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Planispiral Cephalopod Shells as a Sensitive Indicator of Modern and Ancient Bottom Currents: New Data from Flow Experiments with Modern Nautilus pompilius

¹ JSPS Research Fellow, National Science Museum, 3-23-1, Hyakunincho, Shinjuku, Tokyo 169-0073, Japan, wani{at}kahaku.go.jp
² Terrestrial Environment Research Center, University of Tsukuba, 1-1-1, Tennoudai, Tsukuba, Ibaraki 305-8577, Japan

The experimentally determined transportation characteristics of Nautilus pompilius shells by bottom currents reveal that planispiral cephalopod shells can be transported, reoriented, and restrained. Shells of N. pompilius become reoriented when the water flow is about 0.20 m/s, regardless of shell size, and are transported by the flow of 0.25–0.37 m/s, with velocity proportional to shell size. The shells are restrained in a strongly preferred orientation (aperture downstream). The transportation characteristics of various ammonoid groups differ based on differences in shell shape (especially drag coefficient in water). The shells of most ammonoids (especially leiostraca, serpenticones, and cadicones) had reorientation and threshold velocities similar to N. pompilius, and therefore were highly sensitive to paleo-bottom currents. The character and strength of paleo-bottom currents (direction and flow velocity) can be reconstructed from the taphonomic attributes of ammonoids (e.g., three-dimensional orientation within matrix, shell morphology, and ornamentation).