Lineages in early paleogene calcareous nannoplankton

Abstract

A detailed study of the ultrastructure of Paleocene-Middle Eocene calcareous nannofossils has led to the recognition of evolutionary lineages comprising 16 genera and 110 species. The lineages have been established on the basis of evidence from floras derived from closely sampled sections in Spain, Israel, Denmark and Sweden. Information about the ultrastructure of species, and the relations between species, was gained by investigating selected specimens in the Scanning Electron Microscope, and by studying others with a polarization microscope equipped with a gypsum plate. The latter microscope showed up the extinction- and colour patterns of the specimens. The principles and applicability of the latter technique are discussed. It is argued that statements about the evolution of calcareous nannoplankton are necessarily speculative due to the nature of the fossil material and the method of study. Attention was given to the floras in the Cretaceous/Tertiary boundary interval. In this paper it is proposed to define the lower boundary of the lowest nannofossil zone of the Tertiary by the mass occurrence of Braarudosphaera bigelowii and/or Thoracosphaera operculata. With a few possible exceptions, efforts to detect the Cretaceous ancestors of Tertiary taxa were fruitless. The Standard Zonation for the Tertiary was applied for the biostratigraphic subdivision of the investigated sequences. The established lineages added to the refinement of this zonation, as most of the zonal markers belong to the lineages. Additional and, in some cases, new information is given about the ultrastructure and optical behaviour of all taxa encountered. Several new descriptive terms are introduced and some new combinations are made. Nine new species are described: Biscutum parvulum, Braarudosphaera alta, Cruciplacolithus edwardsii, Cruciplacolithus latipons, Fasciculithus bitectus, Fasciculithus magnicordis, Rhomboaster intermedia, Rhomboaster bitrifida and Zygodiscus clausus.