There are a few things about fossils that must be understood to be aware of what they mean in terms of geologic succession of the earth.
Fossils are the remains of previously living organisms.
Most of the fossils found that are related to the early history of the earth are fossils of extinct organisms. Scientists have made detailed graphs or webs of the succession of many species that inhabit the earth today and traced them back to the organisms found in the bedrock of the earth.
These webs show the theory of "survival of the fittest" as they observed that certain traits disappear along the line of succession that proved to aid the species in its livelihood.
So how do geologists know that the earth is approximately 4.5 billion old or more?
Dinosaurs were extinct 65 million years ago and they ruled the earth for 100 million years before that.
(2) Remember earth is estimated to be well over 4.5 billion years old.
An isotope is an atom of the same element with a differing atomic weight from the stable form of the element.
The rate of decay is expressed in terms of an isotope's half-life, or the time it takes for one-half of a particular radioactive isotope to decay.