Morphological variability of <i>Neogloboquadrina pachyderma</i> (Ehrenberg) in the Southeast Atlantic Ocean: Insights from ODP Hole 1085A

Neogloboquadrina pachyderma (Ehrenberg) is one of the few planktic foraminiferal species that thrive in polar waters and serves as an important palaeoceanographic indicator of polar water mass expansion. The species evolved from Neogloboquadrina continuosa during the late Miocene and has undergone several taxonomic revisions—initially described as Aristerospira and later reassigned to Neogloboquadrina . Traditionally, the ratio of its two coiling types (sinistral and dextral) has been used to distinguish glacial and interglacial periods. However, genetic studies have revealed that these two coiling forms actually represent distinct species. As a result, the dextral form was reclassified as Neogloboquadrina incompta . Despite this taxonomic change, earlier palaeoceanographic interpretations based on coiling ratios remain valid. The morphology of N. pachyderma is highly variable, influenced by water mass characteristics and climatic conditions. Variations are observed in aperture shape, chamber arrangement, chamber packing, and other phenotypic traits. In the present study, we identify and describe eight distinct morphotypes of N. pachyderma from the Southeast Atlantic Ocean, based on detailed morphometric measurements and surface ultrastructure analyses.