Other forms: microfossils
A microfossil is a trace left in rock by a plant or animal from a past geological age — a trace so tiny that it must be studied with a microscope, an instrument that enlarges the image of anything placed under it.
The trace that is left behind could be an actual part of the plant or animal that has turned to rock, or just an impression left by it. Either of these is called a fossil, and a microfossil is a very tiny one. Micro- is a combining form meaning "small," and it appears in quite a few words. A microscope, for example, is a scientific instrument for looking at very small things, and microbiology is a branch of biology dealing with very small life forms.