Relative dating uses to estimate how old a fossil is
The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils.
The development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host.
The result is like a radioactive clock that ticks away as unstable isotopes decay into stable ones.
You can't predict when a specific unstable atom, or parent, will decay into a stable atom, or daughter.
Once all the parents have become daughters, there's no more basis for comparison between the two isotopes.
Scientists can't tell whether the clock ran down a few days or millions of years ago.
Fossils may also consist of the marks left behind by the organism while it was alive, such as animal tracks or feces (coprolites).Atoms may have an equal number of protons and neutrons.If, however, there are too many or too few neutrons, the atom is unstable, and it sheds particles until its nucleus reaches a stable state.Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood, oil, coal, and DNA remnants.The totality of fossils is known as the fossil record.