Comets May Have Led to Birth and Death of Dinosaur Era

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At the same strata of rock and ancient soil, the researchers found an increase in the abundance of iridium. Scientists don't know whether it was a comet or an asteroid that plowed into the Earth, but both contain iridium, and on impact, a layer of space dust is laid down. New capabilities to measure at the parts-per-trillion level using high-resolution mass spectrometry enabled the researchers to identify a previously suspected but never found iridium spike.

Just below the Triassic-Jurassic boundary, the scientists found a thin layer of sediment where the Earth's magnetic field is reversed. This reverse polarity happens at random intervals and is very useful to scientists in dating when events happened.

Finally, just above the Triassic-Jurassic boundary, there are massive lava flows. It's possible that the explosive impact created by a comet collision triggers increased volcanism; there is evidence in India and Siberia of massive lava flows coincident with other extinction events. Scientists haven't been able to figure out the mechanism by which this would occur—"it's another point of befuddlement," says Kent—but the fact that the phenomenon is seen at both the K-T boundary (65 million years ago) and the Permian-Triassic boundary (250 million years ago) is suggestive, he says.

"There are similarities to the K-T extinction (Cretaceous-Tertiary Boundary, 65 million years ago)," said Sues. "The fern spike indicating a significant ecosystem disruption, the disappearance of large groups of large land animals, higher concentrations of iridium. All these different pieces of evidence build a picture supporting the idea that a collision occurred."

Bring on the Dinosaurs

"Most of the early mammals came through the extinction—turtles, frogs, salamanders, and of course dinosaurs. The dinosaur's major competitors were wiped out though, and they became the dominant life-form," says Sues.

With many of the earlier life-forms eliminated, the survivors no longer had to compete for food, water, and habitat. In some cases, their status shifted from prey to predator.

The drop in competitive pressure may have triggered both the global spread of dinosaurs and their rapid increase in size. The researchers believe the rapid —a time scale of thousands of years—increase in size was an evolutionary response by the survivors, which may have been quite small prior to the extinction.

The dinosaurs reigned supreme for 135 million years, until another comet colliding with Earth took them out. Once the dinosaurs were gone, mammals had the run of the land, and they flourished.

It may be time to take what we're learning about the Earth's history and apply it to our thinking about evolution and the role of natural selection, says Sues.

"It shows these great mass extinctions are almost a lottery," he said. "The dinosaurs came through and flourished 200 million years ago, but when a similar event happened 65 million years ago they become extinct. It may well be that catastrophic events have a far more profound effect in the shaping of life than people had previously thought."

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