Every fossil tells us something about the age of the rock in which it is found. Index fossils, however, are the ones that tell us the most. Index fossils are also called key fossils or type fossils. They are used to define periods of geologic time.
Characteristics Of An Index Fossil
A good index fossil has four key characteristics: it is distinctive, widespread, abundant, and limited in geologic time. Because most fossil-bearing rocks formed in the ocean, the major index fossils are of marine organisms. However, certain terrestrial organisms are also useful as fossils in young rocks and specific regions.
Boom-And-Bust Organisms
Any type of organism – any plant or animal or single-cell life form – can be distinctive, but not many are widespread in the fossil record. Many important index fossils are of organisms that started life as floating eggs and infant stages. This allowed them to spread around the world by riding ocean currents. The most successful of these became abundant. In doing so, however, they also became the most sensitive to environmental change and extinction. Thus, their time on Earth might have been limited to just a short period. That boom-and-bust population cycle makes for the best index fossils.
Trilobites, Hard-Shelled Invertebrates
Trilobites are a very good index fossil for Paleozoic rocks. They lived in all parts of the ocean. Trilobites were a class of animal, just like mammals or reptiles. Trilobites were constantly evolving new species during their existence, which lasted 270 million years from Middle Cambrian time to the end of the Permian Period, or almost the entire length of the Paleozoic. Because they were mobile animals, they tended to inhabit large areas. They had a global range. They were also hard-shelled invertebrates, so they fossilized easily. In addition, these fossils are large enough for scientists to study without a microscope.
There are several other index fossils of this type of marine organism. They include ammonites, the extinct relatives of today's nautilus; ancient crinoids, relatives to today's feather stars; rugose corals and coral-like bryozoans; and certain shellfish like brachiopods and mollusks.
Small Or Microscopic Fossils
Other major index fossils are small or microscopic. They come from organisms that are part of the floating plankton in the ancient world ocean. These are useful because of their small size. They can be found even in small bits of rock. And because their tiny bodies rained down all over the ocean, they can be found in all kinds of rocks. Therefore, the petroleum industry has made great use of index microfossils in its hunt for oil. Oil reserves formed during very specific periods in geologic history. Geologic time is broken down in quite fine detail based on these floating graptolites, fusulinids, diatoms and radiolarians.
The rocks on the ocean floor are geologically young. This is because they are constantly subducted and recycled into the Earth's mantle. Thus, marine index fossils older than about 200 million years are normally found in sedimentary strata on land. These are areas that were once covered by seas.
Terrestrial Rocks
Terrestrial rocks are rocks that formed on land. These can have regional or continental index fossils. These might include small animals that evolved quickly, like rodents. Continental index fossils can include larger animals that have wide geographic ranges. These form the basis of provincial time divisions.
Defining Ages, Epochs, Periods And Eras
Index fossils are used in the formal architecture of geologic time. They provide us with the boundaries for organizing geologic time periods. These fossils define the ages, epochs, periods and eras of the geologic time scale. Some of these subdivisions are defined by mass extinction events, like the Permian-Triassic extinction. The evidence for these events is found in the fossil record wherever there is a disappearance of major groups of species within a geologically short amount of time.
Other, related fossil types include the characteristic fossil. That is a fossil that belongs to a period but doesn't define it. Another is the guide fossil, one that helps narrow down a time range rather than nail it down.
can you answer this question using this article
What do patterns in the fossil record teach us about changes on Earth? Make a claim. Support your claim with evidence from the article. Then, explain why the evidence supports your claim.
