Tiering of trace fossil assemblages in Plio-Pleistocene bathyal deposits of Boso Peninsula, Japan

Tiering of assemblages of trace fossils produced by the inverted conveyor activity of surface deposit-feeders was reconstructed on the basis of the examples occurring just below volcanic ash layers in the Plio-Pleistocene deep-sea mudstone of Boso Peninsula, central Japan. The trace fossils include Chondrites (subdivided into five morphotypes), Zoophycos, and radiating tunnel structures named herein "star-shaped burrows." All of these structures represent storage bins for fecal material of the tracemakers. Six tiers for the Pliocene sequence and four tiers for the Pleistocene sequence were recognized. No clear-cut relationship between penetration depth of burrows and burrow size (overall and tunnel diameter) was evident. In addition, associated molluscan fossils and lithologic features are indicative of oxygenated bottom water conditions for these sedimentary basins. Consequently, tiering may not be closely related to such environmental factors as dissolved oxygen of the bottom and pore waters or partitioning of the food source in the sediments, but it represents the spatial segregation of the excretory space in the sediment. Stratification of excretory space in the sediment may indicate strategy to avoid interspecific competition of egestion and living space by the burrow-producers that inhabited the substrate simultaneously and utilized the same sea floor as their feeding place.