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November 2 , 2000
Ancient Running Reptile was Bipedal, Say Science Researchers
Long before some dinosaurs, birds, and humans began to make their way through the world on two legs, a 290 million-year old reptile beat them to the bipedal starting line. An international team of researchers reports the discovery of a nearly complete skeleton of this ancient runner in the 3 November issue of the journal, Science.
The exquisitely preserved fossil, discovered in a German quarry, represents a new species, Eudibamus cursoris. 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 bipedal adaptations arose several times independently during reptile evolution, said Science co-author Robert R. Reisz of the University of Toronto in Mississauga, Ontario, Canada.
Just 261 millimeters long, about the length of a sheet of paper, the Eudibamus fossil contains a wealth of clues about the reptile's unusual mode of locomotion. Its skeletal features suggest that the reptile could move swiftly on two legs, probably up on its toes, using an upright posture with the limbs moving pendulum-like and parallel to the long axis of the body. This is similar to the posture used by running humans.
Eudibamus appears to be the earliest known tetrapod, or four-legged vertebrate, to adopt this distinctive posture and gait.
In its slower moments, the reptile probably moved quadrupedally, according to the Science researchers including lead author David S. Berman of the Carnegie Museum of Natural History, Pittsburgh, Pa. Even on four legs, however, Eudibamus' distinct posture would have distinguished its locomotion from the sprawling, arms and legs akimbo gait used by other tetrapods of the time.
Evidence for the reptile's bipedal abilities come from close analysis of the proportions of Eudibamus' arms and legs. Eudibamus' upper limbs are relatively short for its overall size, while its lower limbs are relatively long. The reptile also had an unusually long foot and tail
These proportions are usually indicative of bipedal locomotion, since longer legs lengthen an animal's overall stride, increasing its speed. Eudibamus probably ran up on its toes, maximizing the length of its foot and further lengthening its stride.
The reptile's long tail could have served as a sort of rudder, compensating for changes in the animal's center of gravity during upright, bipedal locomotion, say the Science authors.
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 Eudibamus' legs formed a straight line when the hindlimbs were fully extended.
With this alignment, Eudibamus' ankles and knees were able to flex and extend in only one plane. This is similar to how human knees and ankles move mostly back and forth, but not side to side.
Why did this ancient reptile feel such a need for speed? Eudibamus' teeth indicate that it was a plant eater, so it wasn't chasing down its food. The Science authors suggest that Eudibamus probably used its sprinting speed to escape predators. In fact, recent excavations at the quarry have uncovered a fossil of a small potential predator, said Reisz.
Eudibamus belongs to a 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). The researchers suggest that its unique combination of diet and locomotion may have contributed to its widespread success.
"This find is fascinating because it confirms that bipedalism is an innovation that has happened several times. 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," said Reisz.
Reisz and colleagues plan to continue excavations at the site, which has already yielded a number of other well-preserved fossils.
"It's a super site, a unique locality," said Reisz.
"And it gives us a chance to show that some neat things were happening with reptiles in the Paleozoic, long before the appearance of dinosaurs."
With Berman and Reisz, other members of the research team are A. C. Henrici of Carnegie National Museum of Natural History, Pittsburgh; D. Scott of University of Toronto in Mississauga, Ontario, Canada; S. S. Sumida at California State University, San Bernadino; and T. Martens at Museum der Natur Gotha, Gotha, Germany. This work was supported by grants from National Geographic Society, Natural Sciences and Engineering Council of Canada, Edward O'Neil Endowment Fund and M. Graham Netting Research Fund, Carnegie Museum of Natural History, and Deutsche Forschungsgemeinschaft.