EXTRAPOLATION TO THE POST-MORTEM DISTRIBUTION OF AMMONITES

Reyment (1958) presented a review of the opinions of earlier workers on the taphonomy of fossil cephalopods. A brief resumé of these notes is given below, granted that my original monograph is not widely available. Walther (1897, p. 258) realized the importance of the problem under review as did, later, Lehmann (1964, p. 14). Walther referred to, in his opinion, the inexplicable lack of ammonite soft parts allowing that impressions of body parts are known of belemnites and other cephalopods and was led to assume that empty ammonite shells could float for a period of time (Walther 1897, p. 261, 262). Rothpletz (1909) monographed the fossils of the Solnhofen beds. In this work he observed that aptychi are present in the body chambers of some of the ammonites and concluded that such occurrences represent early burial of individuals in a tidal flat environment. Further discussion of Rothpletz' observations can be found in Reyment (1958, pp. 103-104). Emulating Rothpletz' research, Reyment (1970b) collected field information on vertically imbedded ammonite shells of ceratitids as well as other forms, Choffatia, for example, and ventral impressions in sediment (cf. Chamberlain and Weaver 1978). A specimen of Nautilus macromphalus from New Caledonia received in 1957 from Dr. F. McMichael of the Natural History Museum of New South Wales, and collected in 1882, was still poised upright in formalin in its container when it arrived in Stockholm (Reyment 1958, p. 134) .

Reyment (1973, 1980) constructed models of ammonites to examine their post-mortem spreading capabilities. Models of Ceratites nodosus and Discoceratites sp., based on material from the Triassic of Main-Franken, Germany, were found to float higher in salt water than shells of Nautilus spp. Highly evolute ammonites were modelled on Morphoceras (Middle Jurassic) and Dactylioceras (Lower Jurassic). The empty shells of such forms float horizontally oriented, like a floating pancake as it were. In life, the shell floats vertically and is noticeably static with a marked moment of inertia. By the courtesy of the Trustees of the Swedish Natural History Museum, Stockholm, the text of Reyment (1973) can be accessed at the Museum site. Scott (1940) proposed a bathymetrical zonation based on the shape and ornament of Texan Cretaceous ammonites. His ideas were based on the assumption that nekroplanktonic dispersal of empty shells would have been an exceptionally rare event. Scott seems to have been largely unaware of observations on living nautiloids. Occasionally, Scott's Texan model shows up in general textbooks on palaeontology by "non-experts" but for the most part it has been consigned to oblivion. Arkell (1949) was uncharacteristically ambivalent with respect to the post-mortem dispersal of ammonite shells. He thought (1949, p. 408) that on death, shells sank to, or remained on, the bottom and stayed there. Further on in the same publication, however, he (Arkell 1949, p. 413) contemplated the wide dispersal of many species. Later, Arkell (1952, p. 15) changed stance entirely and concluded that drifted shells of Jurassic species ended up as part of stranded material along the shoreline, mixed with plant and vertebrate remains. The Jurassic locality at Brora (Scotland), interpreted for me in 1968 by Professor Michael House, is particularly informative; here kosmoceratid ammonites occur abundantly together with coalified wood (Reyment 1970b). Haas (1949, p.7) is a further useful reference for examining the extrapolation of observations on living Nautilus to ammonites. He noted that relatively compressed acanthoceratid shells occurred together with fossil wood, here and there charred, in the mid-Cretaceous near Greybull, Wyoming. This type of association mirrors what is found in the Pacific Ocean in the cast up flotsam on the shorelines of atolls. Similar occurrences can be observed in the Jurassic of southern Sweden (Skåne), as for example, at Katslösa.

The experimental studies reported in Reyment (1958) showed that the single most important factor among all interacting variables for deciding the post-mortem buoyancy capabilities of a cephalopod shell is the length of the body-chamber. The shape of the shell comes next in importance. A graphical procedure for appraising the buoyancy for ammonite and nautiloid shells was introduced by Reyment (1973, figure 32) using a multivariate Q-mode graphical procedure known as Principal Coordinates (Gower 1966). This figure was constructed using data on 42 species of coiled nautiloids and ammonites of a variety of shapes. This chart supplies a means of estimating, approximately, the likelihood of a coiled shell possessing a good nekroplanktonic dispersal capability. The statistical distances between "points" yield information on degrees of morphological similarity.