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Comets May Have Led to Birth and Death of Dinosaur Era

Hillary Mayell
for National Geographic News
May 16, 2002
 
Comets slamming into the Earth may be responsible for both the birth and the death of the dinosaur era, an international group of researchers report.

There is a considerable amount of evidence that a bolide (a comet or asteroid) collision with Earth triggered the end of the dinosaur era 65 million years ago.

An international team of scientists has assembled a compelling collection of evidence that a giant ball of ice, rock, and gases smashed into the supercontinent Pangaea 200 million years ago, ending the Triassic era and beginning the Jurassic. The impact was devastating to ocean life, and more than half of land-based species disappeared. Once the dust had cleared, dinosaurs began their 135 million year reign as the planet's dominant life-form.

"We have been able to show for the first time that the transition between Triassic life-forms to Jurassic life-forms occurred in a geological blink of an eye," said Paul Olsen, a geologist at the Lamont-Doherty Earth Observatory of Columbia University. "The change was associated with an iridium anomaly suggesting the cause was the result of a giant impact of an extraterrestrial body."



Crocodile relatives were dominant during the Triassic, while dinosaurs were dominant in the Jurassic. Olsen is the lead author of the study published in the May 17 issue of the journal Science.

"Different lines of evidence, including dinosaur footprints, skeletal remains, and a spike in iridium levels exactly at the change in plant spores from the strata that mark the Triassic-Jurassic boundary, are all suggestive that a disaster befell the land and it may have been a comet or an asteroid," said Dennis V. Kent, a geologist at Rutgers University and a co-author of the study.

Piecing Together the Evidence

Since the first appearance of life on the planet around 4.6 billion years ago, there have been at least five mass extinction events, when 50 percent or more of the species on Earth disappeared. On the other side of these biodiversity crashes, new life-forms appear and thrive. The mass extinction of the dinosaurs 65 million years ago, for instance, lead to the age of mammals.

"The disappearance of one set of plants and fossils, and the appearance of new forms of life, are what characterize geological boundaries," said co-author Hans-Dieter Sues, a paleobiologist at the Royal Ontario Museum in Canada.

The Earth was still one big landmass 200 million years ago, a supercontinent called Pangaea that was getting ready to break apart. As the Earth's crust stretched and thinned, great rift valleys appeared. One, called the Newark Basin, was formed just prior to the separation of North America from Europe and Africa. Stretching through parts of New York, New Jersey, and Pennsylvania, it is a geological treasure trove. The researchers examined footprints, bones, and plant spores at 70 locations in the Newark Basin.

"The Triassic-Jurassic boundary is identified very precisely by a fern spike," said Dennis V. Kent, a geologist at Rutgers University and a co-author of the study. "An increase in the spores of ferns is indicative of an ecological calamity. Ferns are the first plant to invade and colonize when everything else has been wiped out. You can see a similar phenomenon today at Mt. St. Helens after the volcano erupted."

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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