The dinosaurs were wiped out by an asteroid that smacked the Earth 65 million years ago, but they survived another cataclysmic event — perhaps another asteroid impact — that snuffed out 80 percent of all species about 200 million years ago, scientists said today.
By studying the fate of a type of marine plankton, single-celled organisms called Radiolaria, researchers found that the mass extinction was a sudden event, not the prolonged die-off that experts previously had thought. The extinction occurred at the boundary between the Triassic and Jurassic periods during the Mesozoic era.
The event provided the death knell for most species and helped crown the dinosaurs, which arose earlier in the Triassic, as the rulers of the Earth, said Peter Ward, a University of Washington paleontologist who led the study.
Ward said this calamity had tremendous similarities to two of the other five mass extinctions that have ravaged Earth over the past 500 million years. Like those, Ward said it appears this mass extinction was caused by a giant rock from space.
"We know now that asteroid impact can cause rapid extinction," Ward said in an interview. "It may not be an asteroid. But if it isn't an asteroid, it acts like an asteroid, put it that way."
Wiped Out in a Second?
Most scientists believe an asteroid strike caused the mass extinction at the end of the Cretaceous period that killed the dinosaurs and ushered in the age of mammals. In February, scientists presented evidence that an asteroid or comet impact also caused the even bigger extinction at the boundary between the Permian and Triassic periods 250 million years ago.
Ward's team gathered evidence about the extinction 199.6 million years ago at two remote sites in the Queen Charlotte Islands off Canada's British Columbia coast, examining fossil samples indicating a collapse of the plankton population.
The researchers found an abrupt drop in the rate at which inorganic carbon was turned into organic carbon by life forms through processes such as photosynthesis.
The organic carbon decline coincided with the disappearance of more than 50 species of radiolarians, which served as a food source for numerous marine species and whose disappearance was an indicator of a major biological crisis.
The study was published in the journal Science.
Ward said the research indicated it took less than 10,000 years for the mass extinction to unfold. It could have taken place even more quickly — perhaps in an instant, he added.
"This thing was real fast," Ward said.
At the time, most dinosaurs were relatively small, and they were locked in a survival-of-the-fittest battle with other well-adapted animals, including the mammal-like reptiles — the biggest of which were among the major herbivores of their day.
"These suckers are huge, they're hulking," Ward said.
But the mammal-like reptiles — whose earlier forms gave rise to the first true mammals — perished in the calamity.
"One of the great mysteries has been … why would these creatures, which are seemingly better adapted for eating a variety of plant sources, die out and the dinosaurs not? And the answer is: Mass extinction doesn't give a hoot about your adaptations for everyday life. There's a lottery involved, for whatever reason," Ward said.
Also nearly wiped off the planet were the ammonoids — marine predators that resembled a giant squid in coiled cone shell.
Death From the Sky
Ward said there are ongoing studies to try to confirm an asteroid as the cause. Ward said he has found evidence of little carbon molecules called buckminsterfullerenes — or buckyballs — that hint at a space rock as the culprit.
He said a massive crater in Quebec called the Manicouagan structure, which measures 60 miles (100 km) wide, could be the impact site. The crater has been dated to 214 million years ago, but Ward said the date may be too old.
Ward said alternative theories include an explosion of a nearby star that could have blown off the Earth atmosphere's ozone layer and sent temperatures soaring, or massive volcanic activity, possibly related to the breakup of the archaic super-continent known as Pangea.
Scientists know very little about the mass extinctions that took place 350 million and 420 million years ago, Ward said.