| 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. |
| They are very abundant in some Paleozoic formations and include useful index fossils. |
| They make excellent index fossils because they underwent rapid evolution and attained a broad distribution. |
| The spire-bearing spiriferoids were perhaps the most common and have been used as index fossils. |
| Anisian marine strata are correlated worldwide by seven biozones containing ammonoid cephalopod index fossils. |
