What Killed Tyrannosaurus?

During the Cretaceous period, when mammals were still furtive, slinking creatures, the mighty dinosaur, king of the reptiles, ruled the earth. Then about 60 million years ago, the last of the dinosaurs vanished. The mammals inherited the earth and have bossed it ever since. Some geologists believe that the dinosaurs were too specialized to adapt themselves to a sudden change of climate. The trouble with the theory has always been that there is no way to prove that the climate really changed. Last week the University of Chicago's Dr. Harold Urey told a Los Angeles meeting of the American Association for the Advancement of Science how he hopes to solve the ancient whodunit.

Famed Chemist Urey (Nobel Prize, 1934) proved three years ago that certain fossil sea shells can be used as fossil thermometers to measure "paleotemperatures." His method takes advantage of the fact that normal oxygen contains two stable isotopes, oxygen 18 and oxygen 16, in the proportion of 1 to 500. When a sea mollusk takes up calcium carbonate (CaCO3) to build its shell, the proportions of the oxygen isotopes in it vary with the temperature of the sea water. The warmer the water the less oxygen 18 is built into the shell.

By measuring the percentage of oxygen 18 in fossils, Urey showed that the seas around Britain, now cold, could have been as warm in Cretaceous times as tropical seas are today. This indicates that the whole earth was warm at that time, and therefore fine for furless, cold-blooded dinosaurs. Urey's next job will be to measure the temperature of the Eocene period that followed the Cretaceous.

As a start on the job, Dr. Urey this year went to Egypt, which was covered by the sea during both geological periods. In many parts of Egypt and Israel, the rock layers formed during the transition from the Cretaceous to the Eocene are very easy to identify. They contain few fossil shells, for life was apparently scarce at that time. But Egyptian and Israeli paleontologists have promised to send him suitable specimens from above and below the dividing line. If the climate-change theory is correct, the fossils will show that Cretaceous mollusks lived in warm water and Eocene mollusks in colder water.

Testing the shells for oxygen isotopes will take a long time. But when the job is done, Urey hopes to know whether a change of climate, or some other disaster, wiped out the mighty dinosaurs.

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