Index fossils are specific fossils that are used to define and identify particular time periods in Earth’s geologic history. These fossils are typically widespread geographically and have a short-lived existence, making them useful for dating and correlating the age of rocks and formations. Keep in mind that new discoveries and research may lead to updates in the list of index fossils. Here are some examples of index fossils associated with different geologic periods:
Cambrian Period (541-485 million years ago):
Trilobites (e.g., Olenellus, Paradoxides)
Ordovician Period (485-443 million years ago):
Graptolites
Brachiopods (e.g., Rafinesquina)
Trilobites (e.g., Isotelus)
Silurian Period (443-419 million years ago):
Graptolites
Brachiopods (e.g., Pentamerus)
Trilobites (e.g., Calymene)
Devonian Period (419-359 million years ago):
Brachiopods (e.g., Spirifer)
Trilobites (e.g., Phacops)
Fish (e.g., Dunkleosteus)
Carboniferous Period (359-299 million years ago):
Brachiopods (e.g., Productus)
Ammonoids
Fusulinids
Permian Period (299-252 million years ago):
Brachiopods (e.g., Dictyoclostus)
Ammonoids
Synapsids (early mammal-like reptiles)
Triassic Period (252-201 million years ago):
Ammonoids
Conodonts
Archosaurs (e.g., early dinosaurs)
Jurassic Period (201-145 million years ago):
Ammonoids
Dinosaurs (e.g., Allosaurus, Brachiosaurus)
Belemnites
Cretaceous Period (145-66 million years ago):
Ammonoids
Dinosaurs (e.g., Tyrannosaurus rex, Triceratops)
Foraminifera (e.g., Globotruncana)
Paleogene Period (66-23 million years ago):
Mammals (e.g., early primates)
Foraminifera (e.g., Nummulites)
Neogene Period (23-2.6 million years ago):
Mammals (e.g., mastodons, mammoths)
Foraminifera (e.g., Globigerina)
Quaternary Period (2.6 million years ago – Present):
Humans (Homo sapiens)
Pleistocene megafauna fossils (e.g., saber-toothed cats, mammoths)
It’s important to note that this list is not exhaustive, and the use of index fossils can vary depending on the region and specific geological context. Additionally, ongoing scientific research may lead to the discovery of new index fossils or updates to existing classifications.
