Piecing together evidence from footprints, fossils and a dusting of a rare metal found in asteroids, the experts said they had concluded that a huge impact wiped out most of the plants and crocodilian creatures that ruled the world during the Triassic era.
It opened the way for the age of the dinosaurs in the Jurassic, just as the later asteroid allowed mammals to evolve.
"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 Dennis Kent, a geology professor at Rutgers University in New Jersey.
"Dinosaurs went on to dominate for the next 135 million years."
Lacking competition, the dinosaurs were free to evolve into the monsters that delight children in museums and films -- the Tyrannosaurus rex, velociraptor and others.
"They replaced a whole suite of other kinds of carnivores that, for the most part, are remotely related to living crocodilians," said Paul Olsen of Columbia University in New York, who led the team of researchers whose findings were reported in Friday's issue of the journal Science.
Looking at footprints and fossilized bones from nearly 80 different sites, Olsen's team concluded that it only took about 50,000 years for dinosaurs to start growing big -- really big.
Before the Jurassic, the biggest dinosaur around was probably something like plateosaurus, a long-necked, two-legged plant-eater 20 feet long.
Afterward, giant carnivores such as tyrannosaurus reached lengths of 41 feet and more -- Argentina's — giganotosaurus was more than 45 feet — long.
"You want to get as large as you can so you can eat what you want," Olsen said. Living examples include the Komodo monitor lizards of Indonesia and a similar creature that lived in Australia until humans arrived and somehow wiped them out.
In an evolutionary race for survival, plant-eaters grew even larger, developing into the thundering 160-foot — long behemoth, as yet unnamed, dug up in Argentina in 2000, and the more familiar apatosaurus.
It was believed it took millions of years for this to happen but Olsen says the evidence points to a quick explosion of size and diversity among dinosaurs.
Other fossil evidence shows plants also changed. Could it have been an asteroid, like the one that, it is now widely accepted, hit what is now Mexico's Yucatan peninsula 65 million years ago? It would have sent up a plume of dust and smoke that obscured the sun for centuries, killing off plants and the animals that depended on them.
Vital evidence of an earlier impact comes from iridium, a rare metal that is abundant in asteroids - chunks of rock that orbit the Sun in a belt of debris between Mars and Jupiter and which occasionally fly loose.
"Finding the element iridium, which is common in space objects, creates a time marker for comet or asteroid impacts." Kent said.
The team used a high-resolution mass spectrometer at the lab of Christian Koeberl of the University of Vienna in Austria to find iridium traces from geologic layers dating back 200 million years, to the boundary between the Triassic (248 to 208 million years ago) and the Jurassic (208 to 146 million years ago).
Still to be found is what would be a vast crater, said Olsen, although there are candidates in Canada and Australia. "We don't actually have an impact structure yet," he said, but pointed out it took 15 years to find the 65 million-year-old crater in the Yucatan.