Radiogenic dating definition
That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age.
Some of the most useful fossils for dating purposes are very small ones.
This matching process is called correlation, which has been an important process in constructing geological timescales.
Some fossils, called index fossils, are particularly useful in correlating rocks.
For example, microscopic dinoflagellates have been studied and dated in great detail around the world.
Correlation with them has helped geologists date many New Zealand rocks, including those containing dinosaurs.
The 235U–207Pb cascade has a half-life of 704 million years and the 238U–206Pb cascade is considerably slower, with a half-life of 4.47 billion years.So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero.