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Biostratigraphy is the science of dating rocks by using the fossils contained within them. Usually the aim is correlation, that is, demonstrating that a particular horizon in one geological section represents the same period of time as another horizon at some other section. The fossils are useful because sediments of the same age can look completely different because of local variations in the sedimentary environment. For example, one section might have be made up of clays and marls while another has more chalky limestones, but if the fossil species recorded are similar, the two sediments are likely to have been laid down at the same time. Ammonites, graptolites and trilobites are index fossils that are widely used in biostraigraphy. Microfossils such as acritarchs, chitinozoans, conodonts dinoflagellate cysts, pollenSEM image of pollen grains from a variety of common plants: sunflower Helianthus annuus , morning glory Ipomea purpurea ,hollyhock Sildalcea malviflora , lily Lilium auratum , primrose Oenothera fruticosa , and castor bean Ricinus communis . Pollen is a f, spores and foraminiferaAllogromiida Carterinida Fusulinida extinct Globigerinida Involutinida Lagenida Miliolida Robertinida Rotaliida Silicolocunida Spirillinida Textulariida The Foraminifera are a large group of amoeboid protists with reticulating pseudopods, fine strands thans are also frequently used. Different fossils work well for sediments of different ages; trilobites, for example, are particularly useful for sediments of CambrianThe Cambrian is a major division of the geologic timescale that begins about 542 million years before the present (BP) at the end of the Proterozoic eon and ended about 490 million years BP with the beginning of the Ordovician period. It is the first peri age. To work well, the fossils used must be widespread geographically, so that they can occur in many different places. They must also be short lived as a species, so that the period of time during which they could be incorporated in the sediment is relatively narrow. The longer lived the species, the less accurate the correlation, and so fossils that evolve rapidly, such as ammonites, are favoured over forms that evolve much more slowly, like nautiloidNautilida Bactrida Nautiloids are a group of marine animals which all possess an external shell, the most well known example being the modern nautiluses. They first appeared in the Cambrian period, some 570 million years ago, and flourished in the ancients.
PaleontologyPaleontology (palaeontology is the British spelling) is the study of the developing history of life on earth, of ancient plants and animals based on the fossil record, evidence of their existence preserved in rocks. This includes the study of body fossils
StratigraphyStratigraphy a branch of geology, is basically the study of rock layers and layering. It is primarily used in the study of sedimentary and layered volcanic rocks. The subject was essentially invented and first rigorously applied by William Strata Smith in
Fossils
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