Index fossils (zone fossils) are the most reliable biostratigraphic tools because they: (1) belong to species with wide geographic distribution (good correlation potential), (2) have short stratigraphic ranges (precise time control), (3) are abundant and easily preserved, and (4) have well-understood taxonomy. Classic examples: graptolites (Ordovician–Silurian), ammonites (Jurassic–Cretaceous), foraminifera (Cretaceous–Cenozoic). Derived fossils are reworked and give misleading ages.