4. Why are the time periods of the Paleozolc and Mesozolc subdivided into early, middle, and late Instead of named epochs, as they are in the Cenozelo The fossil record for the Paleozoic and Mesozoic eras is not sufficient to subdivide the periods based on the presence of organisms.

The number of fossils discovered outnumbers the quantity needed to subdivide the periods based on the presence of organisms.

The rocks of these time periods are too young to provide sufficient data to subdivide the periods based on the ages of rocks.

The terms early, middle, and late are easier to remember, which removes some confusion for those studying the geologic time scale. ​