Graptolites
Colonial drifters leave saw-blade fossils that become the finest clock in Paleozoic rock.
- When
- Known to
- ± 5 million years
- Era
- Ordovician, Paleozoic
Graptolites are colonial hemichordates whose chitinous skeletons, preserved as pencil-like marks on shale, evolved rapidly and drifted worldwide in the plankton. That combination makes them exceptional index fossils: Ordovician and Silurian rocks can be correlated between continents to within a few hundred thousand years using graptolite zones alone.