Predominant equilibrichnial Rosselia socialis are present at up to thirty stratigraphic levels in middle Pleistocene, siliciclastic inner shelf deposits on the Boso Peninsula, Japan. The ichnofabric composed of R. socialis is interpreted to have formed by a considerably dense population of R. socialis animals (terebellid polychaetes?). The ichnofabric interval represents a transgressive inner shelf deposit strongly affected by a high-frequency, 5th- or 6th-order sea-level rise that probably was rapid enough to influence the ecology of benthic communities. The dense colonization by the R. socialis animals is interpreted to result mainly from rapid transgression caused by short-term sea-level rise. Coastal erosion induced frequent pulses of sedimentation in to the shelfal environment and probably prevented colonization by most benthic animals, except for R. socialis animals, which are thought to be tolerant of such conditions. Ravinement also provided organic detritus, derived mainly from organic-rich coastal deposits of salt-marsh origin, which enabled the detritus-feeding and stress-tolerant R. socialis animals to thrive.