The Lower Ordovician Cephalopod Faunas of the Durness Group, North-West Scotland

ABSTRACT The cephalopod assemblages of the Lower Ordovician portion of the Durness Group of North-West Scotland are described and referred to 46 taxa across six orders and 15 families. A total of 30 genera are described, of which Expandocassinoceras and Augeoceras are new. Thirty-four species are described, although only 13 are named; of these, Pionoceras peachi, Allopiloceras bealachense and Allopiloceras lonachonense are new. A further nine taxa are treated in open nomenclature at the order or family level, and another three are regarded as indeterminate at the present time but are included here for completeness. The older cephalopod assemblages of the Durness Group (Sailmhor and Sangomore formations) are relatively sparse, but dominated by ellesmerocerids and bassleroceratids. By contrast, the younger-assemblages (in particular those of the Croisaphuill and Ben Suardal Limestone formations) are relatively abundant and dominated by endocerids, tarphycerids and orthoceratoids. The cephalopod assemblage appears to be strongly affiliated to those of western Newfoundland, Quebec, Ontario and New York State. Members of the Orthoceratoidea occur in the Durness Group at horizons somewhat older than those previously recorded from the south-western United States and are similar in age to occurrences in Avalonia (England and Wales). This suggests that whatever the earlier history of the Orthoceratoidea, a rapid diversification and dispersal appears to have taken place during the early Floian.