[BPT04-P03] Morphologic changes of a calcareous nannofossil group, Reticulofenestra group, from Eocene to Miocene in the Atlantic Ocean
Keywords:Calcareous nannofossil, Morphologic change, Scaning electronic microscope
In this study, we investigated morphologic characters and sequential size variations of Reticulofenestra group during the Eocene–Miocene based on the scanning electron microscope (SEM) observation. Samples from Ocean Drilling Program (ODP) Leg 208 in the South Atlantic Ocean were used. The ages of the core used here range from ~42 Ma to ~10 Ma. One hundred specimens of Reticulofenestra group were examined and morphometric analysis (measurements of diameters of overall coccolith and its central part) were done using a software ImageJ.
Specimens in the Reticulofenestra group can be divided into three genera, Cyclicargolithus, Dictyococcites, and Reticulofenestra based on their morphologic characters. Moreover, dominated ranges of larger forms of each genus were clearly different; Reticulofenestra in the Eocene–lower Oligocene, Dictyococcites in the Oligocene–lower Miocene, and Cyclicargolithus in the lower Oligocene–lower Miocene, respectively. The size of the central opening of Cyclicargolithus can be a good morphologic character to separate it from other forms of Reticulofenestra group. After the middle Miocene, another abundant occurrence of large forms of Reticulofenestra are observed with some morphologic changes. Detailed observations will enable us to clarify specific characters of each Reticulofenestra.
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