Strength of concave septa and depth limits of fossil cephalopods

Abstract

Simple septa with spherical curvature are present in the shells of all Endocer‐oidea, Actinoceroidea, Bactritoidea, and most Nautiloidea and Coleoidea. Such septa act as quasi‐hemispherical concave membranes when subjected to hydrostatic pressure. Since the tensile strength of a spherical membrane is directly proportional to the ratio of its thickness and radius of curvature, measurements of these parameters on polished and thin sections of septa can be used to obtain strength of the septum against implosion. Depth limits of fossil cephalopods can be made by calibrating these measurements in terms of recent implosion data on ‘living’Spirula and Nautilus. Estimates of septal strength are augmented by strength estimates for long septal necks and cylindrical to globular connecting rings.