Whorl Surface Expansion

The surface expansion rate is directly dependent on the WHER and WWER, and consequently, the trends observed in those characters can also be seen in the WSER plot (Fig. 3.6G). A prominent skewing is visible, but the box-and-whiskers diagram (Fig. 3.6H) illustrates an ontogenetic increase up to 10 mm conch diameter, followed by a reduction and a slight late ontogenetic increase.

The growth trajectories of the four selected specimens display striking differences. The curves of all four specimens perform, at different sizes, remarkable spontaneous fluctuations, visible in specimen 275 at approximately 8 mm conch diameter, in 218 at 40 mm, in 276 at 60 mm, and in specimen 219 at 70 mm.

The expansion rate of the whorl cross-section surface shows drastic ontogenetic changes in Manticoceras. In juvenile and preadult stages, only a slow increase in this rate occurs. Usually beginning with a value of 3.0 at 2-3 mm conch diameter, there is a more or less rapid increase, and a preadult maximum of WSER 6-8 is reached between 5 and 30 mm conch diameter (Fig. 3.6G, H). This maximum is followed by stagnation (specimens 219, 276) or a decrease (218, 275).

It is mainly the change of the whorl height that is responsible for the changes in the WSER. In the four specimens, the curve of the WHER largely parallels the WSER curve. WWER and WHER are seldom positively correlated. The WHER typically shows a more rapid preadult acceleration.

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