Adaptive radiation, survival and extinction of brachiopods in the northwest European upper cretaceous-lower paleocene chalk

Abstract

The Upper Cretaceous-lowermost Paleocene white, monotonous chalk of northwest Europe was deposited more or less continously through a period of at least 24 million years. It contains a rich, well-preserved fauna of minute brachiopods which comprises at least 80 species representing 28 genera and 14 families in the mid-Coniacian to Early Danian interval. In order to understand the adaptive patterns of this fauna, the time-specific diversities of the brachiopod fauna are broken down into taxonomic and environmental components. The brachiopod fauna apparently initially colonized the chalk sea bottom earlier than mid-Coniacian times probably as early as Cenomanian-Turonian times. From mid-Coniacian times, however, there was a slow build-up of both specific and generic diversity, whereas the number of families remained essentially constant. A climax in diversity was reached in the Late Maastrichtian. During the Coniacian-Maastrichtian interval mean rates of extinction and origination, and the percentage of turn-over are low for both species and genera. For families these rates are practically zero. A sudden mass extinction eliminated more than 70% of the species at the Maastrichtian-Danian boundary, more than 40% of the genera, but only about 1% of the families. A survivorship curve at the species level based on longevity history reveals an essentially linear trend indicating that new and old lineages became extinct indiscriminately through the mid-Coniacian to the top of the Early Danian, and that the mass extinction at the Maastrichtian-Danian boundary hit all lineages alike irrespective of their age. By comparing the variety of modes of life to brachiopod species richness at different times, major adaptive themes of the evolving brachiopod fauna and their degree of ecospace exploitation through time can be illustrated. Here are thus defined ten ecological groups of species each of which is defined without regard to their taxonomic composition. It appears that the main diversification took place preferentially in some ecological groups rather througout the Late Cretaceous, and that some groups rather than others were affected by the mass extinction at the Cretaceous-Tertiary boundary. Adaptive radiation took place primarily within a group of minute pedically attached mainly cancellothyridid brachiopods and within a heterogeneous group of secondarily free-living, hemispherical reclining brachiopods. These groups were also the most strongly affected at the Cretaceous-Tertiary boundary, the hemispherical reclining forms apparently became totally extinct and the minute pedically attached forms displaying very high rates of species turnover across the boundary. Environment apparently contributed only little to the large-scale evolution observed. Except for a complete, short-term cessation in chalk deposition at the Cretaceous-Tertiary boundary, the chalk sea represents an unusually stable environment through the iinterval studied. The fauna appears to have developed forms well adapted to the chalk already in Coniacian times. Species increase took place primarily by species packing within already existing ecological groups. The only major ecological novelty was the hemispherical reclining brachiopods which do not appear till later in the Late Cretaceous.