Wednesday, March 03, 2010

Dinosaurs Older Than Previously Thought?

Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. 2010. S. Nesbitt, et al. Nature 464: 95-98.

Asilisaurus kongwe. Image by M.H. Donnelly (Field Museum).
Paleontologists announced the discovery of a dinosaur-like animal—one that shared many characteristics with dinosaurs but fell just outside of the dinosaur family tree—living 10 million years earlier than the oldest known dinosaurs. The researchers conclude that dinosaurs and other close relatives such as pterosaurs might have also lived much earlier than previously thought.

At least 14 individuals of the new species Asilisaurus kongwe (a-SEE-lee-SOAR-us KONG-way) were recovered from a single bone bed in southern Tanzania. The individuals stood about 0.5 to 1 m tall at the hips and were 1 to 3 m long. They weighed about 10 to 30 kg. Asilisaurus walked on four legs and most likely ate plants or a combination of plants and meat. They lived about 240 million years ago.

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. 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 silesaurs and the dinosaur lineage had already diverged from common ancestors by 240 million years ago. Silesaurs continued to live side by side with early dinosaurs throughout much of the Triassic Period (between about 250 and 200 million years ago).

This new animal 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. link