Packing process for the filling material inChondrites

Abstract

The trace fossil Chondrites from Plio‐Pleistocene deepsea sediments of the Boso Peninsula, central Japan, can be classified as small and giant forms. Fillings generally consist of mud for the small form and pellets for the giant form; both forms are lighter colored than, and contrast with, the host sediment. In some burrows occurring below a tuff layer, the fillings consist of the same pyroclastic material as the overlying tuff layer, although the structures lack a distinct vertical shaft. These characteristics are indicative of the fecal origin of the material constituting the fillings. Under normal conditions, Chondrites producers probably ingested detritus on the seafloor and subsequently excreted the fecal material in sediment below the sediment‐water interface. In the giant form, a void space for packing fecal pellets might be formed by swelling and retracting of the animal's body. Considering the marked similarities in overall morphology and mode of occurrence between the two forms, the same interpretation can be applied to the small form. Absence of a well‐defined vertical shaft in the Chondrites specimens examined apparently reflects either destruction during a bioturbational process or postmortem filling by muddy material from above.