The connecting rings of Nautilus and Mesozoic ammonoids: implications for ammonoid bathymetry

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Abstract
The fresh conchiolinous (horny) connecting rings of N. pompilius decrease ontogenetically in the strength index (100 .times. wall thickness h/inner radius r), i.e. from .gtoreq. 16 to .apprx. 12. A similar decrease of 100 h r is the rule in Ammonitina. This may reflect depth migration but, more likely, and in analogy with Nautilus, is compensated for by increased conchiolin strength. The mature ammonoid siphuncle provides the more reliable indicator for depth limits. No marked shrinkage of connecting rings is evident in fossil nautiloids, but Mesozoic nautilid connecting rings are rarely preserved. Postmortem alteration of the abundantly preserved ammonoid connecting rings entails mechanical distortion and fragmentation by differential filling of the siphuncle and camerae with sediment, and probable microbial decomposition, proceeding from the body chamber and resulting in pitting and perforation; shrinkages occurred rarely. This is probably related to the high phosphate content as documented for Haplophylloceras. If the phosphate prevalent in ammonoid connecting rings is a primary constituent, rather than of early diagenetic origin, calibration of the ammonoid strength index on the non-phosphatic Nautilus connecting rings may not be justified. The strength indices of ammonoid connecting rings would thus provide only a relative scale of depth limits. Another example of a phylloceratid, Ptychophylloceras, has been found with a connecting ring extending well into the body chamber.