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Stratigraphic variations in the morphometry of the genus Gephyrocapsa (calcareous nannoplankton) were quantitatively investigated in the Quaternary sequence of Hole 709C recovered from the tropical Indian Ocean. Gephyrocapsa can be roughly divided into groups of small and large Gephyrocapsa, mainly on the basis of their significant differences in coccolith size. Four lineages were recognized among the large Gephyrocapsa, and members of these lineages were tentatively named Gephyrocapsa sp. A, B, C, and D. These species are approximately 3 /xm large at their incipient stages, and the latter three species become progressively larger thereafter. In the case of Gephyrocapsa sp. B and C, well-developed large forms disappeared abruptly, only to be succeeded by a new cycle of evolutionary size increase, respectively performed by Gephyrocapsa sp. C and D. The first cycle observed in Gephyrocapsa sp. B occurred at the 16.02-12.02 mbsf interval (around 1.6-1.1 Ma), and Gephyrocapsa sp. A occurred simultaneously within this cycle. The second and third cycles, performed respectively by Gephyrocapsa sp. C and D, occurred at 9.62-6.02 mbsf (around 0.9-0.5 Ma) and above 5.62 mbsf (0.5 Ma to the present). The percentage abundance of large Gephyrocapsa species that showed an evolutional size increase conversely decreased with an increase in overall size. An interval lacking the large species that roughly corresponds to the "small Gephyrocapsa zone" was found between the first and second cycles, but changes in abundance of the small and large forms seem to be independent. The appearance and disappearance of the large forms as well as their changes in overall size and abundance play major roles in the floral change throughout the Quaternary. The six nannoflora assemblages dividing the last 1.3 m.y. reported from the subtropical Pacific Ocean were also recognized in the tropical Indian Ocean. A new additional assemblage was identified in the basal part of the Pleistocene.

Time-Progressive Morphometric Changes of the Genus Gephyrocapsa in the Quaternary Sequence of the Tropical Indian Ocean, Site 709