Dinosaurs dominated life on Earth for millions of years and then disappeared
very suddenly. In the experimentation and data-collecting stage,
paleontologists studied fossils and skeletons found in rocks in various layers
of Earth's crust. Their findings enabled them to map out which species existed
on Earth during specific 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. 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
floated through the air and blocked the sunlight for months or perhaps years.
Without ample sunlight most plants could not grow, and the fossil record
confirms that many types of plants did indeed die out at this time.
Consequently, of course, many plant-eating animals gradually perished, and
then, in turn, meat-eating animals began to starve. Limitation of food sources
obviously affects large animals needing great amounts of food more quickly and
more severely than small animals. Therefore, the huge dinosaurs vanished
because of lack of food.
(a) How does the study of dinosaur extinction illustrate the scientific
method?
(b) Suggest two ways to test the hypothesis.
(c) In your opinion, is it justifiable to refer to the asteroid explanation as
the theory of dinosaur extinction?
(d) Available evidence suggests that about 20 percent of the asteroid's mass
turned to dust and spread uniformly over Earth after eventually settling out
of the upper atmosphere. This dust amounted to about \(0.02 \mathrm{~g} /
\mathrm{cm}^{2}\) of Earth's surface. The asteroid very likely had a density of
about \(2 \mathrm{~g} / \mathrm{cm}^{3}\). Calculate the mass (in kilograms and
tons) of the asteroid and its radius in meters, assuming that it was a sphere.
(The area of Earth is \(5.1 \times 10^{14} \mathrm{~m}^{2} ; 1
\mathrm{lb}=453.6 \mathrm{~g} .\) ) (Source:
Consider a Spherical Cow-A Course in Environmental Problem Solving by J.
Harte, University Science Books, Mill Valley, CA, 1988 . Used with permission.
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