Echinoderm microfossil dating
Today, due to the speed of preparation, calcareous nannofossils have bec ome the preferred tool for quick accurate stratigraphic age determination in post-Palaeozoic calcareous sequences.
There are two forms of coccoliths, the holococcoliths which are formed from calcite crystals which are essentially identical in shape and size and the heterococcoliths which are formed from larger calcite crystals which vary in size and shape.Most living forms are known to produce only heterococcoliths and then only during the non-motile stage of their life cycle.Those that do produce holococcoliths do so only during their motile stage.Today coccolithophores are one of the most important forms of phytoplankton found in the oceans, and may be described as the grass of the sea.The classification of calcareous nannoplankton is carried out under the International Code of Botanical Nomenclature.They are formally classified in the Kingdom Protoctista, Phylum (or Division) Haptophyta, Class Prymnesiophyceae.
Classification is complicated by the fact that some species are dimorphic, that is they possess more than one coccolith on a single coccosphere.
It was not untill the second half of the nineteenth century when Wallich found coccoliths joined to form coccospheres that an organic origin was suggested.
Even after the publication of Sorby's 1861 paper, following which the organic origin of coccoliths was generally accepted, Ehrenberg remained unconvinced.
The organism which creates the coccosphere is called a coccolithophore, they are phytoplankton (autotrophs that contain chloroplasts and photosynthesise).
Their calcareous skeletons are found in marine deposits often in vast numbers, sometimes making up the major component of a particular rock, such as the chalk of England. Formally coccolithophores are separated from other phytoplankton such as diatoms by the presence of a third flagella-like appendage called a haptonema, although the flagella bearing stage is often only one of a multi-stage life cycle.
The locations from which the earliest nannofossils are found include; the Northern and Southern Calcareous Alps, Timor, North-West Australia and Queen Charlotte Islands (Canada), all low latitude sites at the time.