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What Killed The Dinosaurs? New Evidence Perplexes Scientists

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By JOHN NOBLE WILFORD, The New York Times

Published: November 11, 2007

FREEHOLD, N.J. - Splashing through a shallow creek in suburban New Jersey, the paleontologists stepped back 65 million years to the time of the last mass extinction, the one notable for the demise of the dinosaurs.

The stream flows over sediment laid down toward the end of geology's Cretaceous period. The clay at water level holds meaningful traces of iridium, the element more common in asteroids and other extraterrestrial objects than in the earth.

Scientists associate the iridium anomaly with the asteroid impact or impacts thought to have set off the extinctions. The thin layer, which has been detected worldwide, also is considered the marker for the end of the Cretaceous and beginning of the Tertiary period, known as the K-T boundary.

At the time, sea levels were higher and New Jersey was warmer. The proto-Atlantic waters reached the center of the current boundaries of New Jersey, standing more than 60 feet deep here, where on a recent day the paleontologists were up to their ankles in a creek. They had their eyes on the sediments in the bank just above the iridium clay. They call this the Pinna layer.

On previous visits, they had found in the Pinna rock and soil a surprising number of marine fossils, including small clams, crabs and sea urchins. There was an abundance of ammonites, considered index organisms of the uppermost Cretaceous environment. Somehow, here at least, life appeared to have not only persisted but also flourished for tens, perhaps hundreds, of years after the putative asteroid impact.

The discovery of thriving communities of survivors at the end of the Cretaceous is giving some scientists second thoughts about the extinction's causes and effects. Some question the conventional explanation of a single large impact that enveloped Earth in a cloud of dust and almost instantaneously brought on a deadly global winter. They contend that this may be an oversimplification, and that the real story behind the dinosaur-ending disaster is more complicated and as yet unclear.

"It is undeniable that the iridium spike at the base of the Pinna layer was produced by the impact," Landman said. "That's amazing and makes it hard to explain the ammonite abundances we find above the iridium anomaly."

Gerta Keller, a paleontologist and professor of geosciences at Princeton University, said the research by Landman's group "shows the complexity of this extinction event and the difficulty explaining it by the currently popular impact theory."

At a meeting of the Geological Society of America two weeks ago, Keller reported marine fossil evidence that she said linked the mass extinction to widespread volcanic eruptions that swept India at the end of the Cretaceous.

In other words, the world's ecosystem was under widespread stress for an extended time. The extinctions might have had multiple causes, not the single asteroid impact and almost instant death as hypothesized in 1980 after the detection of the global iridium layer.

At least 110 species of near-shore marine organisms have been identified in the Pinna layer, experts say.

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