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A fossil, as of a pollen grain or unicellular organism, that is too small to study with the naked eye.
(Palaeontology) a fossil generally less than 0.5 millimetre in size, such as a protozoan, bacterium, or pollen grain
mi•cro•fos•sil(ˌmaɪ kroʊˈfɒs ɪl)
a fossil so small that it can be studied and identified only with a microscope.