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Thursday, 16 May, 2002, 18:06 GMT 19:06 UK
Impact led to dino rule
Dino, BBC
An asteroid strike may have led to their demise
The impact of an extraterrestrial object may have led to the downfall of the dinosaurs, but now it seems another, much earlier collision, could have been the factor that allowed the great beasts to rise to prominence in the first place.

An international group of scientists scoured 70 sites in North America, examining dino footprints and other fossils and looking at the chemical traces in rocks.

Our research adds to the speculation that there was a comet or asteroid impact about 200 million years ago

Prof Dennis Kent
They say this work shows that the larger of the ancient reptiles began to proliferate at the beginning of the Jurassic Period.

This would have been just after the time when geologists think life on Earth experienced a mass extinction.

The great loss of species - a little under half disappear from the fossil record - just over 200 million years ago would have opened up ecological opportunities for surviving dinosaurs to exploit.

"What we see are relatively small dinosaurs and a lot of other reptile groups prior to the Triassic-Jurassic boundary; and then the rather abrupt appearance of large carnivorous dinosaurs immediately afterwards," Professor Paul Olsen, from Columbia University, near New York, US, told the BBC.

Time marker

One of the key findings of the new research, published in the journal Science, centres around the discovery of an iridium "spike" in rocks laid down at the time.

Iridium is an element that is rare on Earth but common in asteroids and comets.

The researchers say its presence strongly suggests that the Earth was hit by something extraterrestrial - as does the presence of abundant fern spores, also found in the sedimentary layers. Ferns are the first plants to move back into devastated areas.

"Finding the element iridium, which is common in space objects, creates a time marker for comet or asteroid impacts," said co-researcher Professor Dennis Kent, from Rutgers, the State University of New Jersey.

"Correlating the finds with evidence of plant and animal life helps to tell us what happened."

Mammals rise

That evidence indicates the large dinosaurs appeared relatively quickly after the end of the Triassic (about 210 million years ago), and that the transition from Triassic-type to Jurassic-type footprints occurred within the relatively short period of just 50,000 years.

Maybe the iridium and fern spore anomalies they found were from a local hit. I don't believe they offer the evidence of great environmental change

Dr Norman MacLeod
"Our research adds to the speculation that there was a comet or asteroid impact about 200 million years ago, followed relatively quickly by the rising dominance of dinosaur populations of the Jurassic period," said Professor Kent.

He suggested that the effects of the impact killed off or reduced many competitive species, clearing the way for dinosaurs to adapt and flourish.

Many scientists now believe a massive impact off the Yucatan peninsula in Mexico 65 million years ago (the end of the Cretaceous) transformed the Earth's climate, making it impossible for the dinosaurs to continue; it was mammals on that occasion which took advantage.

Difficult to test

However, many scientists may need further proof before accepting the new analysis.

Dr Norman MacLeod, the keeper of palaeontology at the Natural History Museum, London, UK, told the BBC that the iridium anomaly could be attributed to other factors.

"It could be volcanic in origin," he said. "They also imply that there was an asteroid impact that precipitated a global catastrophe and I believe they are almost certainly wrong about this.

"The iridium anomaly is a thousand times smaller than the one at the end of the Cretaceous Period, so the size of any impacting body must have been much smaller. The much smaller size of the impactor suggests the impact didn't cause a global event.

"Maybe the iridium and fern spore anomalies they found were from a local hit. I don't believe they offer the evidence of great environmental change around the globe occurring at this time".

Dr MacLeod said it would be difficult to test the theory because so few rocks from the late Triassic had been retained; most have been recycled by the Earth's tectonic processes.

The BBC's Sue Nelson
"Volcanic origin can't be ruled out"
See also:

08 Mar 01 | Sci/Tech
'Quick' demise for the dinosaurs
25 Sep 01 | Sci/Tech
How reptiles survived the big one
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