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

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Abstract
Septal strength index has become a prime means of establishing maximum living depths of fossil cephalopods with simple, concave septa. This method calculates palaeobathymetric limits from the septal geometry of Nautilus, based upon the premise that septa are the weakest structural .unit of the shell. We examined this proposal by evaluating data relating to ontogeny, septum formation, septal geometry, and microstructure of Nautilus. Our main results are: 1) septal strength index of embryonic and juvenile septa are not representative of the depths at which such specimens live, 2) septa become fully functional in resisting hydrostatic pressure at thicknesses significantly less than that used to compute strength index, 3) no correlation has been demonstrated between septal strength index and rupture pressure, and 4) shell microstructure does not show sufficient uniformity among cephalopods to permit an undocumented assumption of uniformity in mechanical properties of cephalopod shell material. These observations indicate that: 1) Nautilus is an improper paradigm upon which to base depth calculations for fossils, 2) strength index misidentifies septa as the weak link in cephalopod shell design, and 3) the strength index approach must be refined to accommodate the effects of sutural characteristics and other factors, before it can be regarded as providing accurate estimates for living depths of fossil cephalopods.