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When the world-girdling ice came at the end of the Ordovician period roughly 440 million years ago, only a few species of graptolite survived the mass extinction.

There is a golden spike hiding in a rock face outside the village of Moffat in Scotland that marks the end of the Ordovician, denoted by the appearance of these graptolite survivors—Akidograptus ascensus and Parakidograptus acuminatus, to give them their scientific names.

Many Graptolite zones, showing a constant uniformity of succession, paralleled in this respect only by the longer known Ammonite zones of the Jurassic, have been distinguished in Britain and northern Europe, each marked by a characteristic species.

The Graptolite polyparies vary considerably in size: the majority range from 1 in. to about 6 in. in length; few examples have been met with having a length or more than 30 in.

These consist of greywackes, flags and shales with bands of dark graptolite shales, the finer sediments being often well cleaved.