Index fossils (also known as guide fossils or indicator fossils) are fossils used to define and identify geologic periods (or faunal stages). Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends. Another term, "zone fossil", is used when the fossil has all the characters stated above except wide geographical distribution; thus, they correlate the surrounding rock to a biozone rather than a specific time period.
| Fossil | Scientific Name | Geological time interval | Million Years Ago | |
|---|---|---|---|---|
Calico scallop | Argopecten gibbus | |||
| Neptunea tabulata | ||||
| Viviparus glacialis | Tiglian (Early Pleistocene) | |||
| Calyptraphorus velatus | ||||
| Venericardia planicosta | ||||
Scaphites | Scaphites hippocrepis | |||
Inoceramus | Inoceramus labiatus | |||
Perisphinctes | Perisphinctes tiziani | 201.3 to 145 million years ago | ||
| Nerinea trinodosa | ||||
| Tropites subbullatus | ||||
| Monotis subcircularis | ||||
Leptodus | Leptodus americanus | |||
| Parafusulina | Parafusulina bosei | |||
| Dictyoclostus americanus | ||||
| Lophophyllidium proliferum | ||||
| Cactocrinus multibrachiatus | ||||
| Prolecanites gurleyi | ||||
Mucrospirifer | Mucrospirifer mucronatus | |||
| Palmatolepis unicornis | ||||
| Tetragraptus fructicosus | ||||
| Paradoxides | ||||
| Billingsella corrugata | ||||