Geochemical composition of faecal pellets as an indicator of deposit-feeding strategies in the trace fossil Phymatoderma

摘要

Phymatoderma is a branching burrow system consisting of tunnels filled with faecal pellets, and it has been interpreted as a product of a surface deposit-feeding animal. Elemental analyses of Phymatoderma were conducted to reveal the specific feeding mode of its tracemaker, using samples from the Lower Jurassic epicontinental shelf deposits in the Dotternhausen section, southern Germany and from the Upper Pliocene continental slope deposits in the Shioura section, central Japan. Elemental compositions of the pelletal infill of Phymatoderma and its overlying mudstone from the Dotternhausen section show no significant difference, suggesting that the tracemaker was a non-selective deposit feeder. In contrast, elemental compositions of the tuffaceous pellets of Phymatoderma from the Shioura section and its overlying volcanic ash show a difference in trends: CaO is significantly concentrated in the pellets. Because microfossils such as foraminifera and coccoliths are occasionally found in the tuffaceous pellets, CaO accumulation in the pelletal infill indicates the Phymatodermaproducer that lived in the Pliocene slope setting selectively ingested particles with higher biomass of such microorganisms (or ingested microorganisms themselves) when feeding the surface sediments. Although two types of feeding modes of the tracemaker were recognized in Phymatoderma between different bathymetrical settings, each feeding mode seems to be an effective strategy to intake nutrients from the surface sediments and to reflect an adaptation of the tracemaker to the food contents in the surrounding substrate. This study suggests that geochemical composition of faecal pellets of trace fossils can be a useful indicator of grain-selectiveon-selective depositfeeding strategies of ancient animals. □ Deposit feeding, faecal pellet, geochemistry, indicator, Phymatoderma, trace fossils.