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The guide fossils are used for the relative dating of rocks.

It is the remains of organisms, which fulfil specific requirements:

  • a wide geographical distribution;
  • a relative abundance of large populations and therefore readily found in sedimentary rocks in the period of their existence;
  • evolved rapidly: thus have a very limited time duration and allow to achieve high accuracy in dating.

The best-known guide fossils are:

  • For the Palaeozoic (542 million to 251 million years ago): Trilobites, Graptolites, primitive Ammonites (Ammonoidea) and microfossils (Conodonts and Fusulinid).
  • For the Mesozoic (251 – 65.5 million years ago): Ammonites (Ceratitiidae and Ammonitida, such Arietites) and other microscopic fossils (eg. benthic and planktonic Foraminifera for dating within the Cretaceous, Tintinnids).
  • For the Cenozoic (65.5 million years ago – until now): microfossils, especially benthic and planktonic Foraminifera (Nummulites) and limestone nanofossils.


Photography: Drotops sp., Trilobite of the medium Devonian.

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