Arguably the most important new species in the Ordovician period were the Graptolites. Graptolites are microscopic individuals grouped together in colonies. They have been found in wide spread regions all over the world. Their broad distribution is believed to indicate that they were floating colonies. Because they evolved very briskly and spread worldwide so quickly, and then became extinct just as fast, their fossilized appearance is a great indicator of the age of a rock.

Fossilized Graptolytes