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set: fossils // series: chalk // picture: COCCOLITHSCoccoliths continued.
The planktonic organisms which bore these coccoliths belong to a group of plant protists (photosynthetic, single-celled organisms) termed calcareous nannoplankton which first appeared in the Late Triassic. The living relatives can occur at up to 60 million cells per cubic meter of water and ancient equivalents are recorded at up to 1 billion - 1 trillion coccoliths per cubic centimetre in marine sediments. Apart from forming the very fine-grained limestone known as chalk, these plankton showed rapid evolution and their platelets are amongst the most important microfossils for relative dating of Mesozoic and Cainozoic rocks. Their occurrences in latitudinally controlled assemblages also make them very useful in palaeobiogeography and palaeooceanography. These and other microfossils are studied in specialist 4th year courses in Applied Palaeontology whilst sediment forming organisms are studied in their sedimentological context in 1st year work.