By MATT KAPLAN - SCIENTIFIC AMERICAN
Added: Thu, 28 Jul 2011 01:33:41 UTC

Analysis of fossil traits suggests that Archaeopteryx is not a bird at all. The latest discovery of a fossil that treads the line between birds and non-avian dinosaurs is leading paleontologists to reassess the creature that has been considered the evolutionary link between the two.

Archaeopteryx has long been placed at the base of the bird evolutionary tree. It has traits that have helped to define what it is to be a bird, such as long and robust forelimbs. Yet in recent years, the discoveries of numerous small, feathery dinosaurs have created a conundrum for paleontologists, raising questions about which animals are the ancestors of modern birds and which are just closely related cousins.

The fossil that is driving the latest Archaeopteryx rethink is called Xiaotingia zhengi, and is described in Nature today by Xing Xu, a paleontologist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, and his colleagues. It was found in western Liaoning, China, in rocks dating to the Late Jurassic epoch, 161 million-145 million years ago. Like many similar fossils, it is surrounded by feather impressions in the rock, but has claws on the ends of its forelimbs and sharp teeth.

These traits by themselves do little to help place the fossil in the dinosaur-bird transition, but Xu reports that it also has extremely long middle and last finger bones and a wishbone with an L-shaped cross-section at one end. These characteristics, Xu argues, identify Xiaotingia as very closely related to Archaeopteryx and another feathery relative, Anchiornis.

TAGGED: EVOLUTION, PALEONTOLOGY