Taphonomy of radiolarian and diatom fossils preserved within Oligocene trace fossils

This study deals with the taphonomy of siliceous microfossils preserved in trace fossils. With this objective, we chose to study trace fossils (Tasselia ordamensis) from the Oligocene Okubo Mudstone Member of the Nishinoomote Formation (Kumage Group), exposed on Tanegashima Island, Kagoshima, southwest Japan. Specifically, we examined microfossil distribution patterns, their mode of occurrence, and state of preservation, within the infill of the trace fossil and as well as from the surrounding host mudstone. This has enabled us to establish that there is a significant siliceous microfossil preservation advantage within trace fossils in mudstones that have been subject to burial diagenesis. The following three conclusions can be made (1) Relatively well-preserved radiolarian and diatom fossils are distributed unevenly within the trace fossil infills and most of them are interpreted to be derived from passive infilling sediment that was not affected by feeding and excretion activities of the trace maker. (2) Comparing preservation from inside and outside of the trace fossils, the radiolarians from inside the trace fossil are much better preserved than those from outside. (3) Microfossils that were transported into the trace fossil within infill sediment were protected from subsequent burial compaction and diagenesis by preferential cementation to produce a concretion of the trace fossil, and this has enabled the siliceous microfossils to be relatively well-preserved. These results support the idea that trace fossils offer significant microfossil preservation potential. Trace fossils may also be considered as natural archives of valuable micropaleontological records that would usually be lost during burial, compaction and diagenetic processes. Taphonomic analysis of siliceous microfossils within the infills of trace fossils, such as Tasselia ordamensis, has the potential to reveal significant new paleontological insight from sedimentary rocks throughout the fossil record.