Dinosaurs dominated life on Earth for millions of years and then disappeared very suddenly.
To solve the mystery, paleontologists studied fossils and skeletons found in rocks in various
layers of Earth’s crust. Their fi ndings enabled them to map out which species existed on Earth during specifi c geologic periods. They also revealed no dinosaur skeletons in rocks formed immediately after the Cretaceous period, which dates back some 65 million years. It is therefore assumed that the dinosaurs became extinct about 65 million years ago.
Among the many hypotheses put forward to account for their disappearance were disruptions
of the food chain and a dramatic change in climate caused by violent volcanic eruptions.
However, there was no convincing evidence for any one hypothesis until 1977. It was then that
a group of paleontologists working in Italy obtained some very puzzling data at a site near
Gubbio. The chemical analysis of a layer of clay deposited above sediments formed during the
Cretaceous period (and therefore a layer that records events occurring after the Cretaceous
period) showed a surprisingly high content of the element iridium (Ir). Iridium is very rare in
Earth’s crust but is comparatively abundant in asteroids.
This investigation led to the hypothesis that the extinction of dinosaurs occurred as follows.
To account for the quantity of iridium found, scientists suggested that a large asteroid several
miles in diameter hit Earth about the time the dinosaurs disappeared. The impact of the asteroid
on Earth’s surface must have been so tremendous that it literally vaporized a large quantity
of surrounding rocks, soils, and other objects. The resulting dust and debris fl oated through the
air and blocked the sunlight for months or perhaps years. Without ample sunlight most plants
could not grow, and the fossil record confi rms that many types of plants did indeed die out at
this time.
Consequently, of course, many plant-eating animals perished, and then, in turn, meateating
animals began to starve. Dwindling food sources would obviously affect large animals
needing great amounts of food more quickly and more severely than small animals. Therefore,
the huge dinosaurs, the largest of which might have weighed as much as 30 tons, vanished due
to lack of food.
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