| These fossils are indexes of relative geologic age and may be termed index fossils. |
| Their distinctive scales are used as index fossils for the relative dating of rock layers. |
| Carboniferous and Permian strata often contain useful index fossils belonging to this group. |
| Induan marine strata are correlated worldwide by six biozones containing ammonoid cephalopod index fossils. |
| Thus, the Silurian-Devonian boundary is anchored to the first occurrence of specific index fossils. |
| Ostracods—small crustaceans radiated during the Jurassic and are used today as index fossils. |
