[BPT04-P03] Morphologic changes of a calcareous nannofossil group, Reticulofenestra group, from Eocene to Miocene in the Atlantic Ocean
キーワード：石灰質ナノ化石、形態変化、電子顕微鏡
Coccolithophores, uniceller marine phytoplankton, produce calcite plates called coccolith, consisting of two elliptical discs arranged a lot of tiny, tabular calcite plates (e.g., Young et al., 1992). The Reticulofenestra group, composed of a genus Reticulofenstra and its related taxa, is one of the major coccoliths during the Cenozoic. Specimens have been classified by their shape and central structures of a coccolith. Size variations during the Miocene and Pliocene have been studied (Young, 1990; Kameo and Bralower, 2000) because frequent size reductions can be considered to correlate globally. Changes in size and morphologic characters of Reticulofenestra group have been studied mainly in the Neogene and Quaternary, and a few in the Paleogene to the Early Miocene. So, we applied biometric measurements to Reticulofenestra group in the Paleogene to the Early Miocene in order to clarify evolutionary trend of this taxon.
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.
[Reference]
Kameo, K. and Bralower, T. J., 2000. Neogene calcareous nannofossil biostratigraphy of Sites 998, 999, and 1000, Caribbean Sea. In Proceedings of the Ocean Drilling Program, Scientific Results, 165, 3-17. College Station TX.
Young, J. R., 1990. Size variation of Neogene Reticulofenestra coccoliths from IndianOcean DSDP cores. Journal of Micropaleontology, 9, 71–86
Young, J. R., Didymus, J. M., Brown, P. R., Prins, B., & Mann, S., 1992. Crystal assembly and phylogenetic evolution in heterococcoliths. Nature, 356, 516-518.
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.
[Reference]
Kameo, K. and Bralower, T. J., 2000. Neogene calcareous nannofossil biostratigraphy of Sites 998, 999, and 1000, Caribbean Sea. In Proceedings of the Ocean Drilling Program, Scientific Results, 165, 3-17. College Station TX.
Young, J. R., 1990. Size variation of Neogene Reticulofenestra coccoliths from IndianOcean DSDP cores. Journal of Micropaleontology, 9, 71–86
Young, J. R., Didymus, J. M., Brown, P. R., Prins, B., & Mann, S., 1992. Crystal assembly and phylogenetic evolution in heterococcoliths. Nature, 356, 516-518.