Advances in Scottish graptolite biostratigraphy: an introduction

Graptolites were colonial organisms that first appeared in the Cambrian. In early Ordovician times they migrated from the benthos into the plankton, becoming some of the most significant zooplankton of the Lower Palaeozoic. Planktonic graptolites radiated quickly, reached an acme of variation in the late Ordovician and Silurian, then declined into extinction by the mid-Devonian. The benthic graptolites followed them to extinction in the Carboniferous