Fossil Leaves Suggest Asteroid Killed Dinosaurs

By John Roach
for National Geographic News
June 17, 2002

A team of scientists says evidence from fossilized leaves indicates that dinosaurs appear to have become extinct as a result of the catastrophic impact of an asteroid and not volcanic activity.

Dinosaurs, along with an estimated 70 percent of all life on Earth, are believed to have gone extinct 65 million years ago as a result of a series of dramatic temperature changes. The extinctions are known as the K-T extinctions because they fall on the boundary between the Cretaceous (geological symbol K) and the Tertiary periods.

Some researchers believe that a burst of volcanic activity at the Deccan Traps in India is to blame for the climate changes, while others insist that fallout from an asteroid impact was the cause.

Garland Upchurch, a biologist at Southwest Texas State University in San Marcos, Texas, and his colleagues analyzed fossilized leaves from gingko trees and ferns that grew about the time of the dinosaurs' demise to determine the state of the climate.

"The work on the fern and gingko leaves, when coupled with geochemical modeling, indicates that there was a mega-greenhouse effect after the terminal Cretaceous event and that this was most likely caused by an asteroid impact," said Upchurch.

An analysis of the K-T boundary fossil leaf research, co-authored by Upchurch and scientists at the University of Sheffield, England, and Pennsylvania State University, appears in the June 11 Proceedings of the National Academy of Sciences.

Clues From Leaf Pores

Upchurch and his colleagues analyzed the fossil leaves to determine how much carbon dioxide was in the atmosphere about the time of the dinosaurs' demise.

Atmospheric carbon dioxide is a greenhouse gas: increased levels of the gas can raise the warmth of Earth. Many scientists believe that today's increasing levels of carbon dioxide from the burning of fossil fuels such as oil and gas are causing Earth's temperature to rise at an accelerated rate.

"Leaf fossils can indicate the amount of carbon dioxide in the atmosphere because of the relationship between the frequency of breathing pores on the leaves—termed stomata—and levels of atmospheric carbon dioxide," said Upchurch.

When there is more carbon dioxide in the atmosphere, leaves need fewer breathing pores to extract carbon dioxide from the atmosphere for photosynthesis. "This has been documented in a number of modern plants grown under controlled conditions at different levels of atmospheric CO2," said Upchurch.

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