| Ostracods—small crustaceans radiated during the Jurassic and are used today as index fossils. |
| They are very abundant in some Paleozoic formations and include useful index fossils. |
| The spire-bearing spiriferoids were perhaps the most common and have been used as index fossils. |
| These fossils are indexes of relative geologic age and may be termed index fossils. |
| Thus, the Silurian-Devonian boundary is anchored to the first occurrence of specific index fossils. |
| These rocks are interbedded in Emsian marine sedimentary successions that are well documented by index fossils. |
