Population Paleoecology of the Zoophycos-Producing Animal

Well-preserved specimens of the trace fossil Zoophycos are found abundantly in upper Pliocene middle bathyal deposits (Shiramazu Formation) of the southernmost Boso Peninsula, central Japan. The Shiramazu Zoophycos represents the product of an inverted conveyor activity of the tracemaker throughout its growth. Some of the Zoophycos occur in mudstone beneath a volcanic ash layer. These specimens contain tuffaceous pellets composed of pyroclastic material alone or of mud mixed with pyroclastic sand grains. Such specimens allow the recognition of synchronous individuals within the complex ichnofabric. This is because the accumulation of pyroclastic material on the seafloor, and its subsequent stuffing into the burrows via the gut by the burrowproducer, are thought to be a short-term event. The census fossil assemblage of Zoophycos containing tuffaceous pellets is analyzed and indicates a high population density, patchy spatial distribution pattern, and a markedly biased age structure consisting only of adult individuals. Factors considered important in controlling the spatiotemporal pattern and age structure of the Zoophycos animals include the following: the enriched organic matter for food and unstable chemicophysical bottom-conditions, both caused by the frequent influx of turbidity currents and rapid sedimentation of turbidites; and the episodic recruitment. It follows that the mud bottom was colonized only by individuals derived from an episodic recruitment during the interturbidite period and these subsequently formed patchy aggregations at the nutrient rich seafloor.