Yet there is nothing that directly links meteorite impacts with the extinction of entire species.

Scientists can recite a long list of the devastating environmental consequences of a large meteorite impact, but they cannot prove these effects have led to the simultaneous loss of life around the globe.

Answering the question of how and why such a large variety of species died out at the same time is one of the greatest mysteries in paleontology.

While the exact reason for the Cretaceous-Tertiary (K-T) mass extinction is still under debate, other past extinctions have been clearly linked with climate change.

As species become increasingly specialized to their environment, a substantial or sudden change will tend to threaten their survival.

The Earth has gone through many cycles of extreme warming and cooling in its history, with an associated rise and fall of species.

Could such climate change have played a role in the K-T extinction? One proponent of this theory is Dewey McLean, a geologist at Virginia Polytechnic University who first published the idea in the 1970s.

McLean thinks the Chicxulub impact in Mexico just added more stress to an environment that was already upset by the release of copious amounts of volcanic gases.

His culprit for the outgassing is the Deccan Traps, an ancient lava flow in west central India. This flood basalt volcanism, says McLean, upset the Earth's carbon cycle and led to long-term global warming.

McLean suspects the dinosaurs gradually became extinct through heat-induced reproductive failure. He says that the higher temperatures, along with pH changes in ocean water, led to the extinctions seen in marine life at the time.

But Simon Kelley, a geologist at Open University in England, disagrees that the volcanic gas from the Deccan Traps could have caused such warming.

He says the traps could have released, at most, only 2 percent of the carbon dioxide (CO2) already in the atmosphere - not enough to trigger global warming.

In addition, he notes that volcanoes release sulfur dioxide (SO2), which causes cooling rather than heating.

"The most recent example of massive volcanism, the Laki eruption of 1783-4 in Iceland, caused cooling in Europe and the northern USA, not heating," says Kelley.

"The bitter winter in Paris was documented by Benjamin Franklin, envoy from the newly formed United States of America. Although SO2 is washed out rapidly, the signal of volcanism should be a combination of the both cooling and heating."

Kevin Pope, a geologist with Geo Eco Arc Research, says there is no evidence for global warming following the K-T extinction. "In fact, the best records show an abrupt cooling in the earliest Tertiary," says Pope.

Although McLean says that oxygen isotopes in ancient rocks indicate the Earth endured long-term global warming from the Cretaceous through the Tertiary eras, he admits the climate signal is mixed overall, with some rocks indicating cooling instead of warming.

"We have much work to do in straightening out the K-T climate record," McLean says.

Dinosaurs first appeared and flourished during the Mesozoic, a generally warm era that lasted from 248 to 65 million years ago. Geologists split that large chunk of time into the Triassic, Jurassic and Cretaceous periods.

In the Triassic, all the continents were joined together in one huge landmass called Pangea.

Such large swaths of land tend to influence their own climate, resulting in very dry conditions, with greater seasonal fluctuations than in coastal areas.

Deserts spread across the continent during the Triassic, but there were still oases of tropical and temperate forests.

In the Jurassic, the great continent Pangea broke in half, creating Laurasia in the north and Gondwanaland in the south. Rainfall increased, ocean levels rose, and lush rain forests began to displace the deserts.

These tropical forests eventually blanketed much of two continents.

By the end of the Cretaceous, the two continents had separated into even smaller landmasses that were well on their way toward their present continental shapes.

The late Cretaceous experienced extreme climate fluctuations, where temperatures would drop and then rebound. This stressed the environment and likely resulted in the extinction of many species.

Not only did the breakup of large continents into smaller chunks of land alter the global climate, but all that tectonic movement also must have affected the ocean cycles that help regulate climate.

The El Nino and La Nina ocean cycles of our own time are testament to how strong this marine influence can be.

After the extinction of the dinosaurs and 50 percent of the world's species, the early Tertiary (or Paleocene) era begins. Temperatures continued to fluctuate during this time period, although they were generally cooler than the end of the Mesozoic.

This cooling may have been due in part to the gasses and debris that was thrown into the atmosphere by the Chicxulub meteorite impact.

But the continental shuffling in the late Mesozoic also suggests a great deal of volcanic activity must have been occurring at the time, throwing out gases that could have changed the balance of atmospheric gases.

