Type material of calcareous nannofossil index species Coccolithus crassus and two geographically widespread species Toweius magnicrassus and T. callosus have been studied by both light and SEM microscopy and morphometric measurements were made. Coccolithus crassus resembles Coccolithus pelagicus but has a raised cycle of elements around the centre of the distal shield. It probably evolved from C. pelagicus. Both T. magnicrassus and T. callosus have three cycles of elements in distal view, which is a characteristic of Toweius. Toweius magnicrassus is larger than T. callosus. Differentiation of T. magnicrassus from T. callosus is possible and useful because there is generally a size gap between them in a given sample and they have different stratigraphic ranges. However, both T. callosus and T. magnicrassus appear to increase in size from high to low latitudes. Toweius callosus most probably evolved from Toweius pertusus in the latest Palaeocene and gave rise to T. magnicrassus in the early Eocene.
- Received October 1, 1992.
- Accepted February 1, 1993.
- © 1993 The Micropalaeontological Society