When animals gave up the buoyancy of water to fight gravity and crawl about on land, it marked a major step in vertebrate evolution. Another significant victory came when they began to walk on two legs instead of four.
Now, scientists say that a newly described fossil of a 10-inch reptile pushes back the advent of terrestrial bipedalism by about 60 million years. The researchers, who report their find in the Nov. 3 Science, have dubbed the 290-million-year-old animal Eudibamus cursoris, which means primitive two-legged runner.
Evidence of bipedalism in the well-preserved fossil includes forelimbs that are much shorter than its hindlimbs, says David S. Berman, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh and lead author of the report. Also, the animal’s hindlimbs and tail are relatively long compared with its body.
The bone surfaces in the hip, knee, and ankle joints in the animal’s hindlimbs were arranged so that the legs were straight when they were fully extended. Also, the third, fourth, and fifth toes of the rear feet were greatly elongated, allowing E. cursoris to run up on its toes. All these features, Berman notes, indicate the animal’s strides were long, a major contributor to speed among bipeds.
Berman describes E. cursoris as facultatively bipedal—in other words, it was a tetrapod that ran on two legs when it needed to. According to its teeth, the reptile was a vegetarian, so it wasn’t running to chase prey. Rather, Berman contends, it was running to escape predators.
E. cursoris doesn’t appear to be an ancestor of later bipedal reptiles, Berman notes. This means that the ability to strut one’s stuff on two legs has likely evolved several times among different types of animals.
“It doesn’t surprise me that a creature figured out how to [run bipedally] in the Paleozoic,” says James O. Farlow, a paleontologist at Indiana-Purdue University in Fort Wayne.
The fossil remains of the ancient reptile, which Berman and his colleagues found in a sandstone quarry near the village of Tambach-Dietharz in the former East Germany, were missing only a few parts of the skull, forelimb, and tail. The fossil’s remarkable state of preservation stems from the type of sediments in which it was entombed—fine-grain clays that had been laid down by seasonal rains in a shallow, undrained intermountain basin.
E. cursoris lived in a completely terrestrial environment, says coauthor Stuart S. Sumida, a paleontologist at California State University in San Bernardino. Even though German researchers have been removing fossils from the quarry for more than 25 years, they haven’t found a single fish—”not so much as a scale”— stresses Sumida, who described the ancient environment at the Society of Vertebrate Paleontology’s annual meeting in Mexico City last week.
The rocks in the quarry provide a “rare snapshot of life away from water, an environment that normally doesn’t fossilize,” Sumida told Science News. Other fossils found at the site include insects, ferns, high-fiber plants, and diadectids, the most primitive vertebrates that ate such plants. The diadectids are so well preserved that you can see the wear marks on their teeth, Sumida adds.
A top predator in the basin’s food chain was likely Dimetrodon, a carnivorous reptile whose fossils were found in Europe for the first time at this quarry, Berman notes. When it comes to terrestrial vertebrates, Berman says the number and the diversity of species recovered from the German site, as well as the quality of preservation of the fossils, “outstrips all of Europe combined.”
Some of the fossils found in the quarry are virtually identical to those found in the western United States. Berman contends this is overwhelming biological evidence to support the idea that 290 million years ago North America and central Europe were part of one supercontinent.