Scientists have discovered 243-million-year-old fossils of dinosaurs' closest relatives, pushing back the origin of dinosaurs by at least 10 million years*.*

The dinosaur-like creature, Asilisaurus kongwe, was about the size of a Labrador retriever and had teeth and jawbones ideally shaped for eating plants, indicating it ate a mostly vegetarian diet.

"This shows that the lineage leading to dinosaurs goes a lot further back in time than we thought. The second thing is that it shows that there’s this real ecological diversity," said paleontologist Randy Irmis, co-author of the study appearing Mar. 3 in Nature. "No one thought that the closest relatives to dinosaurs were these four-legged, herbivorous animals. We thought they were small carnivores."

The earliest known dinosaur fossils are around 230 million years old. The new findings indicate that the dinosaurs and the silesaurs, the group that encompasses genus Asilisaurus, diverged more than 243 million years ago. That means dinosaurs must have originated sometime before then.

The team found more than a dozen partial skeletons of Asilisaurus in bare patches in the Tanzanian grasslands. During the Triassic period, the area was warm and lush, with a mixture of woodlands and lower plants like ferns.

"Back then it was a very large river system, maybe something like the Mississippi today," said lead author and University of Texas at Austin paleontologist Sterling Nesbitt. During that time, Africa, South America, Antarctica, Australia and India were all one giant continent called Gondwana.

Though silesaurs are very closely related to dinosaurs, they lack the open hip-sockets that are universal in dinosaurs. The Asilisaurus was a small, four-legged creatures with a long tail. Their beak-like jaws and leaf-shaped teeth helped the animals eat the soft, fibrous leaves of the primordial palms, ferns and conifers that were prevalent during the Triassic period. That suggests that, while the animal may not have been exclusively vegetarian, a good portion of its diet came from plants, he said.

"In a carnivorous animal, the teeth are pointed, or they are serrated, like a knife to cut meat. In order for this to be efficient, the serration has to be perpendicular to the edge of the tooth so that it functions like a knife," said paleontologist Gilles Cuny of the the Natural History Museum of Denmark, who was not involved in the study. "In these leaf-shaped teeth, you have some very vague serrations, but they are oriented towards the top of the teeth."

The findings overturn the previously held idea that the closest relatives to dinosaurs were two-legged, cat-sized predators, Irmis said.

The team also found that similar teeth and jaws evolved separately in the line of dinosaurs that includes apatosaurus, as well as in another line that included triceratops and stegosaurus. All these changes occurred within 10 million years of each other.

"We were really surprised," Irmis said. "These are three different groups that are really closely related to each other, so you’d expect that maybe their common ancestor had this tooth form. And no, it evolved independently in these three groups."

That suggests that each of these lineages evolved separately to take advantage of a large, untapped food source, he said.

Images: 1) Left: A team member excavates a fossil in Tanzania. Right: Silesaur bone/L. Tsuji, Tibia, R. Smith

2) Skeletal reconstruction, with missing bones in gray./S. Nesbitt

Citation: "Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira," Sterling J. Nesbitt, Christian A. Sidor, Randall B. Irmis, Kenneth D. Angielczyk, Roger M. H. Smith and Linda A. Tsuji. Nature Vol. 464 (4) Mar. 2010.

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