Résumé
Study of conodonts indicates a decrease in their size in response to the Upper Kellwasser crisis. This result can be interpreted as a progenetic effect or a dwarfism, which may result from a decrease in surface-water productivity during this event. The maximum of size variability is different for the deep-water and the shallow-water genera and may indicate the maximum of environmental stress. Finally, a delay in the response of the deep-water genera to the event may explain their difficulties in re-colonizing the environment after the crisis.