.The results, which need to be validated by further research, will be published in the May 17 issue of the journal Science.
Rocky times
Researchers have thought for more than a decade, and with growing certainty, that an impact played a critical role in the ultimate death of dinosaurs. Other species perished, too, in an event that allowed mammals to prosper in a world where there were fewer large creatures to step on them or swallow them whole.
However only in recent years has evidence turned up linking earlier mass extinctions to impacts.
The new research documents a widespread die-off that occurred at the boundary of the Triassic and Jurassic periods in time, when dinosaurs had begun to gain a foothold in what is now North America. The study found iridium in layers of rock in Earth's crust at several sites that have been traced back to the same point in time.
The element is prevalent in asteroids and comets and can be left behind as a global signature when an incoming object vaporizes on impact and kicks up a dust storm that circles the planet.
Though there is no evidence of a crater, the iridium "creates a time marker for comet or asteroid impacts," said Dennis V. Kent, a Rutgers University geologist and part of the research team. "Correlating the finds with evidence of plant and animal life helps to tell us what happened."
Paul E. Olsen of the Lamont-Doherty Earth Observatory of Columbia University led the study.
Asteroid or volcanism?
An asteroid could certainly do the trick, experts say, creating years of winter-like conditions caused by tons of dust that blots out the Sun.
Yet, other researchers have speculated that the extinction 200 million years ago was instead a result of increased volcanic activity, which would have pumped choking chemicals into the atmosphere. Some experts say a combination of an impact and increased volcanism might provide a one-two punch necessary to cause extinctions of many species.
Either way, the case is not firm.
"Considerably more geochemistry is needed to rule out a volcanic origin," Olsen told SPACE.com. "And much more sampling over a broader temporal and geographic range is needed to confirm that what we found is really a global event tied to an impact."
Olsen said, however, that the team found iridium at levels two and three times higher than normal "background" levels found in sedimentary rock. This anomaly, or difference in levels, is not as high as has been documented for the so-called K-T impact that occurred 65 million years ago (and for which
)."The magnitude of our anomaly is small compared with that at the K-T boundary, but it is similar to anomalies at other known impacts," Olsen said.
The iridium spike was found to be coincident with a spike in fern spores, thought to be a signal of recovery from an impact.
Paul Sereno, the noted dinosaur hunter from the University of Chicago, said the study provides "critical new data on the origins and early evolution of dinosaurs that support a sharp break in the fossil record between the early and rarer dinosaurs of the Triassic period, and the larger dominant dinosaurs of the early Jurassic."
The number and extent of the Triassic-Jurassic extinctions have been much debated.
"Which is why this well-dated, well-supported find of iridium and a fern pollen spike in association with footprints is such key new information," Sereno told SPACE.com. "Doubtless it will spark new research into the question of how and when dinosaurs rose to dominance on land -- a question that may ultimately become as resolved as the events at the end to the dinosaur era."
Next Page: Other impacts, and how this study was done
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