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THE ROLE OF DEEP-WATER OCEANIC CURRENTS
Reyment (1958, pp. 158-160) investigated experimentally the effect of temperature and salinity stratification on the buoyancy of shells. Briefly summarized it could be shown that even a difference of 2-3oC was sufficient to either bring an almost waterlogged shell to sinking condition or, conversely, raise a partially waterlogged chambered shell from the bottom. Thermal stratification in modern oceans is well known (Münster-Strøm 1936) in which a treatise on the subject of salinity reaction currents was also treated.
The potential for submarine dispersal of shells by oceanic currents is probably great. In one experiment (Reyment 1973), it was found that a shell poised upright on the floor of the experimental tank responded quickly to even slight currents, being lifted from the bottom and sent skimming. Experiments on partially waterlogged shells by
Wani and Ikeda (2006, p. 291) confirm
Reyment's (1973)
findings obtained using the flumes in the Department of Physical Geography at
Uppsala University. Photographs of experimental results are given in
Reyment, 1973.
Seilacher (1963, p. 599) deduced that something of the kind could have happened with fossilized ammonite material he studied but he invoked an unlikely mechanism of "bowling" i.e., shells being bowled along by currents as hoops in the manner of a popular children's game. The experiments accounted for in
Reyment (1973) also showed that the more evolute shell of N. macromphalus, when propelled by a current over a bottom of unconsolidated sand, tends to fasten owing to the braking effect of sediment scooped into the body chamber. It requires a strong current to cause the shell to move all. The ammonite models of
Reyment (1980) of the types Paravascoceras, Pseudaspidoceras, Dactylioceras, when poised on the bottom have the aperture raised further upward than the above-mentioned species of Nautilus, implying that a dragging motion over bottom sediment would have been less likely to be hindered than in the case of N. pompilius.
 Related physically and hydrostatically to thermal stratification there is salinity stratification and reaction currents (Münster-Strøm 1936). Experiments showed that a waterlogged shell placed in saline water of two densities, less saline above and normal saline below, sank until it encountered the denser layer where it floated stably on the salinity interface (Reyment 1958). A detail of an experiment on buoyancy at a saline interface is shown in
Figure 4. Post-mortem floating of chambered shells along a salinity gradient has yet to be explored in Nature. The salinity reaction current is a possible mechanism whereby sunken shells can be transported far inland into a bay owing to the outflow of freshwater. A classical site is the marine shell-mounds at Uddevalla in Bohuslän, west Sweden, where marine life was moved far inland by reaction currents, coming to rest beneath freshwater (Hessland 1943). The combined factors of thermal and salinity stratification coupled with the deflecting effects of sea currents takes the credibility of the "crash-dive" model of submarine maneuvering for nautiloids out of consideration.
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