Paleoecology of the Zoophycos producers

Well-preserved Zoophycos and Spirophyton-like burrows occur in the Upper Pliocene deep-sea sediments exposed along the southern coast of Boso Peninsula, central Japan. They consist of an axial tunnel and helically coiled spreite with fecal pellets. In most cases the two kinds of trace fossil are found separately. In several complete specimens, however, the upper and lower portions represent a Spirophyton-like burrow and Zoophycos, respectively. The downward increase in size of the spreite and fecal pellets in a single burrow suggests that the complete Zoophycos was built successively as the producing animal grew. The occurrence of incomplete Zoophycos may have resulted from post-mortem destruction by echinoid locomotion and turbidity currents. In some cases, the uppermost portion of the axial tunnel is covered with a thin tuff layer. Fecal pellets in such burrows consist of the same tuffaceous material which must have originated from the overlying tuff layer. This fact indicates that the burrow producer did not feed on organic matter within the sediments but foraged detritus on the sea floor. The animal could probably stretch a part of the body from the top of the axial tunnel for feeding and systematically pack the fecal pellets into the sediments. Such segregation between the feeding place and the excretory space is interpreted as an efficient feeding strategy for the detritus-feeding burrowers in the deep sea.