Small acritarchs from the Upper Cretaceous: taxonomy, biological affinities and palaeoecology

Abstract

A systematic study of small acritarchs (most commonly around 10 μm) from the Late Campanian (Duwi (Phosphate) Formation) and Maastrichtian (Dakhla Shale) of Egypt, based on scanning electron miscroscopy, resulted in the identification of nine genera and 27 species, among them the new genus Recticystis and 14 new species, namely Operculites bigranulatus, Bacillidinium partitum, Mecsekia acuta, M. curvispina, M. triangula, Recticystis hemispinosus, Clavaticystis septatus, C. furcatus, Odontothrix multiornatus, Filisphaeridium paucigranulosum, Cymatiosphaera parvirugosa, C. muralis, C. polyornata and C. triradiata. In addition, the following new combinations are proposed: Cymatiosphaera coronis (Habib and Knapp), Mecsekia minor (Takahashi) and Mecsekia ariakense (Takahashi). Small acritarchs are thought to include cysts of dinoflagellates because of significant morphological similarities (cyst shape, chasmic and tremic archeopyles, spinose and septate ornaments) to certain organic-walled and calcareous dinoflagellate cysts. Small size in acritarchs is regarded as an adaptation to a planktonic and possibly to an interstitial mode of life. Small acritarchs attain the highest diversity and abundance in the well agitated environment of the phosphoritic sandstones. In the pores between the sand grains they were protected from the wave action and currents which moved away most of the larger dinoflagellates. Dinoflagellates are common in the associated shales representing a more quiet marine environment. The interpretation of some small spinulose acritarchs as interstitial dinoflagellates may also be supported by the presence of spinose cysts with a chasmic archeopyle in the modern dinoflagellate Polykrikos which includes interstitial forms.