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Fossil Leaves Suggest Asteroid Killed Dinosaurs

National Geographic News (06/17/2002)

By John Roach

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.

These controlled experiments have resulted in what scientists term the stomotal index, which shows an inverse relationship between the amount of carbon dioxide in the atmosphere and the number of breathing pores on the leaves.

The researchers compared the fossilized fern and gingko leaves with a stomotal index derived from the closest living relatives of the fossil plants, which allowed them to reconstruct past levels of atmospheric carbon dioxide for analysis.

The analysis indicates a sudden and dramatic increase in carbon dioxide levels equivalent to injecting 6,400 billion metric tons of carbon into the atmosphere, which is enough carbon to warm the Earth by 12 degrees Fahrenheit (7.5 degrees Celsius).

“6,400 billion metric tons of carbon is, by at least a factor of five, more than the entire carbon pool of either modern or latest Cretaceous vegetation,” said Upchurch. “If our calculations are correct, a significant quantity of the carbon had to come from the vaporization of limestone rock by the asteroid impact on the Yucatan Peninsula,” he said.

Another Theory

According to the analysis of the leaf fossils, the increase in carbon occurred over a period of 10,000 to 20,000 years, too short of a time period to lay the blame on volcanism at Deccan Traps, which scientists have said lasted from 500,000 to several million years.

Dewey McLean, emeritus professor of geology at Virginia Polytechnic University in Blacksburg, Virginia, who proposed the Deccan Traps volcano theory in 1981, said the fossil leaf database that Upchurch and colleagues used for their analysis is too small to accurately depict the timing of the K-T boundary record.

“At one collecting site, they have one leaf,” he said. “And for each of the other sites only a few leaves.”

According to McLean’s analysis of the Deccan Traps, 70 to 90 percent of the entire lava pile from the eruptions at the Deccan Traps began 65 million years ago, right at the K-T boundary.

The Proceedings of the National Academy of Sciences paper does not change Dewey’s belief that the Deccan Traps volcanism is the major culprit behind the global warming that led to the mass extinctions at the K-T boundary.

“I believe that a number of factors combined to trigger a major K-T transition greenhouse, of which Deccan Traps was the primary contributing factor,” he said.