Van Niel, Brigitta E.; (1992) Early Cretaceous Nannoconus (Calcareous nannofossil, Incertae sedis) in NW Europe. Doctoral thesis (Ph.D), University of London.

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Abstract

Early Cretaceous nannofossil assemblages were studied, with particular emphasis on the calcareous nannolith genus Nannoconus. A total of 409 samples were observed from 16 Lower Cretaceous sections from NW Europe, consisting of 221 sidewall cores, 113 core chips and 75 ditch cutting samples. Coverage was from the Danish Central Graben to the Porcupine Basin, and from the Viking Graben to the Central Netherlands Basin. The problematic morphology of the genus Nannoconus was initially considered as a simple composite cone structure built from a spiral arrangement of plates surrounding an axial canal or cavity. New observations on the fine structure of Nannoconus have been made with the Scanning Electron Microscope. Two distinct cycles and corresponding crystal plate orientations are recognised in longitudinal sections of the nannoconid body. These characteristics can be correlated to the morphological interpretations on the ultrastructure by earlier workers as seen in the Light Microscope. These new interpretations allowed recognition of a total of 5 distinct morphogroups to which the 33 taxa recognised in this study could be assigned. A discussion of the early phylogeny of this genus in the Tethyan Realm is used as an introduction to a new evolutionary model. This model considers the evolution of the taxa within the morphogroup assignments proposed in this study. Samples from the Munk Marl showed interesting associations of nannoconid individuals in the form of double structures. A total of 3 species: N. abundans, N. circularis and N. truitti were observed to show double associations in perfect crystallographic continuity. The occurrence of this phenomena is discussed in relation to ecological controls. The nannoconid biostratigraphical distribution was correlated to the nannofossil zonation scheme, which provided a time-framework for the distribution of Nannoconus in the sections. A compendium of the worldwide occurrence of Nannoconus from published literature and results obtained in this study was used as a data base to determine controls on the occurrence patterns of this genus in NW Europe.

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