Graptolites (grap'-toe-lites) were a very simple kind of marine animal that appeared in the Cambrian Period. They became abundant in Ordovician and Silurian times but gradually died out. The last ones lived during the Mississippian Period.
The animals lived in tiny chitinous cups arranged along slender stems. In some forms, the stem was attached to a round float, and, in others, two, three, or four stems were attached together. Most graptolites floated free in the oceans.
As fossils, they look like little black lines with sawtooth edges. They are found mainly in shales but also occur in limestones. In Illinois, they are most common in the Ordovician rocks of the northern part of the state.