Index fossils facts for kids

Examples of index fossils

Index fossils (also known as guide fossils, indicator fossils or zone fossils) are fossils used to define and identify geologic periods (or faunal stages). They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of the fossil. If the species concerned were short-lived (in geological terms, lasting a few hundred thousand years), then it is certain that the sediments in question were deposited within that narrow time period. The shorter the lifespan of a species, the more precisely different sediments can be correlated, and so rapidly evolving types of fossils are particularly valuable. The best index fossils are common, easy-to-identify at species level, and have a broad distribution—otherwise the likelihood of finding and recognizing one in the two sediments is minor.

Ammonites fit these demands well, and are the best-known fossils that have been widely used for this. Other important groups that provide index fossils are the corals, graptolites, brachiopods, trilobites, and echinoids (sea urchins). Conodonts may be identified by experts using light microscopy such that they can be used to index a given sample with good resolution. Fossilized teeth of mammals have also been used.

Geologists use both large fossils (called macrofossils) and microscopic fossils (called microfossils) for this process, known as biostratigraphy. Macrofossils have the advantage of being easy to see in the field, but they are rarer, and microfossils are very commonly used by oil prospectors and other industries interested in mineral resources when accurate knowledge of the age of the rocks being looked at is needed.

The series of deposits that spans the occurrence of a particular index fossil, is often referred to as that fossil's zone, enabling to relate different faunas through time. An example would be to say that Mesolenellus hyperborea occurs in the late Nevadella-zone.

List of common index fossils

Fossil	Scientific Name	Time Period	Million Years Ago
Calico Scallop	Pecten gibbus Argopecten gibbus	Quaternary Period	1.8 million years ago
	Neptunea tabulata	Quaternary Period	1.8 million years ago
	Viviparus glacialis	Tiglian (Early Pleistocene)	0.5 million years ago
	Calyptraphorus velatus	Tertiary Period
	Venericardia planicosta	Eocene
Scaphites	Scaphites hippocrepis	Cretaceous Period	145 to 66 million years ago
Inoceramus	Inoceramus labiatus	Cretaceous Period
Perisphinctes	Perisphinctes tiziani	Jurassic Period
	Nerinea trinodosa	Jurassic Period
	Tropites subbullatus	Triassic Period
	Monotis subcircularis	Triassic Period
Leptodus	Leptodus americanus	Permian Period
Parafusulina	Parafusulina bosei	Permian Period
	Dictyoclostus americanus	Pennsylvanian Period
	Lophophyllidium proliferum	Pennsylvanian Period
	Cactocrinus multibrachiatus	Mississippian Period
	Prolecanites gurleyi	Mississippian Period
Mucrospirifer	Mucrospirifer mucronatus	Devonian Period	416 to 359 million years ago
	Palmatolepis unicornis	Devonian Period
		Silurian Period
	Tetragraptus fructicosus	Ordovician Period
	Paradoxides	Cambrian Period	509 to 500 million years ago
	Billingselia corrugata	Cambrian Period
	Archeocyathids	Lower Cambrian	529 to 509 million years ago