Evolution in Monograptus priodon

Biometric analysis of the Silurian graptolite Monograptus priodon (Bronn) from Arctic Canada and the British Isles shows that the length of the sicula and the rhabdosome width at the level of theca 5 undergo a progressive increase from latest Llandoverian to late Wenlockian time. This change is attributed to sympatric evolution within the species. Monograptus flemingii (Salter) may have evolved elsewhere from priodon stock and migrated later into the two regions. Monograptus riccartonensis Lapworth probably ranges from latest Llandoverian through the Wenlockian, and may have been ancestral to the Ludlovian to Devonian Monograptus lineages.