“Deep-sea bivalvian highways”: An ethological interpretation of branched Protovirgularia of the Palaeogene Muroto-Hanto Group, southwestern Japan

Well-preserved specimens of the trace fossil Protovirgularia occur in deep sea trough fills of the Palaeogene accretionary complex of the Muroto-Hanto Group in southwestern Japan. The trace fossil, occurring characteristically on the sole of sandstones of turbidite systems, is interpreted as an interstratal trail of a protobranch bivalve having a cleft foot. Among the Muroto-Hanto Protovirgularia, many specimens of branched variants have been found. Based on the morphological analyses, coupled with knowledge of the behaviour of the modern intertidal molluscs, the branched specimens are interpreted to have formed as the result of interstratal trail-following behaviour of the infaunal protobranchs. The animals probably followed trails of other individuals in order to save energy for locomotion. The incipient Protovirgularia was thus interpreted to have functioned as a “highway” for the infaunal mollusc. This is probably the first report of a trail-following behaviour of infaunal invertebrates.