Absolute dating is used to determine a precise age of a fossil by using radiometric dating to measure the decay of isotopes, either within the fossil or more often the rocks associated with it.

The majority of the time fossils are dated using relative dating techniques.

The same rock formation also contains a type of trilobite that was known to live 415 to 425 million years ago.

Protons and neutrons together are called nucleons, meaning particles that can appear in the atomic nucleus.

A nuclide of an element, also called an isotope of an element, is an atom of that element that has a specific number of nucleons.

The rules are the same in all cases; the assumptions are different for each method.

To explain those rules, I'll need to talk about some basic atomic physics. Hydrogen-1's nucleus consists of only a single proton.

Then after another 5,000 years half of the remaining parent isotope will have decayed.

While people are most familiar with carbon dating, carbon dating is rarely applicable to fossils.

Studying the layers of rock or strata can also be useful. If a layer of rock containing the fossil is higher up in the sequence that another layer, you know that layer must be younger in age. This can often be complicated by the fact that geological forces can cause faulting and tilting of rocks.

Absolute dating is used to determine a precise age of a rock or fossil through radiometric dating methods.

Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils.