In this paper we critically examine the biostratigraphy and systematics of late Albian-Coniacian biserial planktic foraminifera using measurements and qualitative observations from SEM and X-radiograph images of exquisitely preserved specimens from hemipelagic sediments of southeast Tanzania, augmented by analogous observations of selected Ocean Drilling Program samples and holotypes and paratypes of nearly all species. We document interspecific differences and intraspecific variability, and propose a revised phylogeny for Cretaceous biserial planktic foraminifera placed within a refined biostratigraphic framework. Taxonomic emendations are proposed for the genera Protoheterohelix Georgescu & Huber, 2009, and Planoheterohelix Georgescu & Huber, 2009, and for the species Pr. washitensis (Tappan, 1940), Pi.. obscura Georgescu & Huber, 2009, Pl. moremani (Cushman, 1938), Pl. reussi (Cushman, 1938), and Laeviheterohellx remformis (Marie, 1941). In addition, a neotype is designated for Pl. globulosa (Ehrenberg, 1840), and Pl. praenuttalli n. sp. is described. Biserial speciation events are recognized in the planktic foraminiferal Rotalipora cushmani Zone (latest Cenomanian), Helvetoglobotruncana helvetica Zone (early Turonian), and at the Helvetoglobotruncana helvetica-Marginotruncana schneegansi zonal boundary (middle Turonian).
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