Early Cretaceous

LATE CRETACEOUS - 75 Mft

Dinosaurs among us?

It is now dear that birds evolved from dinosaurs. But there are still plenty of puzzles about bird evolution.

Mesozoic-era birds were very different from those alive today. Most belonged to a primitive group called the "opposite birds", which appeared about 130 million years ago and went on to dominate the Cretaceous. Yet the opposite birds vanished with the dinosaurs, while the ancestors of modern birds survived. No one knows why.

Another mystery iswhen modern birds appeared. Most of the 30 or so o rd ers of li vi ng bi rds a ppea r in the fossil record around 55 million years ago, leading many palaeontologists to believe that they evolved in a "big bang" some time after the opposite birds had died out.

However, molecular clocks put their origins much earlier, up to 100 million years ago. Some fossil evidence backs this up, notably a skeleton called vegavis from the Cretaceous in Antarctica. In January 2005, a team at North Carolina State University in Raleigh identified it as belonging to the modem group that indudes ducks.

Not everyone agrees. "Vegavis was originally described as belonging to an extinct group; now all of a sudden it's a duck," says Alan Feducaa of the University of North Carolina, who proposed the big bang idea.

Yixian's birds have scarcely helped. The 20 or so spedes have a bewildering mix of primitive and advanced features - the tooth ed j aws a n d I ong ta i Is of dinosaurs, plus the short tails and horny beaks of modern birds. The controversy will only be solved when more fossils come to light. Paul Chambers

Ironically, just as microraptor hit the headlines, another unexpected discovery pushed the argument back in the opposite direction. Biomechanics researcher Ken Dial of the University of Montana in Missoula was studying how baby chukar partridges leam to fly. He noticed that even before they were big enough to take to the air, they flapped their wings when they were running up slopes. And Dial found that the flapping had a purpose - it pressed them down on the surface, giving them a better grip. And the bigger their wings, the steeper the slope they could run up. This was the first experimental evidence that larger wings are an advantage even when you can't fly.

Many palaeontologists think Dial's theory is the better explanation for the origin of flight. It "seems to fit into the evolution of bird flight very well", says Holtz. Others, however, remain undecided. "I'm not sure how the question is going to be resolved, but it's not going to be simply ground-up or trees-down," says Brush. "It's got to be more complicated than that." Currie adds: "The only way we're going to solve the problem is to have a time machine and see who was doing what."

Without a time machine, the Yixian fossils are our best window onto the vanished world of feathered dinosaurs and early birds. And they do reveal something unexpected: birds were part of a broad evolutionary continuum that ranged from unmistakable dinosaurs to creatures that wouldn't look out of place at your bird feeder, like archaeopteryx, these fossils will be icons for future generations of palaeontologists. Perhaps more importantly, they serve as a reminder that the birds you do see on your bird feeder are living, breathing dinosaurs.

No doubt Yixian will continue to deliver surprises. Zhou's lab alone employs io technicians preparing new Yixian fossils. He says there are already many discoveries that remain to be described, and he expects many more to come as thousands of farmers continue to dig up the countryside.

Yixian may not be the end of the story. Xu is planning an expedition to the Daohugou fossil beds in Inner Mongolia, which yielded the enigmatic dino-bird pedopenna. The fossils uncovered so far seem as well preserved as the Yixian finds, but so far dinosaurs and birds are few and far between. The Daohugou beds are clearly older than the Yixian deposits, but how much remains unclear. There may only be a few million years in it, butXu says the beds could be as old as 160 million years- the middle Jurassic period - and many palaeontologists are hoping he's right. Maybe, just maybe, there is another bonanza on the horizon. •

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