How do extinctions help geologists establish the age of something?
Extinctions help geologist to establish relative ages for geological strata
Extinctions supply geologist with index fossils. A fossil of an organism believed to have gone extinct at a point in time provides clues about the age of the strata where the fossil is found.
For example all dinosaurs are believed to have become extinct about 65 million years ago ( perhaps by a catastrophic event). If a fossil of dinosaur is found, it dates the strata as being older than 65 million years.
Some index fossils that were believed to be extinct have been found alive even to the present. Still the theory of descent with modification and the proposed ages of extinctions are used by geologists as the primary means for dating strata containing fossils.