What caused the extinction of the dinosaurs?

Scientist 1

Throughout the long Mesozoic era, hundreds of dinosaur species dominated over smaller animals. Some dinosaurs were meat eaters and others ate only plants. Some lived in deserts, some in swamps. There were even dinosaurs swimming through the oceans and soaring through the skies. Only a worldwide catastrophe could have simultaneously killed all dinosaurs in their diverse environmental niches. The most probable cause of the mass extinction is a close encounter with a comet, which could have abruptly altered the earth's temperatures. Dinosaurs, like all reptiles, were cold-blooded animals and could not adapt to a great temperature change.

Scientist 2

It is almost certain that no single event killed all dinosaurs. Their record during the Mesozoic era is one of species continually evolving to new species; each old species is then said to have become extinct. The extinctions are not at all simultaneous. The early, egg-eating pelycosaurs disappeared in the Permian period. The largest dinosaur of all — Brachiosaurus — became extinct in the Jurassic. The ensuing record during the Cretaceous period is even richer and more complicated, with duck-billed hadrosaurs, predatory tyrannosaurs, and flying pterosaurs appearing and disappearing at various times. The slow disappearance of the dinosaurs occurred as mammals evolved into more habitats. The last dinosaur and the last dinosaur egg were probably eaten by mammals.

Before you answer any questions, reread the opening sentence of the passage to make sure you know the issue in dispute. In this example, it's the cause of dinosaur extinction.

Try not to let the details of evidence keep you from recognizing the main points of disagreement in the theories presented. Circle or underline the main points of agreement and disagreement. Above, Scientist 1 says that all dinosaurs were simultaneously killed, while Scientist 2 states that the different species became extinct at different times — the main point of disagreement between the two theories.

Focus on the details. For example, you might see a question based on how Scientist 1 credits thermal change while Scientist 2 credits deaths by natural causes. This is not a good answer because both scientists are speaking of deaths by "natural causes." A close encounter with a comet is just as "natural" as killing by mammals or extinction from some other environmental cause.

Look for possible weaknesses in an argument. For example, you might be asked:

Question: Which line of evidence, if true, would most weaken the position of Scientist 1?

1. Evidence that dinosaurs became extinct in different types of environments

2. Evidence that dinosaurs became extinct in the same type of environment

3. Evidence that species of dinosaurs became extinct in different geologic periods

4. Evidence that all species of dinosaurs became extinct at the same time

Scientist 1 believes the extinctions happened at generally the same time. Evidence that dinosaurs became extinct at different periods would weaken that position. Choice C is the best answer. Choice A is not the best answer because "types of environments" can refer to many factors, not only temperature. Choice D would strengthen the argument of Scientist 1, not weaken it.

Conversely, look for evidence that strengthens a viewpoint, as well. Also assess additional information that's given in a question.

Question: Current research on extinctions among mammals at the end of the Cretaceous period shows that about 25% of the existing mammals became extinct then. If found to be true, this research would:

1. weaken the views of Scientist 1.

2. undermine the views of Scientist 2.

3. lend support to the views of Scientist 1.

4. neither support nor undermine the views of either scientist.

Since Scientist 1 believes that a comet encounter caused the extinctions of all the dinosaurs, it can be inferred that this scientist would expect this same event to have an impact on all the other life negatively as well. The fact that only 25% of the mammals died off is more supportive of the views of Scientist 2 than of the views of Scientist 1. Choice A. is the best answer.