Index fossils are fossils that help keep a timeline in the fossil record. Their presence gives clues to the geologic time period of the sediment in which they are found, and can help date other fossils based on their proximity to the index fossils.
A good index fossil must be abundant, globally widespread, and durable. An index fossil is often an animal that went through a type of “boom and bust” life cycle – that is, conditions allowed for it to flourish during its time period, but vulnerabilities to environmental changes also quickly led to its demise and extinction. Because these types of fossils only appear for a short period on the pre-historic timeline, they act as markers which other discoveries can be compared against.
Most index fossils are marine organisms, as burial in ocean sediment is much more common than land burial. Ocean currents also helped distribute these creatures worldwide, while many terrestrial animals were bound to smaller geographic areas.
The best known index fossil is the ammonite, an ocean-dwelling cephalopod, somewhat like a shelled squid or octopus. The ammonite’s wide geographic distribution, rapid evolution, and distinctive features make it a model index fossil! Three groups of ammonoids succeeded one another, the goniatite in the Paleozoic Era, the ceratite in the Triassic Period, and the Cretaceous Period left a few surviving ammonoids which were identified by an interwoven pattern called the ammonite pattern.
Other well knows index fossils include graptolites, brachiopods, trilobites, and echinoids. Terrestrial fossils, such as the teeth of certain rodents, are also identified as index fossils.
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