ABSTRACT
Spatangoid trace fossils in the Neogene of Japan are hardly found in well sorted sediments, but are usually found in poorly sorted or laminated sediments. Spatangoids are unique among burrowing animals that form trace fossils, because they utilize spines for burrowing. The urchin moved discontinuously, repeating a stopping action, and the sediment excavated in a onetime progress is accumulated as a layer of backfill on the back of the urchin. In well sorted sediments, on the other hand, the visible grain arrangement or the laminated structure was not formed, and it was impossible to recognize the trace shape, though only a drain tube formed as a small hole could be detectable. These results indicate that apparent lack of spatangoid traces in well sorted sediments may not necessarily mean the absence of their traces.
