Brachiopod extinctions in the Upper Cretaceous to lowermost Tertiary chalk of Northwest Europe

Resumen

The white monotonous chalk from the Upper Cretaceous-lowermost Tertiary of Northwest Europe spans at least 24 million years and contains a rich, well preserved fauna of minute brachiopods. Based on taxonomical range charts and time-specific diversities this fauna is studied in terms of mean species longevity, rate of species origin and rate of extinction. The brachiopod fauna initially appears to have colonized the chalk sea in mid-Coniacian times. From there on there is a slow build-up of species diversity reflecting a gradual niche diversification of the chalk. A climax in adjustment and evolutionary stability is reached in Late Maastrichtian time. During the Coniacian-Maastrichtian interval mean rate of extinction is low (0.07 My-1) and mean rate of origination equally low (0.10 My-1). A sudden mass extinction at the Maastrichtian-Danian boundary, however, eliminated more than 70 per cent of the species. The most specialized species, in particular the secondarily free-lying species, apparently became extinct at the boundary. Only six species are known to cross the Maastrichtian-Danian boundary. These are all relatively featureless, non-specialized forms and gave, together with possible survivors in basin margin areas, rise to a rapidly formed highly diverse Early Danian fauna through adaptive radiation. The Early Danian fauna differs in taxonomic composition from the Maastrichtian fauna both on species as well as on higher levels. A dichotomous classification of extinction seems real for the brachiopods from the chalk as their mass extinction at the Maastrichtian-Danian boundary both quantitatively and qualitatively differ from their Late Cretaceous background extinction.