The main drawback in the use of graptolites is the poor preservation in coarser grained rocks such as sandstones.
They show a wide variety of morphologies and appear to have evolved quite rapidly into taxa with distinct and recognisable characteristics. These exotic and somewhat enigmatic organisms are interpreted as being colonial groups of individuals connected by a skeletal structure.Ammonoids became extinct at the end of the Cretaceous.These also belong to the Mollusca and as marine ‘snails’ they are abundant as fossils in Cenozoic rocks.This phylum includes crinoids (sea lilies) and echinoids (sea urchins).Most crinoids probably lived attached to substrate and this sessile characteristic makes them rather poor zone fossils, despite their abundance in some Palaeozoic limestones.
Search for biostratigraphic dating:
This group is therefore used for zonation and correlation within these strata, despite the fact that they are not generally suitable for biostratigraphic purposes because of the very restricted depositional environments they occur in.