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Mass Extinction That Led to Age of Dinosaurs Was Swift, Study Shows

D.L. Parsell
National Geographic News
May 10, 2001
 
From new fossil evidence, scientists say they now know a mass extinction that preceded the age of large dinosaurs happened relatively fast—perhaps when an asteroid crashed to Earth.



One of several mass extinctions that have occurred over time, this one happened at the time boundary between the Triassic and Jurassic periods, 200 million years ago. It wiped out at least half of all species on Earth, including the last of the mammal-like reptiles, leaving mainly dinosaurs.

This enormous loss of species occurred over a period of less than 10,000 years—almost instantly, in geological time.

"This is the first time ever that we can see how sudden this event was," said Peter Ward, a University of Washington paleontologist who led the team that reported the new research in the May 11 issue of Science. "It was very quick, not a long, protracted episode."

The findings were based on fossil records of one-celled organisms, called protists, which were found at a remote site in British Columbia.

The fossils showed there was sharp decline in the population of a certain organism. It coincided with scientific data indicating that at about the same time, Earth experienced an abrupt drop in levels of organic carbon (which is one measure of the amount of life on Earth).

Enter Jurassic Park

The rapid disappearance of so many species at once had a major influence on the pattern of evolution, shaping the nature of the life that followed, Ward explained.

"This extinction really opens up the age of dinosaurs. This starts Jurassic Park," he said.

"If we didn't have this mass extinction, the age of dinosaurs as we currently know it now may have been very different," he explained. "The big, herbivore dinosaurs we all think of may never have evolved, because other groups might have emerged if the smaller mammal-like reptiles hadn't been wiped out."

Although there is no definitive evidence yet of what caused the sudden demise of so many species, the suddenness of the event is similar to two other, better known mass extinctions. One occurred 250 million years ago at the end of the Permian period, killing off as much as 90 percent of all species. The other happened 65 million years ago at the end of the Cretaceous period, marking the end of dinosaurs.

It's widely believed that the dinosaurs were wiped out by a cataclysmic event such as the crash of a giant meteor that was hurled to Earth. Ward speculates that the same kind of thing may have happened in the mass extinction 200 million years ago, paving the way for dinosaurs.

"The suspicion is this is an impact event, an asteroid-related extinction," he said.

Data From Carbon Levels

The fossil evidence of the extinction was gathered at two very remote and still largely untouched sites in the Queen Charlotte Islands, off the coast of British Columbia, in Canada. "These provide the best record of how hasty the extinction was at this [time] boundary," Ward said.

The mass extinction 200 million years ago occurred just before the breakup of Pangaea, which contained all the land on Earth in one supercontinent. At the time, the Queen Charlotte Islands, which now lie between 52 and 54 degrees north, were probably on the equator or in the southern hemisphere, Ward noted.

By using different isotopes (unusual atomic configurations of a chemical element), scientists can date the age of rocks and measure the amounts of life on Earth, in the form of carbon levels. Samples from a spot called Kennecott Point, in the northern Queen Charlottes, and from Kunga Island, about 100 miles (160 kilometers) to the southeast, showed a sharp decline in the presence of organic carbon about 200 million years ago.

Carbon is an essential component of all living organisms. The drop in organic carbon measured by the scientists correlated with a dramatic decline in radiolarians, a kind of protist, that serve as a food source for a number of marine species.

The scientists actually measured levels of productivity, the rate at which inorganic carbon is turned into organic carbon through processes such as photosynthesis. One example of productivity, Ward explained, occurs in the spring when fertilizer washes into waterways and triggers large algae blooms. The situation 200 million years ago was like a reversal of that process, marked by a huge waning of productivity.

Ward, who has also done research on other mass extinctions, plans to return with a team to the Queen Charlottes in the hope of finding more clues about the Triassic-Jurassic extinction.

The other researchers in the new study were James Haggart and Howard Tipper of the Geological Survey of Canada in Vancouver, British Columbia; Elizabeth Carter, a researcher at Oregon's Portland State University; David Wilbur, an oceanographer at the University of Washington; and Tom Evans, a student at the University of Washington.
 

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