graptolite

Graptolites (pronounced grapt-oh-lites) were colonial marine animals that flourished during the Paleozoic Era, living from roughly 510 to 320 million years ago. They were not single animals in the usual sense. Instead, each one was a colony made up of many tiny individuals called zooids, joined together in a common structure. The part that usually fossilized was the colony's organic skeleton, which often appears as a dark, branching, or saw-toothed mark in rock.

Many kinds drifted in ancient oceans as plankton, while some early forms were attached to the sea floor. They came in various shapes, from simple lines to more complex branching forms, sometimes looking a little like a saw blade or a fine leaf pattern.

While once abundant, graptolites eventually died out during the later Paleozoic. Today, they are very valuable to paleontologists and geologists. Because they were widespread and changed relatively quickly over time, their fossils are important index fossils. That means scientists use them to date rock layers and match geological formations in different places. Studying them also gives clues about ancient ocean conditions and the history of life on Earth.

So, while you will not find one living in today's oceans, their fossils still tell rich stories about Earth's deep past.