Geologic time is divided into units based upon types

Organisms

Explanation:

When we speak of geological time, we refer to a time scale that is usually measured in millions or even billions of years, such as the classification of geological ages and their respective periods. Geological time is a scale obtained from the relative dating of the rocks, either by observing the marks of the events recorded in them, either by the order of superposition of the sedimentary layers or by the fossils they contain, or even by the absolute dating of the rocks, through calculating the rate of disintegration of a radioactive isotope. Radiometric dating of rocks from the Moon and meteorites gave an approximate age of the planet Earth in 4.5 billion years.  In addition, geological time is divided into units based on the types of organisms.

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands