graptolite Definition

graptolite
noun

1. Any of a group of extinct marine animals that were common in the Palaeozoic era, thought to be related to present-day jellyfish and corals.
2. Any of the fossilized skeletons of these animals, often used to date rocks of the Ordovician and Silurian periods.

Etymology: 19c: from Greek graptos written, because on hard shale the fossils look like writing.