Published online by Cambridge University Press: 20 May 2016
Coccolithus crassus was described by Bramlette and Sullivan (1961) from the Lodo Formation of California. This species has a short stratigraphic range within the lower Eocene and its first occurrence is a marker for the CP10/CP11 zonal boundary in the widely used nannofossil zonation of Okada and Bukry (1980). Most nannofossil workers, however, do not report this index fossil, and the CP10/CP11 zonal boundary is often not determined. This is in contrast to the work of D. Bukry, who used the first occurrence of C. crassus for the CP10/CP11 boundary in virtually all lower Eocene DSDP cores that he examined (Figure 1). Apparently, there is considerable confusion about C. crassus. For instance, Romein (1979) considered C. crassus a junior synonym of Coccolithus eopelagicus (Bramlette and Riedel) Bramlette and Sullivan; Steinmetz and Stradner (1984, p. 741, Pl. 42, figs. 1 and 2) figured a specimen of Ericsonia subpertusa Hay and Mohler from Zone CP7 (upper Paleocene) as C. crassus; Perch-Nielsen (1985, p. 433, figs. 3.46 and 3.47, p. 504, figs. 58.12 and 58.13) mistook Toweius callosus Perch-Nielsen as C. crassus, as did Varol (1989, p. 302, Pl. 4, figs. 37 and 38). The main problem that prevents most workers from using this index fossil is that the species concept is largely unclear. It appears to be difficult to distinguish C. crassus from C. pelagicus or T. callosus based on the light micrographs of Bramlette and Sullivan (1961, Pl. 1, figs. 4a and 4c), and scanning electron microscope (SEM) photographs of this species have never been published except one specimen of the species was illustrated in Matter et al. (1974, p. 914, Pl. 4, fig. 8) under the name Ericsonia ovalis Black.