What Killed the Dinosaurs?

One hundred million years ago the monstrous dinosaur was the king of beasts. Then the dinosaurs suddenly died off, leaving dominance of the earth to smaller, warm-blooded mammals. One theory is that the great die-off was caused by a sudden change of climate. Another is that the slow-witted, blundering dinosaurs could not cope with mammals that destroyed their eggs. Biochemist Albert Schatz of National Agricultural College, Doylestown, Pa. has a third theory: that the evolution of modern plants was the death of the dinosaurs.

According to Dr. Schatz, the dinosaurs were sluggish beasts whose metabolism (vital chemical processes) was so slow that they could keep their vast bodies alive without a great deal of food. In their age, he thinks, the earth's atmosphere did not contain so much oxygen as it does today. The dominant plants were mostly gymnosperms (conifers, ginkgoes, etc.) that did not excrete so much oxygen as modern plants do.

Burnout. The dinosaurs evolved in this oxygen-poor atmosphere and were adjusted to it, but when the angiosperms (modern broad-leaved plants and grasses) became dominant, the dinosaurs were headed for trouble. The vigorous angiosperms excreted so much oxygen that they changed the atmosphere. The oxygen-rich air increased the metabolism of the dinosaurs. They were compelled to live at a faster rate, and they could not gather the vast amounts of food their speeded-up bodies called for. So they burned out and died out, while the newly evolved mammals, well-adapted to oxygen-rich air, took over the earth.

Another theory is that supernovae did the dinosaurs in. During the last thousand years, say Astronomers V.I. Krasovsky and I.S. Shklovsky of the Soviet Union, at least five supernovae (exploding stars) have been visible from the earth. Starting with this information, they calculate that every 200 million years or so a supernova explodes not more than 26 light-years (156 trillion miles) away from the earth.

The explosion is quite an event; for a couple of weeks the supernova gives as much light as 200 million suns. The Russian astronomers do not think that a brief burst of light from a supernova 26 light-years away would have much effect on the earth. Much more serious, they think, would be the vast amount of cosmic rays streaming out of the wreckage of the shattered star. For a few hundred or thousand years after the explosion, the number of cosmic rays hitting the earth would be many times greater than it is today.

Siege of Rays. Cosmic rays are held responsible for many of the genetic mutations that make sudden changes in the heredity of plants and animals. So if cosmic rays increase because of a nearby supernova, mutations will probably increase in proportion. Since most mutations are harmful or even deadly, the effects on some forms of life might be disastrous.

Long-lived animals would suffer most because their reproductive tissue would accumulate damage over a long period. Their mutation rate might be doubled, think Krasovsky and Shklovsky, if cosmic rays were stepped up by three to ten times the present number. They might accumulate enough harmful mutations to destroy the species.

The great dinosaurs of 100 million years ago probably reproduced slowly, as big modern animals do. So the Soviet scientists suggest that a few centuries of intense cosmic rays from an exploding star may have killed them off. Small, fast-breeding animals, such as the primitive, ratlike mammals of the time, would not be damaged as much. So the mammals survived the siege of cosmic rays. After the supernova had died down, some evolved into forms almost as big as the dinosaurs.

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