Danish gastropod turnover across the K-T boundary: a tale of changing food supply

Description

The Cretaceous-Paleogene boundary in Denmark represents one of the most complete and biostratigraphically important successions of this time interval in the World. At that time the Danish Basin represented a narrow strait connecting the Protoatlantic Sea to the northwest with the Tethys to the southeast (Håkansson & Thomsen 1999). The sea bottom of the Danish Basin appears to have been at the base of or below the photic zone, but still the deposits contain a very rich shelly macrofauna of especially bivalves, gastropods, cephalopods, polychaetes and brachiopods in addition to anthozoans and spongies. This has made it clear that many of the groups not only survived the K-T extinction event but also experienced a marked resurrection as the carbonate deposition continued for close to another four million years (Håkansson & Thomsen 1999).

The gastropods, preserved as very detailed moulds in the early diagenetic hardground encompassing the Danish K-T boundary, are like most of the other groups characterized by a nearly complete faunal turnover. Thus preliminary results reveal that though a fauna of close to 40 species immediately below the boundary was replaced by one of nearly 50 species in the Paleocene Rødvig Formation and a second of at least five species in the succeeding basal part of the Stevns Klint Formation (above the hardground), no more than about five species actually crossed over from the Cretaceous. In contrast to many other fossil groups, the faunal changes among the gastropods are generally restricted to species level, indicating that the gastropods as such may have been less influenced by the catastrophic event. The changes in the gastropod fauna can to a large degree be explained by changes in food supply as deduced by their feeding strategies. Thus the late Cretaceous fauna was dominated by detrital herbivores and apparently also several species of algal grazers (on algal films), while the Post-Cretaceous fauna of the Rødvig Formation was completely dominated by hunting and grazing carnivores and some detrital herbivores. Algal grazers returned with the development of a hardground prior to the deposition of the succeeding Stevns Klint Formation.

Håkansson, E. & Thomsen, E. 1999: Benthic extinction and recovery patterns at the K/T boundary in shallow water carbonates, Denmark. Palaeogeography, Palaeoclimatology, Palaeoecology 154, 67-85.