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Finite-element analysis of circular septum models indicates that (1) anticlastic fluting weakened the last septa of the same radius of curvature by a factor of about 2.5 relative to the tensile stresses in a sphere of nacre, (2) septa with ammonitic sutures were stronger than those with goniatitic sutures of the same thickness, and (3) septa with more “complex” ammonitic sutures were stronger at the edge between lobes and saddles than “simple” ones. These results contradict recent claims that ammonoid septa became weaker as sutural complexity increased from goniatitic through ammonitic, so that the most complex sutures were limited to the shallowest habitats. The smaller marginal flutes of complex septa were relatively strong, allowing them to be thinner than the central septum and still act as elastic wall supports. Many Mesozoic ammonoids with highly sinuous sutures occurred in deep epeiric and open-ocean habitats, whereas it is those with secondarily reduced, ceratitic sutures that were typically associated with restricted shallow basins.

Westermann, G. E. G.: Strength of concave septa and depth limits of fossil cephalopods. 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. Assuming that actual habitats ranged to approximately two‐thirds of the mechanical limits of the shells, the following maximum depth ranges are indicated from this preliminary survey: Endoceroidea 100–450 m; Actinoceroidea 50–150 m; Nautiloidea, Ellesmerocerida 50–200 m, Orthocerida 150–500 m, Oncocer‐ida <150, Discosorida <100 m, Tarphycerida <150 m, Nautilida 200–600 m; Bactritoidea c. 400 m; Coleoidea, Aulacocerida 200–900 m, Sepiida 200–1000 m, Belemnitida 50–200 exceptionally 350 m.

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