March 3, 2010

Dinosaurs May Have Originated Earlier Than Thought

Paleontologists have discovered a dinosaur-like animal that lived 10 million years earlier than the oldest known dinosaurs, suggesting that dinosaurs and close relatives such as pterosaurs (flying reptiles) originated much earlier than scientists previously thought.

The research also suggests that at least three times in the evolution of dinosaurs and their closest relatives, meat-eating animals evolved into animals with diets that included plants. These shifts all occurred in less than 10 million years, a relatively short time by geological standards.

The description of the new species Asilisaurus kongwe appears in a paper published in the March 4 edition of the journal Nature. The lead author is Sterling Nesbitt, a postdoctoral researcher at The University of Texas at Austin's Jackson School of Geosciences. Christian Sidor, a University of Washington associate professor of biology and curator of vertebrate paleontology at the Burke Museum of Natural History and Culture at the UW, is among the co-authors. Nesbitt conducted the research while a graduate student at Columbia University's Lamont-Doherty Earth Observatory and the American Museum of Natural History.

Asilisaurus is part of a sister group to dinosaurs known as silesaurs. Silesaurs are considered dinosaur-like because they share many dinosaur characteristics but still lack key characteristics all dinosaurs share. The relationship between silesaurs and dinosaurs is analogous to the close relationship of humans and chimps. Even though the oldest dinosaurs discovered so far are only 230 million years old, the presence of their closest relatives 10 million years earlier implies that dinosaurs and silesaurs had already diverged from their common ancestor by 240 million years ago. Silesaurs continued to live side by side with early dinosaurs throughout much of the Triassic period, between about 250 million and 200 million years ago.

This is the first Triassic-aged fossil of a dinosaur-like animal recovered from Africa.

Fossil bones of at least 14 individuals were recovered from a single bone bed in southern Tanzania, making it possible to reconstruct a skeleton nearly complete except for portions of the skull and hand. The individuals stood about 0.5 to 1 meter (1.5 to 3.25 feet) tall at the hips and were 1 to 3 meters (3 to 10 feet) long. They weighed about 10 to 30 kilograms (22 to 66 pounds). Asilisaurus, which lived about 240 million years ago, walked on four legs and most likely ate plants or a combination of plants and meat.

Silesaurs have triangular teeth and a lower jaw with a beaklike tip, suggesting they were specialized for an omnivorous and/or herbivorous diet. The same traits evolved independently in at least two dinosaur lines, and all three involved animals that were originally meat eaters. Although difficult to prove, it's possible this shift conferred an evolutionary advantage. An ecosystem can support far more plant eaters than meat eaters, so being able to eat plants might have opened a broader range of habitats. Not counting modern birds, dinosaurs survived for about 180 million years.

This new species is found along with a number of primitive crocodilian relatives in the same fossil bed in southern Tanzania. The presence of these animals together at the same time and place suggests that the diversification of the relatives of crocodilians and birds was rapid and happened earlier than previously suggested. It sheds light on a group of animals that later came to dominate the terrestrial ecosystem throughout the Mesozoic period, 250 million to 65 million years ago.

"Everyone loves dinosaurs," said Nesbitt. "But this new evidence suggests that they were really only one of several large and distinct groups of animals that exploded in diversity in the Triassic, including silesaurs, pterosaurs, and several groups of crocodilian relatives."

Silesaurus, the first known member of the silesaur group, was discovered in 2003. Since then, specimens of eight other members have been unearthed from Triassic rocks around the globe.

"This goes to show that there are whole groups of animals out there that we've never even found evidence of that were very abundant during the Triassic," said Nesbitt. "It's exciting because it means there is still so much chance for discovery."

The name Asilisaurus kongwe is derived from asili (Swahili for ancestor or foundation), sauros (Greek for lizard), and kongwe (Swahili for ancient).

Besides Sidor, co-authors of the Nature paper include Randall B. Irmis of the Utah Museum of Natural History and the University of Utah), Kenneth D. Angielczyk of The Field Museum in Chicago, Roger M.H. Smith of the South African Museum and Linda A. Tsuji of the Museum fr Naturkunde an der Humboldt-Universität zu in Berlin, Germany.

Funding for the research was provided by The National Geographic Society, The Evolving Earth Foundation, The Grainger Foundation, and The National Science Foundation.


Image 1: Life restoration of Asilisaurus with sail-backed poposauroid in the background. Image by M.H. Donnelly (Field Museum).

Image 2: Asilisaurus compared to a 5'6" human for scale. Image by S. Nesbitt.

Image 3: The tibia of Asilisaurus, following excavation in 2007. Photo by R. Smith.


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