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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.