Which type of microfossils have tests (hard parts) made of calcium carbonate and are useful as index fossils as well as for measuring past sea temperatures and climates since the Cretaceous?

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Coccoliths and foraminifera have tests made of calcium carbonate, which makes them useful for studying past sea temperatures and climates. Coccoliths are plates of calcium carbonate produced by coccolithophores, a type of phytoplankton. Foraminifera are a group of amoeboid protists with calcareous tests. Both are significant in paleoclimatology and biostratigraphy since the Cretaceous period.

Which Type of Microfossils Have Tests (Hard Parts) Made of Calcium