Whorl Cross Section Shape and Flank Convergence Index

After a juvenile stage with an almost circular whorl cross section, an ontogenetic development toward a subtrapezoidal shape takes place during the fourth and fifth whorls. The differences between the specimens and their variability can best be illustrated by the four specimens UMDK 218, 219, 275, and 276 (Figs. 3.6G, 3.8).

Specimens 218 and 275 have similar whorl cross sections between 40 and 50 mm conch diameter. In later whorls, however, specimen 218 enlarged the whorl width remarkably, whereas specimen 275 conserved the preadult outline. Specimen 218 developed a broad umbilical rim while 219 became lenticular with a suboxy-conic venter in the adult stage, and the cross section of 276 grew subtriangular.

The calculation of the convergence of the flanks led to the surprising conclusion that this character is rather uninformative for the analysis of the sample. An inconspicuous ontogenetic change occurs in this character, with a decrease from approximately 0.80 in the initial stage to an average value of 0.70 at 7 mm conch diameter (Fig. 3.9C, D).

Later in ontogeny, this mean value is 0.65 with only minor variability. The only exception is the last whorl of specimen 219, which becomes strikingly triangular and thus has a data point outside of the main range.

Traditionally, separation of Manticoceras species was performed by using the outline of the whorl cross section. M. intumescens, for instance, is defined by its subparallel flanks (as in UMDK275), and M. cordatum possesses converging flanks and a narrower venter (as in specimen 219 up to 50 mm conch diameter). In the material from Coumiac, the traditional subdivision can hardly be applied; all the morphologies are connected by intermediate forms. As shown in Figs. 3.4 and 3.6-9, no clearly definable boundary occurs even between the most striking cross-section shapes. These cross sections shown in Fig. 4 demonstrate that even conchs of adult individuals within the investigated population do not allow clear distinction of morphologies, and adult conchs with similar geometries may possess inner whorls that differ remarkably in their morphology.

The results uncover the major difficulties in the systematics of the genus Manticoceras. The question to be asked is, how many of the 40 erected "species" are in fact justified? According to the study of the material from Coumiac it is obvious that a taxonomy which is based on the principal conch parameters (relative thickness of the whorls, shape of the whorl cross section, umbilical width) does not lead to reasonable results.

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Fig. 3.8 Ontogenetic trajectories (A, C, E) andintraspecific variability(B, D, F) in Manticoceras from Coumiac. The bivariate diagrams (A, C, E) show the ontogenetic development of the conch width index(CWI = ww/dm), whorl width index(WWI = ww/wh), and umbilical width index(UWI = uw/dm); highlighted are the specimens UMDK 218, 219, 275, and 276. The box-and-whiskers diagrams (B, D, F) show the ontogenetic variations ofintraspecific variability of the major conch ratios. Bold lines refer to the total range, boxes refer to the extension of the middle two quartiles, and white lines refer to the median value. Growth stages between 1 mm and 100 mm conch diameter were subdivided into 17 logarithmic intervals.

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