Ancient reptile walked on two legs

Walking on two legs million of years before dinosaurs

By BBC News Online science editor Dr David Whitehouse

A newly discovered fossil of a reptile that walked on two legs shows that bipedal locomotion evolved long before the dinosaurs.

This discovery has shown interesting and exciting things happening in the evolutionary history of reptiles well before the advent of dinosaurs

Prof Robert Reisz

The exquisitely preserved, 290-million-year-old skeleton of Eudibamus cursoris was dug up in a German quarry by an international team of scientists.

It is the oldest known member of the Parareptilia, a major group of primitive reptiles. Eudibamus does not appear to be a direct ancestor to later reptiles, including some dinosaurs, that adopted a bipedal posture and gait.

This may indicate that the ability to walk upright on two legs arose several times independently during reptile evolution.

Built for speed

Although only 26.1 cm (10.3 inches) long, the Eudibamus fossil contains a wealth of clues about how the reptile moved.

The skeleton shows all the signs of bipedalism

Scientists say the creature's upper limbs were relatively short for its overall size, while its lower limbs were relatively long. The reptile also had an unusually long foot and tail, proportions usually indicative of bipedal locomotion.

They think the long tail could have served as a sort of rudder, compensating for changes in the animal's centre of gravity as it moved along in an upright position.

Other evidence of bipedalism comes from the arrangement of the hip, knee, and ankle joints in the reptile's lower limb. The surfaces of these joints are arranged so that the bones in the legs formed a straight line when the hind limbs were fully extended.

Dinosaurs and mammals

This means that the creature's ankles and knees were able to flex and extend in only one plane in a similar way to how human knees and ankles move mostly back and forth, but not side to side.

Eudibamus cursoris appears to be the earliest known tetrapod, or four-legged vertebrate, to adopt this distinctive posture and gait.

"This find is fascinating because it confirms that bipedalism is an innovation that has happened several times," said Professor Robert Reisz, of the University of Toronto at Mississauga.

"It happened in some dinosaurs, and their bird descendants, and it happened in mammals, so it must be a good idea in terms of evolution."

Evolved several times

"There are only a couple of times in evolutionary history when animals have gone from a sprawled posture like that of a four-legged lizard to an upright posture when they tuck their limbs under the body.

The fossil was uncovered in a German quarry

"It happened once in dinosaurs and again with mammals. So to find an example of an animal that did this before dinosaurs or mammals is particularly exciting."

According to the researchers, who report their work in the journal Science, even on four legs, the creature's distinct posture would have distinguished its movement from the sprawling gait used by the other tetrapods of the time.

Eudibamus belongs to an extinct family of early reptiles with an unusually large geographic range, compared to its contemporaries in the early Permian (about 290 to 268 million years ago).

Unique locality

"It was thought that the ability to run on two legs and stand upright first emerged in dinosaurs and their relatives. But this discovery has shown interesting and exciting things happening in the evolutionary history of reptiles well before the advent of dinosaurs," said professor Reisz.

The teeth of Eudibamus indicate that it was a plant eater, so it was not using its speed to chase food. Instead, it probably used its sprinting speed to escape predators.

Professor Reisz and colleagues plan to continue excavations at the German quarry, which has already yielded a number of other well-preserved fossils. "It's a super site, a unique locality," said the palaeontologist,

"And it gives us a chance to show that some neat things were happening with reptiles in the Palaeozoic, long before the appearance of dinosaurs."