Index fossils and relative dating

16-Aug-2019 03:28 by 9 Comments

Index fossils and relative dating

Organisms that lived for relatively short time periods are particularly useful for dating rocks, especially if they were distributed over a wide geographic area and so can be used to compare rocks from different regions. There is no specific limit on how short the time span has to be to qualify as an index fossil.

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By the late Carboniferous, trees had evolved from earlier plants, and reptiles had evolved from amphibians.

The oldest well-understood fossils are from rocks dating back to around 600 Ma, and the sedimentary record from that time forward is rich in fossil remains that provide a detailed record of the history of life.

However, as anyone who has gone hunting for fossils knows, that does not mean that all sedimentary rocks have visible fossils or that they are easy to find.

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Although the recognition of fossils goes back hundreds of years, the systematic cataloguing and assignment of relative ages to different organisms from the distant past — paleontology — only dates back to the earliest part of the 19th century.

The oldest undisputed fossils are from rocks dated around 3.5 Ga, and although fossils this old are typically poorly preserved and are not useful for dating rocks, they can still provide important information about conditions at the time.

In this diagram, the coloured bar represents the time range during which each of the four species (A – D) existed on Earth.

Although each species lived for several million years, we can narrow down the likely age of the rock to just 0.7 Ma during which all four species coexisted.

The Phanerozoic has seen five major extinctions, as indicated in Figure 8.3.1.

The most significant of these was at the end of the Permian, which saw the extinction of over 80% of all species and over 90% of all marine species.

Fossils alone cannot provide us with numerical ages of rocks, but over the past century geologists have acquired enough isotopic dates from rocks associated with fossil-bearing rocks (such as igneous dykes cutting through sedimentary layers) to be able to put specific time limits on most fossils.