17:15 〜 18:30
[BPT02-P04] 竜の口層から産出した放散虫化石：絶滅種の深度分布に関する一考察
キーワード：放散虫、古生態、竜の口層
Radiolarians are marine micro-zooplanktons today living in a wide environmental range from tropic to polar regions as well as from shallow to great depths in the world ocean. Therefore, their fossil siliceous remains in sediments are stratigraphically and ecologically useful material to reconstruct the history of the Earth’s life and surface environments. Going back in deep time, however, living species that are proxies of oceanographic environments become less and less frequent in geological records. Paleobiogeography of extinct species can be a key to understand oceanic biofacies and ecological realms but it is rather difficult to research depth-related distribution in the water column of extinct species.
The Tatsunokuchi Formation is dominated by marine fine-grained sediments deposited in a paleo-bay in northeastern Japan during the latest Miocene to the early Pliocene. This formation yields the Tatsunokuchi Fauna characterized by abundant, diversified fossils including mollusks, crabs, sharks, whales and seals, which indicate cool water condition and water depths shallower than 50 m. This formation provides an opportunity to study ancient planktonic fauna of shallow water environments.
We collected samples from the Tatsunokuchi Formation from the southernmost part of Miyagi Prefecture to the northern border. Most of the samples yielded radiolarian fossils. Some of these samples contain well-preserved extinct species including Stichocorys peregrina, Stichocorys delmontensis, Didymoyrtis penultima and Cycladophora sphaeris. S. peregrina is the commonest component among the samples and the others occurred in rare abundance from one or two samples. The occurred specimens of S. peregrina and D. penultima have well-developed siliceous skeletons. S. peregrina and C. sphaeris are known to be abundant in age-equivalent deep-sea sediments in offshore drilled sites (e.g., DSDP/ODP Sites 438, 1150, 1151). D. penultima is a zonal marker species of the D. penultima Zone. Didymocyrtis tetrathalamus, a modern descendant of the Didymocyrtis lineage, is known to be a surface water dweller.
Our results suggest that S. peregrina and D. penultima lived in surface waters shallower than 50 m. It is likely that both were shallow water taxa, although it cannot be excluded that they had wide depth ranges from the surface to deeper layers. It is not certain that S. delmontensis and C. sphaeris dwelt in abundance in the paleo-Tatsunokuchi bay because of their rare, sporadic occurrence.
The Tatsunokuchi Formation is dominated by marine fine-grained sediments deposited in a paleo-bay in northeastern Japan during the latest Miocene to the early Pliocene. This formation yields the Tatsunokuchi Fauna characterized by abundant, diversified fossils including mollusks, crabs, sharks, whales and seals, which indicate cool water condition and water depths shallower than 50 m. This formation provides an opportunity to study ancient planktonic fauna of shallow water environments.
We collected samples from the Tatsunokuchi Formation from the southernmost part of Miyagi Prefecture to the northern border. Most of the samples yielded radiolarian fossils. Some of these samples contain well-preserved extinct species including Stichocorys peregrina, Stichocorys delmontensis, Didymoyrtis penultima and Cycladophora sphaeris. S. peregrina is the commonest component among the samples and the others occurred in rare abundance from one or two samples. The occurred specimens of S. peregrina and D. penultima have well-developed siliceous skeletons. S. peregrina and C. sphaeris are known to be abundant in age-equivalent deep-sea sediments in offshore drilled sites (e.g., DSDP/ODP Sites 438, 1150, 1151). D. penultima is a zonal marker species of the D. penultima Zone. Didymocyrtis tetrathalamus, a modern descendant of the Didymocyrtis lineage, is known to be a surface water dweller.
Our results suggest that S. peregrina and D. penultima lived in surface waters shallower than 50 m. It is likely that both were shallow water taxa, although it cannot be excluded that they had wide depth ranges from the surface to deeper layers. It is not certain that S. delmontensis and C. sphaeris dwelt in abundance in the paleo-Tatsunokuchi bay because of their rare, sporadic occurrence.