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