Near the end of the Cretaceous period, about 65 million years ago, all the species of dinosaurs somehow became extinct.  Evidence of them exists as fossils.  Two scientists discuss their viewpoints of how and why this extinction occurred.

Scientist I

 The extinction of dinosaurs was caused by a large 6-mile in diameter asteroid.  Once it impacted with the earth it created a huge dust cloud; this phenomenon is known as a nuclear winter.  The dust from the impact prevents plants from producing food through photosynthesis and caused them to die.  After that, the herbivores had nothing to eat and consequently died as well.  And without herbivores to eat, the carnivore dinosaurs also died out.
 Evidence of this asteroid exists as a layer of clay in the earth's crust.  This layer contains high levels of iridium.  Since little iridium exists on the earth, it would be a reasonable inference that the iridium came from some cosmic source - such as an asteroid.  Further calculation can assume that only an asteroid that is as large as 6 miles in diameter could feasibly hold as much iridium.

Scientist II

 The extinction of the dinosaurs, as well as that of many other coexistent species, was not caused by a single catastrophic event, but rather by a series of smaller changes.  Such changes may have included changes in climate, lowering of the sea level, and tectonic plate separation.  Although these changes occurred over a longer period of time, many species of dinosaurs and other creatures were unable to adapt accordingly, and they therefore became extinct.
 If the dinosaurs were destroyed by a sudden catastrophic event, the fossil record would support this.  In such a situation, we would see a great number of dinosaur fossils concentrated within a thin layer of rock - probably one immediately above the layer of iridium.  However, we see no such evidence.  The record suggests a slow but steady extinction, as though creatures were struggling to adapt to new conditions.  When we consider the evidence of the whole fossil record, it is clear that major catastrophe did not play a role in the extinction of the dinosaurs.

1. Which of the following would support Scientist 2's view?
A.  Evidence of an increase of radiation & ultraviolet light reaching the earth.
B.  Fossils showing no mature plants & stunted growth-a sign of nuclear winter.
C.  A study showing that bacteria adjust poorly to changes in laboratory settings.
D.  Previous fossil record findings are erroneous.

2. Which of the following is the best alternate explanation for the dinosaurs' extinction?
A. Small carnivores evolved to attack in packs and were able to overpower the dinosaurs.
B. Insects began to grow in increasing numbers.
C. Larger trees & vegetation became prevalent, inhibiting the movement of dinosaurs.
D. Flora & fauna became parasitic and overran more traditional plants in aquatic environments.

3. According to Scientist 1, iridium
A. Is commonly found in the earth's crust.
B. Exists only on asteroids.
C. Can be bountifully found in the mentioned thin layer of clay.
D. Creates a catastrophic dust cloud which causes nuclear winter.

4. By the information provided, a layer of iridium
A. Occurs naturally.
B. Signifies catastrophe.
C. Would be unnatural for dinosaurs & would kill them all with radiation poisoning.
D. Is a rarity in the earth's crust.