The function of the ammonite fluted septal margins

Abstract

The fluted margins of ammonite septa were thought to resist the hydrostatic pressure upon the phragmocone while the ammonoid dived. However, ammonoids probably did not dive deeper than the extant nautilids, whose conchs, with the simple septa, sustain pressure correlative to depth of about 800 m. The backward and forward stretching lobes and saddles actually provide resistance to pressure perpendicular to the septum. Ammonoids lived for about three to five years, and septa were precipitated in intervals of nearly two weeks to two days, which explain the small dimensions of the scars of the adductor muscles, which were periodically detached and reattached. The weak hold between these small muscles and the buoyant conch was compensated for by the backward branching and expanding folds (forming the sutural lobes), into which the soft tissue penetrated and stiffened for a required period to firmly anchor the body to the conch throughout its whole circumference. The greater the complexity of the septa marginal fluting, the better the ammonoid could withstand the dragging force between the body and the buoyant conch, and hence the more aggressively the ammonoid predated and competed with other creatures.