Rosselia socialis: a dwelling structure of a probable terebellid polychaete

The trace fossil Rosselia socialis Dahmer 1937 is a funnel-shaped structure characterized by a concentrically laminated wall having a narrow cylindrical shaft at the central portion. In association with well-preserved funnel-shaped R. socialis, many spindle-shaped R. socialis-like specimens are abundant in the Middle Pleistocene Kongochi Formation in central Japan. While the former always occur just below an erosional bedding plane or outcrop surface parallel to the bedding, the latter are found in sediment showing no breaks in sedimentation. This suggests that the funnel-shaped ‘R. socialis’ is actually the truncated lower part of the spindle-shaped structure. In one specimen, deformation of laminae appears to have formed synchronously with construction of the wall. Morphology of the structure strongly suggests that the wall of R. socialis was formed through outward expansion caused by accretion of muddy material at the inner surface of the wall by the trace-maker inhabiting the central burrow. A specimen that was covered by a volcanic ash has the wall made up of the ash. This indicates that the maker of the burrow utilized surface sediment to construct the wall; that is, the trace-maker was possibly a detritus feeder. The terebellid polychaetes are reasonable candidates for the trace-makers.