Graptolites
Graptolites are normally found in dark mudstones and shales, and have a shiny look to them, as though they had been drawn onto the rock with a pencil. This is how they get their name, which means 'writing on the rock'.
Like corals they were colonial - each graptolite was made up of many tiny individual animals, all linked together into a single colony. Unlike corals though, most graptolite colonies were not attached to the sea floor, but floated near the surface of the seas, feeding on tiny pieces of food in the water.
Graptolites died out about 370 million years ago. They first appeared about 490 million years ago and quickly evolved into many new forms.
Experts can use graptolite fossils from a rock to tell how old it is, just by looking to see which types are there. This makes graptolites an important tool for geologists.
