The phenomenon of non-random or selective survival across major extinction boundaries in the geologic past is poorly understood but increasingly recognized as a critical area for future research. A current hypothesis, developedfrom a comparison of extinction patterns among Late Cretaceous molluscs, is that biological adaptations of organisms effectual during normal times of earth history are ineffectual during times of mass extinction. Microsampling of laminated biosiliceous marine sediments from the highlatitutde Alpha Ridge (-86? N lat.) provides evidence of an actively exploited biological adaptation by Late Cretaceous planktonic diatoms. Distinctive laminae reveal a life-history strategy comprised of alternating planktonic and nonplanktonic stages. Contrary to the hypothesis that mass extinctions are indifferent to biological adaptations, these data indicate a link between selection for traits in normal paleoenvironmental settings and differential survival during an unexpected episode of paleoenvironmental deterioration during a mass extinction. Planktonic diatoms, adapted locally to surviving periods of stress by leaving the plankton environment, may have differentially survived a global crisis at the end of the Cretaceous as a consequence of a life-history trait. The interaction of this proximate biological adaptation and the postulated end-Cretaceous extinction mechanisms may have played a major role in the unexpectedly high survival of planktonic diatoms, documenting a clear example of causal dependency between a biologic character selected for during times of normal or background extinction and macroevolutionary survivorship during times of crisis.