1. Radiometric Dating: Fossils can be dated using radiometric techniques, providing an absolute age for the rock layer. Radioactive isotopes, such as carbon-14 (for younger fossils) and potassium-argon (for older fossils), decay at known rates. By measuring the abundance of these isotopes and comparing them to standards, scientists can determine the age of the fossil.

2. Uranium-Lead Dating: This method is highly precise and is often used for fossils that are hundreds of millions to billions of years old. It is based on the decay of radioactive uranium isotopes to stable lead isotopes.

Relative Age Determination:

1. Stratigraphy: The principle of superposition states that in undisturbed rock layers, the oldest layers are at the bottom, while younger layers are on top. Fossils found at lower levels in a rock sequence are generally older than those higher up.

2. Index Fossils: Certain fossils are found only within a specific time range and are widely distributed. These index fossils help determine the relative age of rock layers containing them.

3. Biostratigraphy: By identifying and comparing the fossil assemblages in different rock layers, scientists can establish the relative sequence and correlation of sedimentary rock layers. Each fossil assemblage characteristic of a particular time period in Earth's history.