POPULATION PALEOECOLOGY OF THE ZOOPHYCOS-PRODUCING ANIMAL

Well-preserved specimens of the trace fossil Zoophycos are found abund antly in upper Pliocene middle bathyal deposits (Shiramazu Formation) of the southernmost Boso Peninsula, central Japan. The Shiramazu Zooph ycos represents the product of an inverted conveyor activity of the tr acemaker throughout its growth. Some of the Zoophycos occur in mudston e beneath a volcanic ash layer. These specimens contain tuffaceous pel lets composed of pyroclastic material alone or of mud mixed with pyroc lastic sand grains. Such specimens allow the recognition of synchronou s individuals within the complex ichnofabric. This is because the accu mulation of pyroclastic material on the seafloor, and its subsequent s tuffing into the burrows via the gut by the burrow producer, are thoug ht to be a short-term event. The census fossil assemblage of Zoophycos containing tuffaceous pellets is analyzed and indicates a high popula tion density, patchy spatial distribution pattern, and a markedly bias ed age structure consisting only of adult individuals. Factors conside red important in controlling the spatiotemporal pattern and age struct ure of the Zoophycos animals include the following: the enriched organ ic matter for food and unstable chemicophysical bottom-conditions, bot h caused by the frequent influx of turbidity currents and rapid sedime ntation of turbidites; and the episodic recruitment. It follows that t he mud bottom was colonized only by individuals derived from an episod ic recruitment during the interturbidite period and these subsequently formed patchy aggregations at the nutrient rich seafloor.