13:45 〜 15:15
[BPT03-P08] Morphological analysis of a calcareous nannofossil group, reticulofenestrids, around the Oligocene/Miocene boundary in the Atlantic Ocean
キーワード：ナノ化石、形態解析
Around the Oligocene/Miocene boundary, floral and faunal turnovers in various marine organisms had occurred, possibly relating to global glaciations (e.g., Steinthorsdottir et al., 2021). Some bioevents of calcareous nannofossils, one of the major marine algae since the Mesozoic, were also found during this time. Cenozoic nannoplankton had been dominated by a coccolithophore group with elliptical, calcareous exoskeletons, called reticulofenestrids, and time-transgressive variations in their size and shape were characteristic changes of nannofossils during this time. Genus Reticulofenstra and its related taxa, composed of reticulofenestrids, have been utilized as index fossils, and the size and shape of reticulofenestrids specimens are important to recognize individual taxon. In particular, regular size variations of this taxon in chronological order were shown in various places during the Neogene time (e.g., Young, 1990, Kameo and Bralower, 2000). Despite characteristically morphologic changes in reticulofenestrids, detailed size and shape variations and the relationships to their taxonomic classification are still unknown. This study performed biometric analyses of reticulofenstrids in the Oligocene to Miocene to clarify this group's evolutionary trend.
We investigated morphologic changes of reticulofenetrids during the late Oligocene–middle Miocene. Morphological observations of reticulofenestids coccolith were performed under the scanning electron microscope (SEM). In order to investigate morphological changes, the longer and shorter diameters of a single coccolith and its central opening were measured using ImageJ software.
Three genera belonging to reticulofenestrids were identified. Genus Dictyococcites only occurred in the Oligocene, classified by its narrow and closed central opening size. Above the disappearance of Dictyococcites around the Oligocene/Miocene boundary, the genus Cyclicargolithus and Reticulofenestra became the main components of reticulofenestrids. During the Oligocene–earliest Miocene time, the morphologic features of both genera are very similar, and it is difficult to distinguish both taxa. However, elliptical specimens with a large central part corresponding to the genus Reticulofenestra appeared from approximately 22 Ma. After 18 Ma, circular specimens with a tiny central opening had increased, coinciding with the increase of the genus Cyclicargolithus. Our results indicate that the floral turnover from dominant Dictyococcites to Reticulofenestra and Cyclicargolithus are possibly related to the early Miocene climatic changes.
[Reference]
Steinthorsdottir, M., Coxall, H. K., De Boer, A. M., Huber, M., Barbolini, N., Bradshaw, C. D., ... Strömberg, C. A. E., 2021. The Miocene: the future of the past. Paleoceanography and Paleoclimatology, 36(4), e2020PA004037.
Kameo, K., 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 Indian
Ocean DSDP cores. Journal of Micropaleontology, 9, 71–86
We investigated morphologic changes of reticulofenetrids during the late Oligocene–middle Miocene. Morphological observations of reticulofenestids coccolith were performed under the scanning electron microscope (SEM). In order to investigate morphological changes, the longer and shorter diameters of a single coccolith and its central opening were measured using ImageJ software.
Three genera belonging to reticulofenestrids were identified. Genus Dictyococcites only occurred in the Oligocene, classified by its narrow and closed central opening size. Above the disappearance of Dictyococcites around the Oligocene/Miocene boundary, the genus Cyclicargolithus and Reticulofenestra became the main components of reticulofenestrids. During the Oligocene–earliest Miocene time, the morphologic features of both genera are very similar, and it is difficult to distinguish both taxa. However, elliptical specimens with a large central part corresponding to the genus Reticulofenestra appeared from approximately 22 Ma. After 18 Ma, circular specimens with a tiny central opening had increased, coinciding with the increase of the genus Cyclicargolithus. Our results indicate that the floral turnover from dominant Dictyococcites to Reticulofenestra and Cyclicargolithus are possibly related to the early Miocene climatic changes.
[Reference]
Steinthorsdottir, M., Coxall, H. K., De Boer, A. M., Huber, M., Barbolini, N., Bradshaw, C. D., ... Strömberg, C. A. E., 2021. The Miocene: the future of the past. Paleoceanography and Paleoclimatology, 36(4), e2020PA004037.
Kameo, K., 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 Indian
Ocean DSDP cores. Journal of Micropaleontology, 9, 71–86