The evolution of graptolites is the basis upon which much of the Lower Palaeozoic is subdivided. Amongst the clearest evolutionary lineages within graptolites are those that show seemingly unidirectional trends in morphology, such as the sequence of stratigraphically successive taxa of Petalolithus and Cephalograptus, discussed here. Graptolite specimens collected in the 1960s from Coalpit Bay, Northern Ireland are consistent with the succession established in the Czech Republic by Bouček & Přibyl (1941) and Štorch (1998), and include the first UK records of P. aff. krizi. We recognise other intermediate forms within this lineage that may enable further subdivision of the convolutus Biozone.

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