Ethologic and ecologic interpretation of complex stellate structures in Pleistocene deep-sea sediments (Otadai Formation), Boso Peninsula, central Japan

A large stellate structure, which is characterized by long, horizontal radial structures with a single vertical shaft, occurs in mud-dominated lower submarine fan sediments (Lower Pleistocene Otadai Formation, Boso Peninsula, Japan) deposited on an active plate margin during a highstand interval of a eustatic sea-level cycle. A single radial structure is divided into more than 20 meniscate packages filled with pellets of fecal origin. The Otadai specimens closely resemble the trace fossil Phoebichnus trochoides in its configuration, which has been reported only from Mesozoic rocks of shallow marine origin. However, the radials of the Otadai material have no mantle or lining. Hence, the Otadai material can be discriminated from P. trochoides at the ichnospecies level. Thus the Otadai material is introduced as a new ichnospecies, namely Phoebichnus bosoensis. Considering the anatomy and mode of occurrence of P. bosoensis, the trace-making animal adopted a sedentary life-style and apparently fed on detritus at the seafloor and conveyed the fecal material downward into the radials. Internal structures in the radials of P. bosoensis appear to have been produced by the repetition of systematic excretory behavior of the trace-making animal. The vertical shaft is thought to be a dwelling structure. During a highstand interval of a eustatic sea-level cycle, large amounts of terrigenous sediments were deposited by turbidity currents at a lower submarine fan setting of the Otadai Formation, as suggested by sedimentologic data. Large amounts of terrigenous sediment supply to a lower submarine fan setting of the Otadai Formation was due to a narrower shelf and a steeper slope in comparison to a passive plate margin. Therefore sufficient influx to the seafloor of organic matter as the food for the benthos is assumed for the highstand interval of a eustatic sea-level cycle. This inference means that the nutrient level at the seafloor was suitable for surface deposit-feeders. Sufficient food supply must have been available to support the long-term sedentary life-style for the Otadai Phoebichnus producers, which produced the stellate structure as a dump for fecal matter.