Nov. 10, 1994
Vol. 15, No. 6

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    Grad student's research challenges theory on dinosaur extinction

    Contrary to a common theory, the mass extinction of dinosaurs may not have been caused by extreme changes in climate, according to Paul Markwick, a graduate student in Geophysical Sciences. Markwick, who reached this conclusion through a detailed study of crocodile and alligator fossils, presented his research at the 1994 meeting of the Geological Society of America in Seattle late last month.

    Markwick found that crocodiles -- which are coldblooded and extremely sensitive to climate change -- not only survived but flourished at the time the dinosaurs became extinct 65 million years ago. Dinosaurs, along with many other creatures, disappeared at the boundary between the Cretaceous and Tertiary periods, known to scientists as the K/T boundary.

    Based on fossil evidence, the crocodiles showed no change in diversity or distribution across the K/T boundary, Markwick said. "A climate change potent enough to exterminate all dinosaurs would undoubtedly have had a major effect on the climatically sensitive crocodilians, but I don't see any evidence for that," he said.

    Scientists continue to speculate about what caused dinosaurs to become extinct. They have suggested as causes volcanic eruptions, vegetation changes, disease and a meteorite impact that sent large volumes of dust and rock into the atmosphere. The dust would have blocked out sunlight, plunging Earth into a cold, dark winter. Markwick accepts the possibility that a meteorite may have induced a climate change, but, he said, "The survival of the crocodiles indicates that any climate change could not have been as severe as many scientists have claimed."

    Crocodiles are coldblooded, so their body temperature is regulated by the surrounding environment. Markwick points out that crocodile fossils are only found in areas that had mild year-round temperatures. Markwick's examination of the fossil record of crocodilians starting at 100 million years ago shows that they remained numerous and widely distributed across the K/T boundary, indicating that the climate remained relatively mild.

    He also investigated the distribution of crocodiles during times of known climate change. Thirty-five million years ago, for example, geologic evidence indicates that a very sudden global cooling occurred. Crocodile fossils from this time are found only in warmer, more southern latitudes, and the number of genera represented decreases precipitously, he said.

    Markwick compiled existing records of crocodile and alligator fossils into one complete data base -- the largest yet compiled for these fossils. "The survival of crocodiles across the K/T boundary has been noted before, but the implications of these findings for the K/T climate have not been fully appreciated," he said.

    Markwick's research grew out of the Paleogeographic Atlas Project, headed by his adviser, Alfred Ziegler, Professor in Geophysical Sciences. "We are beginning to take this great mass of information we have about the distribution of plant and animal fossils, organize it appropriately and draw some conclusions about climate," Ziegler said. "We can learn a lot about the future of the global climate by taking a detailed look at the past. Paul has looked at the fossil record of an animal that may be a sensitive indicator of past global climatic conditions."

    In the search to understand the dinosaurs' demise, Markwick said the answer may lie in studying which animals survived the K/T boundary, rather than concentrating on the species that disappeared.

    -- Diana Steele NOTE: Includes photo of Markwick and graph