Palaeoenvironmental and palaeogeographical implications of modern Macaronichnus segregatis-like traces in foreshore sediments on the Pacific coast of central Japan

Traces produced by modern animals provide useful information for our understanding of comparable trace fossils. This paper reports on a polychaete worm Euzonus producing traces which fall within the definition of the ichnotaxon Macaronichnus segregatis. Observation on the wave-dominated Pacific sandy coast of central Japan revealed that this animal and its trace occur exclusively in sediments of the mid-foreshore portion of the beach. Owing to such a limited occurrence of the modern trace, the ichnofossil M. segregatis might be useful as an indicator of mid-foreshore environment. The longer axes of modern M. segregatis-like traces on a bedding plane are arranged perpendicular to the shoreline and accordingly, its fossil counterpart can be utilized for reconstructions of the ancient shoreline orientation. In support of this assumption, a similar mode of M. segregatis occurrence is reported from Pleistocene shallow marine deposits in Japan.