The relationship between sutural complexity (estimated as fractal dimension) and body size is analysed for a sample of 204 genera of Cretaceous ammonoids. Although both characteristics are significantly correlated with each other, their evolutionary dynamics are opposite: i) the duration of genera is not correlated with body size but is correlated with sutural complexity and ii) throughout the Cretaceous there is an increase in the body size of the ammonoids in the sample but no increase in the complexity of their septa is detected. This indicates the difficulty of predicting macroevolutionary patterns for a trait on the basis of its correlation with other features on wide stratigraphic scales.
