Good dino. The Labrador retriever–sized, plant-eating Asilisaurus would have come up to your knee.

Marlene Hill Donnelly/Field Museum (main image) S. Nesbitt (inset)

Did Dinosaurs Start Out on All Fours?

How much is that Asilisaurus in the window? That's what prospective pet owners might be asking, if a newly discovered dinosaur ancestor were still alive. "It's a lightweight, four-legged, plant-eating, scampering wonder that might have made a nice pet," says paleontologist Paul Sereno of the University of Chicago in Illinois. The creature's fossilized bones should cause a rethinking about dinosaur ancestors because they suggest that dinos evolved not from two-legged carnivores but from four-legged vegetarians.

For more than a century, researchers have been looking for clues to the origins of dinosaurs. The balance of opinion has alternated between more reptilian ancestors, which walked on all fours, and two-legged animals that had bird-shaped bodies but couldn't fly. Recently, the idea of two-legged dino ancestors had been winning out, but the new find yanks the trend back toward quadrupeds.

Scientists discovered the fossilized remains of 14 individuals in southern Tanzania in 2007—enough to piece together nearly the whole skeleton. They named the new species Asilisaurus kongwe, combining Swahili and Greek words for "ancient," “lizard,” and "ancestor."

After studying the bones for 3 years, the team concludes that Asilisaurus was about the size of a Labrador retriever. The animal walked on four legs, and the shape of its teeth suggests that it ate plants and maybe a little meat. The age of the rocks where the bones were found indicates that Asilisaurus roamed eastern Africa about 240 million years ago—25 million years before the first true dinosaur appeared, the researchers report in tomorrow's issue of Nature.

The animal shares the most characteristics with Silesaurus, a slender, four-legged, long-tailed dinosaur relative that is more recent, explains paleontologist and lead author Sterling Nesbitt of the University of Texas, Austin. Silesaurus coexisted with two-legged potential precursors—hence the competing views about which form gave rise to the dinosaurs.

Asilisaurus adds yet another strange evolutionary link between the earlier reptiles and the dinosaur family tree, says paleontologist Hans Larsson of McGill University in Montreal, Canada. The new fossil also pushes back the origins of the group that gave rise to the dinosaurs by another 10 million years, he says.

Sereno, who wasn't involved in the research, also welcomes the new beast. "It gives us more insight into the variety of animals that were thriving and fighting for survival at the dawn of the dinosaur era," he says. "It would have been impossible to predict that the Asilisaurus line one day would give rise to Tyrannosaurus and the likes of a hummingbird."