[B-PT25_2PM1] Biotic history and its relation to the Earth history

[BPT25-05] Composite trace fossils: Phymatoderma reburrowed by Chondrites/Phycosiphon and its paleoecological implications

*Kentaro IZUMI¹ (1.Department of Earth and Planetary Science, University of Tokyo)

Composite Phymatoderma specimens from the Pliocene deep-sea Shiramazu Formation in Japan, particularly those reburrowed by Chondrites and Phycosiphon, were analyzed to reveal the differences caused by the activities of these trace-makers. Phymatoderma reburrowed by Phycosiphon is significantly larger than non-reburrowed Phymatoderma, whereas Phymatoderma reburrowed by Chondrites shows no significant difference in burrow diameter compared with non-reburrowed Phymatoderma. The recognized size selectivity (i.e., preference for larger burrows) by the Phycosiphon trace-makers can be explained by considering the different feeding strategies of these two ichnogenera; namely deposit-feeding Phycosiphon-makers, which must have processed a significant mass of sediment to obtain sufficient organic matter, whereas chemosymbiotic Chondrites-producers, which did not require a lot of sediment to obtain nutrients. In order to test these interpretations, records of the Phanerozoic trace fossils reburrowed by Chondrites/Phycosiphon were compiled. Consequently, the Phycosiphon -preference toward relatively larger burrows was recognized, which supports the results of this study. The compilation also indicates that the burrow size has become a limiting factor for the Phycosiphon-producers that tried to rework the sediments within previous subsurface burrows, at least for 80 million years.