More than 200 million years ago, toothy crocodilelike creatures stalked a hot, dry mega-continent while squidlike mollusks with spiral shells drifted in the surrounding ocean. Then, in what passes for an instant in geologic time, they vanished – making way for the age of the dinosaurs.
How 50 percent of terrestrial vertebrates and an even larger share of marine life died off in the late Triassic period has become more clear from new research published last month in the journal Science.
The work lends greater validity to the theory that a massive volcanic event tore apart that continent and blanketed Earth’s atmosphere, turning the ocean acidic and snuffing out animals that could not adapt. That geologic event, which created the Atlantic Ocean, ushered in the biggest biological shift in the planet’s history.
“It set the stage for the dinosaurs to take over, biologically,” said Paul Olsen, a geologist at Columbia University’s Lamont-Doherty Earth Observatory, who did much of the field work on which the study is based.
Pushed by the nascent Atlantic, the ruptured pieces of Pangea drifted off and further split, carrying the evidence of ecological collapse to such distant locales as Morocco, Nova Scotia and New Jersey.
Matching the fossil record in sedimentary rock with the dense basalt formed by the volcanic eruptions proved difficult, even with sophisticated tools of the 21st century. It was difficult to say whether the eruptions happened before the mass extinction.
Olsen and others, however, hacked rare zircon crystals from the basalt formations and measured traces of lead and uranium for radiometric dating tests. The results narrowed the margin of error in dating the lava to a mere 15,000 to 22,000 years – stunning precision for geochronology.
The more refined ages put “a sharper tip on that pencil,” said Paul Renne, a geochronologist at the University of California, Berkeley, who has done similar research but wasn’t involved in the new study. “It says all this stuff is true and here’s when it happened.”
The mass extinction occurred 201.56 million years ago, around the age of one of the basalt rock formations dated by the team.
There were several pulses that spewed millions of cubic miles of lava, each separated by tens of thousands of years, said geologist Terrence Blackburn of the Carnegie Institution in Washington, the study’s lead author.
The first of them was probably big enough to cause the extinction, and subsequent ones occurred as life began to recover and dinosaurs exploited the territory left empty by the mass die-off.
For now, the data “strongly imply a causal relationship” between the eruption that broke up Pangea and the extinction that occurred around the same time, the researchers concluded.
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