Vol.23 No.4

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It has been suggested that the pseudopodia of radiolarians play a role in controlling buoyancy for floating behaviour. To understand the function of pseudopodia in terms of planktonic capability, we performed culture experiments on the flat-shaped radiolarian Dictyocoryne. A glass cell, a stereomicroscope and an X-Y-Z stage were used to observe the behaviour of Dictyocoryne from a lateral view. Under static conditions, Dictyocoryne grounded on the bottom of the glass cell extended pseudopodia from both sides of the flat disc surface. Subsequently, these individuals rose slightly by a length equal to that of the extended pseudopodia but remained attached to the bottom. These results suggest that Dictyocoryne lacks the ability to surface by obtaining buoyancy through the emergence of pseudopodia. Under conditions of convection flow, Dictyocoryne moved in the downstream direction only when pseudopodia were completely extended. When the convection flow moved upward, the individuals moved with the flow to just under the surface. In addition, convection flow changed the direction of a long, thick pseudopodium, called the axoflagellum, to be parallel with the flow. Consequently, the axoflagellum was always oriented towards the downstream side, being the disc face in the axoflagellum side on the upturn. Given that the flat-shaped spumellarians have symbiotic algae inside their cells, their unique planktonic capability leads to the stable efficiency of the algal photosynthesis.

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Sedimentary sequences obtained from drilling during Integrated Ocean Drilling Program (IODP) Expedition 320/321, “Pacific Equatorial Age Transect (PEAT)” at eight sites (Sites U1331–1338) in the equatorial Pacific offer an ideal record for reconstructing the evolution of the ocean/climate system throughout the Cenozoic. The sediments drilled at Site U1335 record short-term events of paleoceanographic significance, including the early Miocene climatic optimum (MCO) and the middle Miocene climatic transition (MMCT). Abundant well preserved radiolarians were recovered from the lower Miocene radiolarian Zone RN2 through middle Miocene Zone RN5 at IODP Site U1335. A total of 46 radiolarian datum levels consisting of 20 first occurrences (FOs), 25 last occurrences (LOs), and one evolutionary transition (ET) was recognized within the studied interval at Site U1335. Of these datum levels, 36 radiolarian datum levels were directly tied to the geomagnetic polarity time scale (GPTS) across the early/middle Miocene boundary. The general magnitude of evolutionary change was estimated based on the total turnover rate (the sum of FOs and LOs per 0.5 m.y.) of tropical radiolarians, and two minor faunal turnovers of radiolarian species were recognized between 16.5 and 14.7 Ma and between 13.9 and 13.4 Ma. These faunal turnovers were associated with regional environmental changes such as the increased biological productivity in the equatorial Pacific during the MCO and the MMCT.

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A metric analysis of the morphology of two related Eocene species (Podocyrtis sinuosa and P. mitra) that are part of the Lampterium evolutionary lineage was undertaken in order to evaluate hypotheses related to their mutual taxonomic distinction statistically. All analyses (landmark, outline semi-landmark and landmark-constrained outlines) support an interpretation of statistically significant species-specific shape differences. Moreover, landmark and semilandmark-based morphometric characterizations can be used to identify which regions of the test are best suited for making reliable taxonomic distinctions. These results suggest that both abdomen and thorax shapes represent species-specific characters. While this agrees, in part, with previous, qualitative diagnoses, our results shed light on precisely how abdomen and thorax shape differ between these species. In addition, our investigation demonstrates the taxonomic value of a morphometric approach to character analysis as thorax shape differences had gone unnoticed by previous investigators.