Buoyancy Control in Ammonoids

Abstract

A specimen of Buchiceras bilobatum is shown to have been able to maintain neutral buoyancy in water despite the ever increasing weight of epifaunal oysters which it carried on its shell. It is deduced that before settlement of the oyster spat, the camerae of the ammonite contained several grammes of water and that this was slowly pumped out as the oysters grew. Revised calculations on the densities of other ammonoids suggest that the presence of large quantities of liquid in cephalopod camerae was a common occurrence. The significance of this liquid in the growth, mode of life, and evolution of ammonoids is discussed.

Morphostructural constraints and phylogenetic overprint on sutural frilling in Late Jurassic ammonites

“…In addition, such a relationship operates within a range of depths shallower than usually envisaged, and with no major differences for Ammonitina and the majority of Phylloceratina in neritic and oceanic-epioceanic waters, Lytoceratina being a special case that is poorly represented in neritic waters during the Late Jurassic in Western Tethys. This interpretation is also in agreement with the modi ed Buckland's model for an increase in septal complexity in compressed versus rounded shells (Westermann 1966), as well as with hydrostatic pressure effects proposed by Heptonstall (1970), body movements during swimming (Jacobs & Landman 1993) and body anchorage (e.g. for improving aggressive predation; Lewy 2002).…”

“…Here we show the results from a numerical approach conducted on variables related to those considered more relevant by Westermann (1999) for addressing the 'suture problem': (i) whorl height of phragmocones (estimated at the end-points of the suture measured), which is a parameter more closely related to sutural complexity than whorl diameter, according to partial correlation data (Oló riz & Palmqvist 1995;Pérez-Claros 1999); (ii) taxonomic grouping (at the superfamily level, since most specimens analysed belong to the Order Ammonitina); and (iii) basic planispiral shell shape, as de ned by Westermann (1996), which was presumably related to lifestyle and habitat depth of ammonites. In our approach, the only exceptions for a precise evaluation of shell strength are direct measurements of thickness of shell wall and septa, which were not available in our database, although two observations apply here: (i) the precise thickness measurement in epigenized shells is not unequivocal, nor is the speci c gravity of the shell substance (Reyment & Eckstran 1957;Reyment 1958;Heptonstall 1970;Mutvei 1975), and (ii) stress-maximum values (which are equivalent for connecting rings and the whole shell in cephalopods; Ward 1987) do not provide information about the usual/preferred stress-eld in a biological structure. The health safety factor (Ward 1987) between inhabiting and implosion depths in Nautilus is an appropriate example of this.…”

Ammonoid Habitats and Life History

“…Maeda and Seilacher (1996) reported syn vivo and postmortem epibiont assemblages. The encrustation by inoceramids, oysters or Gervillia of both the flanks and venter in living ammonoids is remarkable (e.g., Jurassic Lytoceras and Harpoceras; Cretaceous Buchiceras ;Seilacher 1960;Heptonstall 1970;Seilacher 1982;Maeda and Seilacher 1996;Larson 2007). One-side-only infestation by oysters was also reported as being caused by horizontal drifting of ammonoid shells on the water surface (e.g., Pectinatites; Donovan 1989).…”

Cameral Membranes, Pseudosutures, and Other Soft Tissue Imprints in Ammonoid Shells

“…Taking the potentially large amount of chamber water (up to 30 % of phragmocone volume ;Heptonstall 1970;Mutvei and Reyment 1973;Reyment 1973;Ward 1979;Tajika et al 2014) in the phragmocone into account, water movement might have altered the orientation of the shell syn vivo. Cameral membranes would have limited the water movement within the phragmocone chambers and thus enhanced stability.…”