Several stratigraphic intervals characterized by crowded occurrence of Rosselia socialis, a vertical fusiform mud-lined burrow probably produced by a terebellid polychaete, are seen in the inner shelf deposits of the middle Pleistocene Ichinono Alternating beds of sand and mud, Kongochi Formation in the Boso Peninsula, Japan. Most of the specimens of R. socialis are of "stacked form" interpreted to be formed as the result of equilibrium behavior of the trace-maker. As the R. socialis-crowded beds are seen characteristically in a transgressive systems tract of a high-frequency depositional sequence, the major factor to allow dense and dominant colonization of the R. socialis animals may be explained as following. During the transgression, shoreface erosion induced frequent sedimentation in the shelf environment, and probably prohibited colonization of most benthic animals, except for the R. socialis-producers being tolerant of such a condition. The shoreface erosion also provided much organic matter, derived mainly from plants in nearshore and coastal area, to the environment, and allowed to thrive the detritus-feeding and stress-tolerant R. socialis animals.
