Is cephalopod septal strength index an index of cephalopod septal strength?

Nautiloid septal strength: revisited and revised concepts

“…Since it is not yet entirely clear whether Nautilus shell has flooded chambers in the egg, it would be rash to enter the debate staged by Chamberlain & Chamberlain (1986). In contrast to juvenile and adolescent septa, embryonic septal spacing and thickness do not fit the optimization originally calculated for growing phragmocone (Westermann, 1977;fig.…”

“…Since septal thickness and curvature radius increase at the same rate, isometry and the strength index are maintained and buoyancy remains constant (Westermann, 1977). No allometry of septal thickness is required as alleged by Chamberlain & Chamberlain (1986). Both the ALCHERINGA NAUTILOID SEPTAL STRENGTH 125 thickness and the curvature radii are linked to undetermined external environmental controls causing illustrated variation in laboratory Nautilus (Spaeth & Hoefs, 1986).…”

“…There is no question of a tensile strength of 131 MPa being significantly less than the maximum tensile strength of nacre. The implosion depth formula for nautiloids is therefore as follows: This equation also differs from that given by Chamberlain & Chamberlain (1986) in taking account of the excess pressure required to hold a vacuum in the most recently completed nautiloid chambers. This only becomes significant in orthocones adapted for life at tess than 100 m water depth and should be ignored during the analysis of implosion test data on dead Nautilus with complete septal thicknesses.…”

“…Denton (1974) and Westermann (1973) have proposed that a somewhat nebulous 'safety factor' is required to translate observed or calculated implosion depths into estimates of the maximum habitat depth limit of particular extant genera such as Spirula and Nautilus (ratio 1.3:1 being proposed originally). Chamberlain & Chamberlain (1986) correctly note that the septal strength index of Westermann (1973) assumes that the 'septa are truly spherical in the engineering sense', i.e., thin shells in a state of tensile membrane stress forming at least part of a spherical 0311/5518/88/010123-6 $3.00 © AAP curvature. This situation applies to many orthoconic nautiloids where the index provides a useful shorthand for comparing the relative bathymetry of fossil species.…”

“…Recent critics of the nautiloid septal strength index concept of Westermann (1973) and the application of this approach to ammonoid septa, have accepted this proposition, while doubting the precise nature of the balance between shell strength and ambient water pressure (e.g., Henderson, 1984;Chamberlain & Chamberlain, 1986). Denton (1974) and Westermann (1973) have proposed that a somewhat nebulous 'safety factor' is required to translate observed or calculated implosion depths into estimates of the maximum habitat depth limit of particular extant genera such as Spirula and Nautilus (ratio 1.3:1 being proposed originally).…”