Strategies of survival during extreme environmental perturbations: evolution of conodonts in response to the Kellwasser crisis (Upper Devonian)

The size and shape evolution of two conodont genera surviving the Kellwasser period, Icriodus and Palmatolepis, has been quantified using a Fourier analysis of the outline of platform elements. Morphological variations of both genera in response to environmental fluctuations are comparable, although slightly shifted in time, suggesting a differential sensitivity to the environmental variations due to differences in favored habitats. The period of the crisis comprises two anoxic events of increasing importance and impact on the biosphere. For each event, the post-crisis conodont elements are smaller than those found before and exhibit extreme morphs, while the recovery period is characterized by an increase in size and inverse changes in shape; morphological trends are expressed with a lower amplitude during the first anoxic event. Strategies of morphological response are different for the two genera, implying heterochronic shifts or changes in the developmental pathway.