2. Ethological interpretation of the trace fossil Zoophycos in the Hikoroichi Formation (Lower Carboniferous), southern Kitakami Mountains, Northeast Japan

The trace fossil Zoophycos was newly discovered in the lower part of the Hikoroichi Formation (Lower Carboniferous) distributed in the southern Kitakami Mountains, Northeast Japan. It consists of a helically coiled spreite, in which black-colored, chevron-shaped lamellae occur in cross section as an internal structure. Apart from the absence of distinct pellets and axial shaft, morphological features of the Hikoroichi Zoophycos are similar to those of Zoophycos known from Tertiary deep-sea sediments. X-ray diffractional analysis and microscopic observation in thin section revealed that the black material of the lamellae in the spreite appears to be derived from a higher level as a result of downward conveyor activity by tracemakers. This inference and morphological similarity to Tertiary specimens imply that the black material in the spreite of the Hikoroichi Zoophycos is probably of fecal origin, although there is no compelling evidence such as pellets. According to this interpretation, the producing animals of the Hikoroichi Zoophycos fed in surface and/or subsurface sediments and deposited fecal matter deep in sediment, as did Zoophycos from Pliocene deep-sea sediments of the Boso Peninsula, Central Japan. Such feeding and excretory styles in the Zoophycos producer may have already been achieved by at least the Early Carboniferous and have remained unchanged over a long peroid.