ORTHOCONIC NAUTILOID SHELLS

Among earlier workers, Lindström (1890, p. 4) recorded the remarkable accumulations of lituitid and ascoceratid shells in Gotlandian sediments, which he interpreted as having been built up by shells stranding in near-shore shallows. In his discussion he related the occurrences to the drifting of the shells of modern Spirula. Kriz and Bogolepova (1995) recorded the parallel orientation of Silurian orthoconic shells due to current action. Laufeld (1974) described the occurrence of orthocones displaying preferred orientation in the uppermost Hemse Beds (Silurian of Gotland), which he could relate to an assumed current direction. Additionally, Laufeld (1974, p. 159) deduced that the calcareous deposits in some of the chambers of the nautiloids is not a primary feature but one deriving from the precipitation of calcium carbonate resulting from the chemical interaction with decaying soft tissue during early diagenesis. (N. B. the post-mortem fate of nautiloids with massive calcareous deposits is not considered in this review.) Laufeld's conclusions with respect to the post-mortem stranding of shells were inferred by applying a procedure based on circular statistics and introduced into palaeontology by Reyment (1971). A work in the same vein is that of Gnoli et al. (1979) concerning the post-mortem distribution and preservation of Silurian orthoceratites from Sardinia. Here, also, the methods of circular statistics as employed by Reyment (1971) were used.

Reyment (1968) analysed occurrences of Rhynchorthoceras in the Ordovician limestones at Såtatorp. Västergötland, Sweden. Preferred orientations of shells were studied by the above-mentioned methods of circular statistics with emphasis on the location of the siphuncle the siphuncular diameter of these shells is about one sixth of the total diameter of the shell. The statistical tests showed that within narrow limits of variation, the nautiloids at Såtatorp are oriented with the siphuncle roughly in the 270 degrees position, and that this orientation is not random (Reyment 1968, figure 2). It was concluded that the shells drifted into unconsolidated lime-mud in a direction at right angles to the shoreline (Figure 2) where they became fixed, largely undisturbed by swash-action (Reyment 1968, 1971).

Preferred orientation of orthoconic shells is not a general rule. Reyment (1970a) studied a superficially similar occurrence to that of Laufeld (1974) in the Ordovician of the Brunflo area in Jämtland, Sweden. That work disclosed that the orthoconic shells were randomly oriented both with respect to the length-axis and, in some cases, to the diametrical axis. It was concluded that current action had not played a decisive role with respect to orientation during the preservation of those orthocones. In paleoenvironmental terms, the random spread of the orientation of the siphuncle is such that it would seem that the shells had been rolled by swash on a relatively unyielding substrate such as algal matting.

Reyment (1973) demonstrated by means of models, that the free-floating (nekroplanktonic) orientation of orthocones and cyrtocones was vertical due to the deadweight of the body chamber. It was concluded by extrapolation, that the orientation of living orthocones and similar uncoiled shell-types, would have been vertical and that the "torpedo-model" is most likely a fanciful construction. A vertically oriented Ordovician cyrtocone from the Såtatorp locality is depicted in Figure 3. This occurrence might be an example of the vertical orientation model deduced by Chamberlain and Weaver (1978) for the final resting phase of cephalopod shells. A note of the vertical floating orientation of post-mortem floating turrilitid gastropods by Krejci-Graf (1932) is an instructive record here. From the aspect of feeding economy, a vertical orientation of the orthoceratid shell and cyrtocones in life is the only reasonable foraging model (browsing) available from the vital viewpoint of energy conservation (Reyment 1988). By way of interest, it is worth mentioning that refloated "mummified" echinoid tests can, under certain conditions, be nekroplanktonically transported (Reyment 1986).