microfossil

/ˈmaɪkroʊˌfɑsəl/
noun
  1. A very small fossil, often of a tiny organism, that can only be seen with a microscope.
    • Paleontologists use microfossils to learn about ancient climates.
    • This rock sample contains microfossils of ancient plankton.
    • By studying microfossils, scientists can date layers of sediment.