Propose reasons why major extinction events (or explosions of life-forms) are good dividing lines for the geologic timescale. Support your answer
What will be an ideal response?
It is sensible to have eras and periods grouped with similar life-forms as found in the fossil record. Using extinction events as a border makes this easy. For example, after the Cretaceous extinction, one would find no dinosaur fossils but would start to see other life-forms such as mammals heavily represented. Similarly, one would see angiosperms in the fossil record after but not before the Triassic extinction. Thus, such events make good dividing lines for the geologic strata and the fossil record found therein.
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