Neogene calcareous nannofossils from the Rio Grande rise and vema seachannel (DSDP-Leg 72, sites 516a and 518-South Atlantic Ocean): Systematics of the genus Discoaster

Abstract

Discoasters are calcareous nanofossils with wide distribution and speciation in the Cenozoic, presenting star or rosette shapes. In the Neogene, this genus guides the detection of several biozones, through its emergences and extinctions. However, as their rays are composed of tabular calcites, they are more susceptible to overgrowth and recrystalli-zation than cocoliths, whereas many individuals, morphologically different, are encompassed in the same species, and presented as preservational variants. During the investigation of Miocene to Pliocene strata in the Rio Grande Rise and Vema Seachannel, we faced with a representative amount of these preservational variants, which are discussed in this paper. Twenty-three taxa were described and eight variants were highlighted, in order to present new observations and discussions, aiming at possible biostratigraphic applications in future investigations.