Before more precise absolute dating tools were possible, researchers used a variety of comparative approaches called relative dating.
A submethod within biostratigraphy is faunal association: Sometimes researchers can determine a rough age for a fossil based on established ages of other fauna from the same layer — especially microfauna, which evolve faster, creating shorter spans in the fossil record for each species.
When it comes to determining the age of stuff scientists dig out of the ground, whether fossil or artifact, “there are good dates and bad dates and ugly dates,” says paleoanthropologist John Shea of Stony Brook University.
The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results.
Paleomagnetism is often used as a rough check of results from another dating method.
Tephrochronology: Within hours or days of a volcanic eruption, tephra — fragments of rock and other material hurled into the atmosphere by the event — is deposited in a single layer with a unique geochemical fingerprint.