Epochs like Pliocene and Miocene are compared to sedimentary exposures in Europe. Just as we ultimately set our watches to the time in Greenwich, England, paleontologists date their rocks by comparing them to these European standards, or "type" localities. These type localities for the epochs are marine; they contain abundant seashells and other fossil animals from ancient oceans.

It is difficult, but not impossible, to determine relative ages of land mammal sites compared to those with marine fossils. At various places in the world, marine fossils will cover or mix with fossil land mammals or vice versa. Marine sediments intermingled with land mammal layers permit paleontologists to establish relative ages for these sediments based on "superposition." For example, a horse fossil is older than the seashell lying over it, but younger than the shells below.

It is also possible to use modern radioactive decay methods to determine the actual age of a particular fossil, not just that it was older or younger than some English clam.