Very birdlike non-avian dinosaur found

Jeff Poling (with material by Darren Naish)

The fossil of a 90-million year old very birdlike non-avian dinosaur have been recovered from Argentina. The fossil, found in northwest Patagonia in January 1996, is described in the journal Nature by Fernando Nova, a specialist in predatory dinosaurs at the Argentine Museum of Natural History in Buenos Aires.

The new maniraptorid dinosaur, named Unenlagia comahuensis, is described from a partial skeleton. Dr. Novas estimates the new animal was nearly 4 feet tall at the hips and nearly 8 feet long. It is from the Turonian-Coniacian Rio-Neuquen Fm of Neuquen Province, Argentina. The name is from the Mapuche Indian words for 'half bird' ('unen' and 'lag'). Comahue, of course, is the Mapuche name for northwest Patagonia.

The paper includes a life restoration in silhouette and a cladogram that places Unenlagia between dromaeosaurids and Avialae (Archaeopterx + Metornithes). The pelvis is complete (I don't know whether to call it opisthopubic or propubic - the pubis is just about vertical) and there is a scapula, humerus, femur, tibia, some dorsal vertebrae and caudal chevrons.

Unenlagia has a laterally directed glenoid - the first time such a structure has been seen outside of Aves - and "forelimbs capable of excursions in the avian manner" were therefore possible. The authors note that this possible preadaptation for flapping renders it unlikely that avian flight evolved after a passive gliding stage. Although the long-legged predatory dinosaur could not fly, its forelimbs were extraordinarily winglike. They were jointed such that they would have been capable of flapping, should the dinosaur have found a need to, and likely were folded close to the animal's body like wings when not in use.

The authors argue that the laterally-directed glenoid and avian-style forelimb motions are preadaptations that only became involved in the evolution of flight when Unenlagia-like avialian ancestors changed their physical proportions and became smaller. Unenlagia does share lack of the fourth trochanter and five other listed characters with avialians but is thought to be less derived than Archaeopteryx because it lacks a transversely reduced pubic apron, still has an obturator process on the ischium and doesn't have a tibia that is 25% longer than the femur.

"We don't know whether it had feathers or not," Dr. Novas said, "but it probably used the forearms to maintain equilibrium when it was running, much like a human skater or surfer does to keep balance, or even as a modern ostrich does." The fossil was not found in sediment that would preserve feathers or other integument had they existed.