Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
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.
But, how can we determine how old a rock formation is, if it hasn’t previously been dated?
For groundwater, this means that C is a widely used tool to establish chronologies for groundwater flow systems and climate records for the Holocene and Pleistocene.
It is considered to be the most important tool for age dating of ‘old’ groundwater.
Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of 415 to 420 million years.
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.
After one half life, it would have had 1/2 the carbon.