Article: Changes in life orientation during the ontogeny of some heteromorph ammonoids

To understand the mode of life of Eubostrychoceras muramotoi Matsumoto, 1967, and some other heteromorph ammonoids from the Upper Cretaceous of Hokkaido, Japan, their life orientation and growth patterns were restored using a hydrostatic model and differential geometry. The adequacy of these restorations was tested using the obliquity of ribs and its change during ontogeny. Rib obliquity parallels the aperture at every growth stage and corresponds well to the inferred changes of life orientation. The pattern of rib obliquity was also deduced from a computer simulation, programmed so that growth occurs only at the aperture which keeps a constant angle to the sea floor. In most heteromorphs, the computer-produced profiles approximate well to actual rib patterns. Although the rib obliquity of these ammonoids may appear to change somewhat capriciously, it must be functionally regulated. Except during the early orthoconic stage of growth, a floating or lightly touching benthonic mode of life is strongly suggested.