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Index fossils are fossils that are known to only occur within a very specific age range.
Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils.
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.
This makes it ideal for dating much older rocks and fossils.
The simplest and most intuitive way of dating geological features is to look at the relationships between them.
It can only be used to date fossils younger than about 75,000 years.
Then after another 5,000 years half of the remaining parent isotope will have decayed.For example if you have a fossil trilobite and it was found in the Wheeler Formation.The Wheeler Formation has been previously dated to approximately 507 million year old, so we know the trilobite is also about 507 million years old.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.