Many investigation of Neogene nannofossils often ignore specimens smaller than 2.5-4.0 µm. Smaller nannofossils are sometimes grouped as small placoliths. Because small placoliths often dominate the flora in many stratigraphic intervals (e.g., the middle Pleistocene "small Gephyrocapsa Zone" of Gartner, 1977), they cannot be ignored in floral assemblage studies.
Studies of living calcareous nannoplankton reveal blooms of very small (<2 µm) placoliths, Gephyrocapsa crassipons, Gephyrocapsa ericsonii, Reticulofenestra pulvula, and Reticulofenestra punctata, in upwelling regions of the tropical Pacific Ocean (Okada and Honjo, 1973; Okada and McIntyre, 1977). Also, a morphometric study of Gephyrocapsa through the Quaternary revealed the occurrence of very small-size clusters separable at ~2.0 µm in some time intervals (Matsuoka and Okada, 1990). Therefore, a size of 2.0 µm was used to separate very small and small placoliths. Because of the difficulty of identifying the very small placoliths at species level in the light microscope, they were combined in counts. Very small nannofossils that are not placoliths occur only rarely and were included in the miscellaneous category.
Many morphotypes or variations exist in the Gephyrocapsa lineage (e.g. Matsuoka and Okada, 1989, 1990), and the classification and taxonomy of the genus Gephyrocapsa is currently in a state of confusion. Because it is not the purpose of this investigation to refine the classification of this genus, specimens of Gephyrocapsa are classified into 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). As mentioned above, very small Gephyrocapsa are incorporated into the very small placolith category. The small and large Gephyrocapsa were not subdivided, but the medium Gephyrocapsa were further divided into two categories: Gephyrocapsa caribbeanica for the medium forms with a very small or completely closed central opening, and Gephyrocapsa oceanica (medium) for the forms with a relatively large central area. Species of genus Reticulofenestra are similar to Gephyrocapsa in coccosphere construction and size distributions throughout the late Pliocene to Quaternary. Coccoliths of the small form of the genus, Reticulofenestra minuta, and the medium form, Reticulofenestra minutura, are separated at the coccolith size of 3.0 µm (Backman, 1980). Because specimens of Gephyrocapsa that lost or didn't develop a bridge are difficult to distinguish from specimens of Reticulofenestra under a light microscope, the separation of medium and small Gephyrocapsa at 3.0 µm is a reasonable as well as practical solution.
Small specimens of Helicosphaera, including Helicosphaera minuta, Helicosphaera pacifica, and Helicosphaera pavimentum are combined as small Helicosphaera. Finally, specimens of genera Acanthoica, Scyphosphaera, and Syracosphaera were not identified at the species level.
Gartner (1967) originally described Umbilicosphaera aequiscutum as Cyclicoccolithus aequiscutum from the Pliocene of Jamaica and a Pliocene core taken from the Gulf of Mexico. Cohen and Reinhardt (1968) have transferred this species to the genus Umbilicosphaera. Aubry (1993b) noted its consistent occurrence in the middle Miocene cores in the Gulf of Mexico, and by claiming invalidity of Cohen and Reinhardt's transfer of this species to Umbilicosphaera rightfully reclassified it to genus Umbilicosphaera (Aubry, 1993b). The coccolith size illustrated in the original description ranges between 3.6 and 3.8 µm, but the specimens encountered in this investigation range from 2.0 to 4.4 µm, with an average size of ~3.0 µm. The relative size of the central opening varies, but most specimens found in this study have a much smaller central opening than the original illustrations. The cross-polarized light image of this taxon illustrated by Aubry (1993a) is similar to the specimens observed here, but the size of Aubry's (1993a) specimens are larger than encountered in this study. SEM observation of U. aequiscutum confirmed that the proximal disc is equal to or slightly larger than the distal disc, which is the key point for the classification to genus Umbilicosphaera. A review of the taxonomy and biogeography of the species will be included in a separate article (Okada and Hagino, pers. comm., 1998).