Scientists previously had to make do with relatively inaccurate fossil dating techniques. Now Carl Swisher III of the Insitute of Human Origins in Berkeley, California, has used argon radioisotopic dating - the most accurate dating technique known - to work out the age of volcanic ash found in Argentinian rocks containing dinosaur fossils. His figure of 227.8 million years is the first firm date for early fossils, and marks these as the world's oldest.

Fellow team member Paul Sereno of the University of Chicago says the fossils reveal that the first dinosaurs probably lived only two to three million years earlier than that date. He adds that early dinosaurs were rare, making up only about 6 per cent of the reptile and amphibian fossils in the northwestern Argentinian deposits. But the riddle of how dinosaurs came to dominate the land 10 to 25 million years later still remains.

Dinosaurs evolved in the Triassic period (245 to 202 million years ago). Many details are missing because fossils are rare and the ages of deposits from different areas are difficult to correlate. This has led to fierce debate about whether dinosaurs replaced other reptiles gradually, or after one or more mass extinctions. The new findings indicate the early dinosaurs were a stable but small fraction of the reptile population for millions of years in Argentina.

There were so many fossils in the deposits that a team from Chicago and the National University of San Juan in Argentina were able to identify and count individuals. Most of the fossils were remains of non - dinosaur plant-eating reptiles, with a few fragmentary remains of plant-eating 'bird-hipped' dinosaurs. Of the carnivore fossils, 37 percent were 'lizard-hipped' dinosaurs - two-legged predators with muscular arms. The remainder were mostly mammal-like reptiles which later became extinct. The presence of both carnivorous and herbivorous dinosaurs indicates that the division between them came very early, about two million years before the oldest Argentinian fossils.

Sereno says the fossil census shows dinosaur numbers stayed constant during the several million years that the Argentinian fossils accumulated. This casts doubts on the idea that early dinosaurs quickly established their evolutionary superiority. But dinosaurs, including plant-eaters, were more common near the end of the Triassic, and dominated the land in the following Jurassic period. Why?

Paul Olsen of the Lamont-Doherty Geological Observatory blames the mass extinction of other reptiles at the end of the Triassic, while Michael Benton of the University of Bristol thinks the cause is an additional earlier mass extinction in the late Triassic.

Sereno thinks 'something may have opened up' for dinosaurs, and hopes that younger rocks in Argentina may hold more clues as to what it was.

New Scientist, 15 May 93, Volume 138, Issue 1873