In this paper, we demonstrate that Buckman’s law of covariation, describing the cases of extreme variability observed in ammonoids, can be explained in a simple way by analysing the internal shell geometry. This geometry can be characterized by the amount of lateral and ventral curvature of the shell which controls the thickness of the mantle and the concentration of morphogens present in the shell-secreting epithelium. The most salient ornamentation is present where the whorls are most curved, shells with slight angular bulges often being spinose or carinate and flat ones being almost smooth. These observations agree with a morphogenetic model based on Meinhardt’s reaction – diffusion mechanisms [Meinhardt 1995].

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