The greatest accumulation of lava on the Earth's surface at the time was in the Deccan Traps of India. The Deccan lava first appeared millions of years before the K-T extinctions.

Sankar Chatterjee, a paleontologist at Texas Tech University in Lubbock, Texas, says the fossil evidence shows that dinosaurs lived quite happily right near these lava flows.

"There are layers of sediment, lava, sediment, lava, and so on, indicating the lava stopped and then started again over a long period of time," says Chatterjee.

"We find dinosaur eggs and bones throughout these layers, right up to the K-T layer. So they lived around the Deccan Traps while this lava was erupting."

But then, right about 65 million years ago, the intermittent trickle of lava became a vast flood. Geologists estimate that 90 percent of the lava in the Traps was released at that time.

The K-T extinction is not the first mass extinction event to coincide with a large outpouring of lava. An extensive lava flow in Siberia occurred about 250 million years ago, around the same time as the Permian-Triassic (P-Tr) extinction event, the largest extinction of life in Earth's history.

The P-Tr extinction is often referred to as the "Great Dying," because 90 percent of marine and 70 percent of land species perished.

In 2004, a group of scientists announced that the Bedout crater, buried off the northwestern coast of Australia, is about 250 million years old, and therefore may coincide with the P-Tr extinction.

The scientists say the Bedout crater was created by a meteorite similar to the one that made the Chicxulub crater in Mexico's Yucatan Peninsula.

If Bedout does prove to be an impact crater, it would point to another instance where both a giant meteorite impact and massive flood basalt volcanism occurred around the same time as a mass extinction.

Yet the evidence indicating Bedout is an impact crater is not as firmly established as the impact evidence for Chicxulub, and some scientists say there is little proof Bedout is anything but a volcanic structure.

Peter Ward, a geologist with the University of Washington in Seattle, doesn't think the P-Tr extinction was caused by impact.

He recently published a paper in the journal Science that blames global warming on the Permian extinction. Just as McLean thinks the Deccan Traps led to the K-T extinctions, Ward thinks the Siberian lava flows could have eventually led to the P-Tr mass extinction.

Ward says that the extinction rate in the P-Tr was much more gradual than the K-T, occurring almost imperceptibly over millions of years. Yet McLean believes the K-T extinction rate also was slow and incremental.

"I use the term K-T transition' because the biological turnover actually began during the Late Cretaceous, and extended into the Early Tertiary," says McLean.

"As indicated by the geobiological record, there was no global catastrophic extinction of most of Earth's life at the K-T boundary 65 million years ago."

Most paleontologists disagree, however. The consensus is that the K-T extinctions took place over a relatively brief point in time.

"The best paleontological data bases, like marine foraminifera and dinoflagellates and terrestrial pollen, all point to an abrupt catastrophic event at the K-T boundary," says Pope.

These microscopic fossils are preferable to dinosaur bones when it comes to determining the time scale of the K-T extinction, says Pope, because dinosaur fossils are so rare that they can not reliably indicate whether an extinction was sudden or gradual. (A "short" period or "sudden" event in the fossil record can describe something that occurred over hundreds or thousands of years, due to the margin of error in the dating methods.)

Looking at the bone record we do have, it's clear that many dinosaur species died out as a part of the natural extinction cycle long before the Chicxulub meteorite hit the Earth, and other species were in decline.

But species that seemed to have robust populations before the impact suddenly disappear as the fossil record enters the Tertiary.

In fact, the fossil record indicates that dinosaurs achieved their greatest species variety only a few million years before they became extinct. This suggests that something dramatic must have occurred to cause such a definitive end to the reign of the dinosaurs.

Most scientists who study the K-T believe the Chicxulub impact alone caused the extinction, because the preponderance of evidence suggests the two events are closely linked in time.

But, says Kelley, "we still have a lot to learn." He notes that, historically, "there is a better correlation between volcanism and mass extinctions than impact and mass extinction."

Even if the K-T was triggered by atmospheric changes due to massive volcanic outgassing, that does not answer many other questions about the extinction event.

For instance, while the dinosaurs and many other species perished 65 million years ago, a variety of other animals survived, including the rodent-like mammals that eventually became human beings.

If the environment became so hostile that half of all life on Earth died, then how did animals like birds, frogs, crocodiles, and mammals live on?

Read parts 1, 2, and 3 of this series on the K-T extinction.

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