September 25, 2009

Discovery Fuels Evolutionary Debate Between Birds And Dinosaurs

Fossil experts say the stunning remains of a "four-winged" dinosaur have confirmed that birds owe their ancestry to two-footed dinosaurs that lived millions of years ago, AFP reported.

The well-preserved fossil of a bird-like dinosaur called Anchiornis huxleyi may be the deciding factor, according to Xing Xu of the Chinese Academy of Science in Beijing.

Based on incomplete fossils, A. huxleyi was thought to have been a near contemporary of Archaeopteryx, the first recognized bird, which flew around 150 million years ago.

However, the new, almost-complete specimen suggests that A. huxleyi is millions of years older than Archaeopteryx and has both dinosaur and avian features.

Xu argues that it is the long-sought evidence that proves birds descended from theropod dinosaurs, as it is was thought to be a dinosaur with long feathers covering its arms, tail and feet.

Xu says it was likely "four-winged", although there is no guarantee that the creature had aerial ability, and its elongated lower legs suggest it was a good runner, according to the study published by the British journal Nature.

A four-winged condition may have played a role in the origin of flight, according to some evolutionary biologists.

The study suggests that the plumage attachment is particularly important because it shows how bird-like dinosaurs developed skeletal and other features enabling them to have feathers.

The evolutionary line taken by birds has been the subject of debate by many evolutionary specialists.

The "temporal paradox" states that bird-like dinosaurs appear too late in the fossil record to be the true ancestors of birds, but the debate has raged for years mainly because the fossil evidence is so rare or fragmented.

The new fossil was uncovered in Daxishan, in Jianchang county in northeastern China, where it was found in rock dated to the early part of the Late Jurassic, between 151 and 161 million years ago -- meaning it is clearly older than Archaeopteryx.

The study argues that A. huxleyi is a late member of the Troodontidae, a category of dinosaurs closely related to avians.

The authors of the paper believe the new find refutes the temporal paradox argument and provides significant information on the temporal framework of theropod divergence.

"Drawing the tree of life, it's fairly obvious that feathers arose before Archaeopteryx appears in the fossil record," said renowned Bristol paleontologist Michael Benton.

He told BBC News that these new discoveries by Professor Xu Xing are maybe 10 million years older than Archaeopteryx.


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