極めて保存良好な中期更新世着生珪藻Arachnoidiscus ornatus : その化石化過程

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Exceptionally well preserved frustules of a single diatom species, Arachnoidiscus ornatus Ehrenberg, were found on mid-Pleistocene seaweed-like remains. The distribution of frustules on the remains were compared with the distribution of A. ornatus cells on modern seaweeds. This comparison indicated that the fossil frustules were likely preserved epiphytic assemblages that occurred on seaweeds. Almost all modern A. ornatus cells are attached to seaweeds with epivalves, and 97% of the fossil A. ornatus frustules had epivalves on the exposed upper face. This finding and the occurrence of flattened thin frustules of a single naviculoid diatom speices on the fossil epivalve surfaces suggested a possible taphonomic process for the exceptional state of preservation. Seaweeds, which acted as impermeable films within sediments during the taphonomic process, probably prevented interstitial waters from permeating the tiny pores of the frustules. These films were peeled off when the fossils were excavated. To infer the possible physicochemical conditions of preservation, the framboidal pyrite (FeS_2) preserved in the fossil A. ornatus valves was compared with cytoplasmic masses in living A. ornatus cells collected from modern coastal waters off Japan. The following two scenarios were inferred: the cytoplasmic masses in Arachnoidiscus cells may be the source of the framboids, and the rapid burial of cells in mud under saline conditions probably led to the superior preservation.
国立科学博物館の論文
2005-12-22

極めて保存良好な中期更新世着生珪藻Arachnoidiscus ornatus : その化石化過程

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