The buoyancy, stability, and orientation of a shelled cephalopod in water are the predictable products of shell geometry, body chamber length, and such physical parameters as shell, tissue, and water densities. Given such physical characteristics as shell geometry, shell, tissue, and water densities, and shell thickness, the hydrostatic characteristics of planispiral shelled cephalopods, including orientation, centers of mass and buoyancy, stability, and neutrally buoyant body chamber length, can be calculated and simulated using microcomputer-based techniques. Individual variables such as geometry, body chamber length, and shell thickness are linked in a calculable manner to orientation, neutral buoyancy, and stability. Living Nautilus provides a means of testing the model and for making hydrostatic comparisons between ammonoids and nautiloids. The close agreement between calculated versus observed body chamber lengths in five species of Mississippian ammonoids shows that neutral buoyancy, and (with one exception) Nautilus-like orientations, were at least feasible for these species.

Morphologic analysis of 281 species of ammonoids from Great Britain, the North American mid-continent, and the South Urals, at eight successive levels within the Namurian Series (ca. 18 Myr duration), using bivariate plots and principal-components analysis, permits definition of morphologic diversity and identification of morphotypic patterns in time and space. Namurian ammonoids exhibit the same general range of shell geometry that characterizes ammonoids as a whole; there were few post-Namurian innovations in the basic geometry of planispiral ammonoids. Within this overall range of geometry, there are eight preferred morphotypes: two were phylogenetically monopolized by long-ranging forms; three were generalized and reoccur in successive horizons; two others were homeomorphically utilized at different times by different lineages; and one represents morphologic innovation followed by radiation. Such patterns seem to represent combined effects of function, phylogeny, and ecology. Synchronous variations in isolated successions suggest global controls such as eustatic sea-level fluctuations, whereas provincial differences in diversity may be attributable to paleogeographic and ecologic factors. We predict that the Namurian record of ammonoid morphologic diversity and change will be found to be distinctive and differentiable from earlier and later intervals.

The evolution of septal complexity in fossil ammonoids has been widely regarded as an adaptive response to mechanical stresses imposed on the shell by hydrostatic pressure. Thus, septal (and hence sutural) complexity has been used as a proxy for depth: for a given amount of septal material greater complexity permitted greater habitat depth. We show that the ultimate septum is the weakest part of the chambered shell. Additionally, finite element stress analyses of a variety of septal geometries exposed to pressure stresses show that any departure from a hemispherical shape actually yields higher, not lower, stresses in the septal surface. Further analyses show, however, that an increase in complexity is consistent with selective pressures of predation and buoyancy control. Regardless of the mechanisms that drove the evolution of septal complexity, our results clearly reject the assertion that complexly sutured ammonoids were able to inhabit deeper water than did ammonoids with simpler septa. We suggest that while more complexly sutured ammonoids were limited to shallower habitats, the accompanying more complex septal topograhies enhanced buoyancy regulation (chamber emptying and refilling), through increased surface tension effects.

This study confirms earlier findings of minimal overall differences in symptom frequency between African-American and non-African-American community-dwelling older adults in controlled studies.