Image: DIANE SCOTT

Although humans aren¿t particularly fleet-footed as compared with, say, horses, there are instances in which two legs are faster than four. Indeed, for certain dinosaurs, among which bipedalism was long thought to have first evolved, two-legged running probably arose as a means of chasing down prey. The terrestrial reptiles that ruled the earth before them, in contrast, appeared to have been limited to relatively slow, quadrupedal locomotion, owing to their short, stocky limbs and sprawling posture. But according to a report published today in the journal Science, a 290-million-year-old fossil from central Germany reveals that in fact bipedalism first evolved at least 60 million years before the rise of the dinosaurs, in a little known group of reptiles called bolosaurs.

The remarkably complete fossil (right), discovered in an abandoned sandstone quarry, represents a previously unknown species of bolosaur dubbed Eudibamus cursoris. Researchers infer that Eudibamus was bipedal based on its skeletal proportions, which resemble those of living lizards that can move bipedally and dinosaurs that traveled on two legs. Specifically, Eudibamus had relatively short forelimbs, and relatively long tails and hindlimbs--a telltale combination. Unlike the bipedal dinosaurs, however, this creature was herbivorous, judging from its dentition. Eudibamus therefore most likely used its two-legged speed to keep from being eaten. "Since this bolosaur was able to tuck in its limbs under the body and run on two legs, it was basically faster than anything else around at the time," notes team member Robert R. Reisz of the University of Toronto. "And for a small herbivorous animal that makes a lot of sense, especially when there are lots of larger predators around."

The bolosaurs eventually died out, but they were quite successful at the time, having colonized North America, China, Russia and, apparently, Germany. If Eudibamus is representative of the other bolosaurs, which are known only from very fragmentary fossils, it might explain how the group achieved such a wide distribution. "Perhaps this precocious dispersal and early success," the authors write, "were related to the unique combination of bipedalism and herbivory."