Certain sea shells are examples of index fossils.
Index fossils are the remains of an organism that lived in a particular time period. Index fossils help to date other fossils that are in a similar sedimentary layer. Because index fossils are categorized according to a limited time span, a fossil of an unknown time period may be determined in comparison to the index fossil. Types of index fossils are classified into three different eras.
Types of index fossils found as representative of the Cenozoic Era are considered recent fossils. This era of fossils includes fossils found in the Quaternary period, such as pecten gibbus and neptunea tabulata, and the Tertiary period, calyptraphorus velatus and venericardia planicosta. Pecten gibbus are index fossils found in the Cenozoic Era in the Quaternary period, 1.8 million years ago. Pecten gibbus fossils are similar to sea shells we might find today.
Index fossils found in the Mesozoic Era, the age of medieval life, include the Cretaceous period (scaphites hippocrepia and inoceramus labiatus), the Jurassic period (perisphinctes tiziani and nerinea trinodosa) and the Triassic period (trophites subbullatus and monotis subcircularis). An example of an index fossil found during the Mesozoic Era is the ammonite fossil, found from 65 to 245 million years ago.
The Paleozoic Era, or ancient life, index fossils include fossils from the Permian period (leptodus americanus and parafusulina bosei), the Pennsylvanian period (dictyoclostus americanus and lophophyllidium proliferum), the Mississipian period (cactocrinus multibrachiatus and prolecanites gurleyi), the Devonian period (Mucrospirifer mucronatus and prolecanites gurleyi), the Silurian period (cystiphyllum niagarense and hexamoceras hertzeri), the Ordovician period (bathyurus extans and tetragraptus fructicosus), and the Cambrian period (paradoxides pinus and billingsella corrugata). Trilobites, found during the Paleozoic Era of 245 to 540 million years ago, make up half of the Paleozoic fossils. These index fossils are marine invertebrates with hard exoskeletons belonging to Kingdom Animalia. Trilobites went extinct approximately 250 million years ago during the Permian period.