Fusulinids are one-celled protozoans which evolved very early in geologic history and then stretched out all the way from the cambrian to the the tertiary, when they were finally extinguished forever. They were very plentiful in their time, and their species are found all over the world.

The sizes of fusulinds ranged from a fraction of a millimeter to 60 millimeters or more, yet they were each only one cell. They seemed to congregate in large groups in fresh water, not far from marine environments. This predisposition to congregation in large groups leaves us fossil markers that help us determine the time of the layer in which particular species of fusilinids are found.

Different species of fusilinids were abundant at different geological times and so they make excellent time markers for certain stratigrigraphic zones.