Late Quaternary calcareous nannoflora usually contain abundant placolith-type nannofossils that are produced by Emiliania huxleyi and Gephyrocapsa species. Placoliths of Gephyrocapsa species display a great degree of size variation (Bollmann, 1997; Matsuoka and Okada, 1989, 1990). Downcore size variation of Gephyrocapsa species is useful for stratigraphy (Matsuoka and Okada, 1989; Erba 1995; Okada and Wells, 1997); however, classification and taxonomy of the genus Gephyrocapsa is in a state of confusion. Because this study does not intend to refine the classification system, Gephyrocapsa species are classified into the following four size categories: very small (<2.0 µm), small (2.0-3.0 µm), medium (3.0-5.0 µm), and large (>5.0 µm).
In the small Gephyrocapsa specimens, only Gephyrocapsa caribbeanica is identified at the species level based on the closed central area. The other small and very small placoliths having a bridge and a wide central area are classified into Gephyrocapsa (small) or Gephyrocapsa (very small) categories based on size.
Under the light microscope, it is difficult to distinguish the small and very small Gephyrocapsa specimens that were lost or did not develop bridge elements from the small and very small Reticulofenestra specimens, respectively. Moreover, it is difficult to identify the small and very small placoliths at species level under the light microscope. Therefore, the small and very small Gephyrocapsa specimens without bridge and the small and very small Reticulofenestra specimens are classified into the placolith (small) or placolith (very small) categories based on size.