One way that helps scientists place fossils into the correct era on the Geologic Time Scale is by using radiometric dating.Also called absolute dating, scientists use the decay of radioactive elements within the fossils or the rocks around the fossils to determine the age of the organism that was preserved.Using relative dating the fossil is compared to something for which an age is already known.For example if you have a fossil trilobite and it was found in the Wheeler Formation.Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.Index fossils are fossils that are known to only occur within a very specific age range.
It is an accurate way to date specific geologic events.
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.
There are many radiometric clocks and when applied to appropriate materials, the dating can be very accurate.
As one example, the first minerals to crystallize (condense) from the hot cloud of gasses that surrounded the Sun as it first became a star have been dated to 4568 plus or minus 2 million years....!! Other events on earth can be dated equally well given the right minerals.