Changes in morphology within the biostratigraphically important Oligocene nannofossil lineage, Sphenolithus predistentus, S. distentus and S. ciperoensis were investigated in carbonate sediments from the palaeo-equatorial Pacific Ocean Site 1218 in order to determine the nature of this evolutionary lineage. Using differences in their morphology and stratigraphical ranges, the aim of this study was to determine whether these taxa represent an anagenetic evolutionary lineage or a set of discrete species with overlapping stratigraphical ranges. A total of 1215 specimens from 12 samples were analysed morphometrically and the basal ratio, i.e. the ratio between the basal width and proximal cycle height, was identified as a key parameter for the study of this lineage. We conclude that S. distentus and S. predistentus are intergradational species forming an anagenetic lineage but that S. ciperoensis is a discrete species which evolved relatively abruptly in the Late Oligocene.