AP BIOLOGY

Detailed explanation-1: -Mass extinctions were first identified by the obvious traces they left in the fossil record. In the strata corresponding to these time periods, the lower, older rock layer contains a great diversity of fossil life forms, while the younger layer immediately above is depauperate in comparison.

Detailed explanation-2: -Evidence suggests an asteroid impact was the main culprit. Volcanic eruptions that caused large-scale climate change may also have been involved, together with more gradual changes to Earth’s climate that happened over millions of years.

Detailed explanation-3: -The fossil record, however, describes sudden, global extinctions that affect many species. These dramatic events are known as mass extinctions. Some populations seem more vulnerable than others: tropical forms of life have been drastically affected, while land plants tend to be highly resistant to mass extinctions.

Detailed explanation-4: -BP: Nowadays, scientists are aware of five mass extinction events in the past, starting with the End-Ordovician Extinction 450 million years ago and up to the End-Cretaceous Extinction that killed off the dinosaurs 66 million years ago (see chart).