structures seen in loons and grebes. These crests anchor powerful muscles needed for diving under water and swimming."

According to Harris, these adaptations all demonstrate how the Gansus branch of the family tree, the structurally modern birds called ornithuromorphs, split from the enantiornitheans (or "opposite birds"). Enantiornitheans were among the feathered fossils found in northeastern China during the 1990s.

"The enantiornitheans had the best adaptations for perching, so they were able to dominate the ecological niche that we would associate with songbirds, cuckoos, woodpeckers or birds of prey," Harris said. "Gansus appears to have had adaptations for a lifestyle centered around water, based on things like the proportions of the leg and foot bones."

While the enantiornitheans are now long gone, their perching lifestyle has now been taken over by the descendents of birds like Gansus. What remains a mystery for now, according to the researchers, is how the amphibious lifestyle of birds like Gansus helped enable them to survive the cataclysmic end of the dinosaurs 65 million years ago.

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