Theoretical morphology of Nipponites : a heteromorph ammonoid

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  • 異常巻きアンモナイト Nipponites の理論形態
  • イジョウ マキ アンモナイト Nipponites ノ リロン ケイタイ

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Abstract

Nipponites Yabe, 1904, is a Cretaceous ammonoid genus conspicuous for its meandering whorls. The coiling pattern appears very complicated but it is never irregular, as was pointed out by Yabe (1904) and Matsumoto (1977). On both theoretical and empirical grounds, I attempt to model the coiling pattern and to simulate some actual specimens of this genus by computer graphics. Such theoretical-empirical approaches are, I believe, effective not only for an understanding of coiling regularity but also for the quantitative evaluation of intraspecific variation and interspecific differences among heteromorph ammonoids. It is anticipated that they will be informative for taxonomic and functional considerations. The coiling geometry of three species (including one variety) of Nipponites, except for their helical early growth stage, can be expressed by a common function, which is fundamentally an exponentially amplifying sine curve in an equiangular spiral roll (see Figs.2 and 6). Some slight modifications are, however, necessary to realize a satisfactory simulation (see Figs.7 and 8). Nine coefficients were estimated on the basis of measurements and allometric analyses of actual specimens. These may be used to express the variability of whorl shape among and within species. A number of projections from various angles have been obtained by means of a micro-computer (e.g. Fig.9). They are successfully similar to the actual specimens of Nipponites mirabilis, N. mirabilis "var. sachalinensis", and photographs of specimens of N. occidentalis. It is important to note that the calculated values of six of the nine coefficients that determine the basic pattern of coiling are very stable for all of the specimens. On the other hand coefficients for 1) whorl expansion rate, 2) strength of whorl meandering, and 3) position of critical point between the early and middle-late stages (which strongly affects the "space occupying ratio") are greatly variable among these species and varieties of Nipponites.

Journal

  • Fossils

    Fossils 36 (0), 37-51, 1984

    Palaeontological Society of Japan

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