By Sean Henahan, Access Excellence

A group of researchers from Argentina found a  a small theropod from the Cretaceous period with a number of bird-like characteristics. The anatomical features the creature, dubbed Unenlagia (indian dialect for "half-bird") may begin to enter one of the great lingering questions in paleontology, how wings evolved from forelimbs. The animal held its forelimbs much like a bird holds its wings, furling them so that the humerus lay close to the body, and the limb as a whole was capable of the 'upstroke' needed for flapping flight.

Although the two-meter long Unenlagia was probably flightless, the researchers note that the structure of the all of the anatomical requirements for powered, flapping flight were present in the animal. Moreover, several features, including the shape of the pelvis and scapula resemble the morphology seen in Archaeopteryx.

"Unenlagia may therefore be the closest thing we have to an intermediate between dinosaurs and Archaeopteryx," the researchers note.

It is unlikely that Unenlagia flew, owing to its large size and relatively small forelimbs. The anatomy of the newly discovered animal suggests that there may have been an evolution from a  flapping stage to a flying stage. It is unknown whether the animal had feathers.

Archaeopteryx lithographica, the well-known feathered dinosaur, was discovered only two years after Darwin published his "Origin of the Species". Scientists have been arguing ever since about the evolutionary transition between reptiles and birds. The growing consensus has been that birds were   morphologically closest to  theropods such as Deinonychus and Veolciraptor.

Unenlagia has characteristics of both birds and dinosaurs, and it may help to explain the functional transition from reptiles to birds. The new findings will refine the ongoing argument as to the origin of birds, or where the dividing line is between dinosaurs and birds, comments Lawrence Witmer, department of biological sciences, Ohio University, adding:

"Regardless  of the final interpretation of the functional transition, Unenlagia is a critical testament of the phylogenetic transition, and it is only after the phylogenetic players are better known that the drama of the adaptive story will be truly comprehensible."

The research appeared in the May 22, 1996 issue of Nature