異常巻アンモナイトの着底時における生息姿勢の復元法 : Eubostrychoceras muramotoi Matsumotoを例にして

抄録

To restore the living attitude of Eubostrychoceras muramotoi Matsumoto, 1967, from the Upper Cretaceous of Hokkaido, Japan, computer simulations based on a newly developed hydrostatic model with hypothesizing negative buoyancy were carried out. The adequacy of restorations was tested by the inclination of ribs which depends tightly on the life orientation of the ammonite during growth. The results of computer simulations suggest that a rapid upside-down of the shell would take place at about one and a half whorls after the shell apex when the ammonite had a small and light phragmocone. This stage corresponds well to the changes in rib inclination observed in the actual specimens, whereas it is significantly late comparative to the previous calculation with hypothesizing neutral buoyancy. These results suggest that E. muramotoi was negatively buoyant and probably had a nektobenthic mode of life. Since the life orientation is essential information for living animals, ammonites would have used it as a "guide" for morphogenesis. The establishment of the new technique covering negatively buoyant shells enables us to make some farther computer simulations in order to understand the meanings of ammonites' shell forms.