The chondrichthyan faunal evolution across the Cretaceous/Tertiary-boundary, have been analysed from collected fossil shark teeth from Stevns Klint, Faxe Kalkbrud and a few other localities in Denmark. The teeth have been collected from the upper Maastrichtian Chalk, the K/T-boundary clay Fiskeler, the early Danian Cerithium Limestone and the middle Danian Bryozoan Limestone.
Furthermore oxygen isotopes extracted from the teeth revealed the temperature development across the boundary and yielded indications on paleobathymetry. The chondrichthyan fauna at Stevns Klint during the Late Maastrichtian was very diverse with at least 27 different species. A major reduction of fauna diversity took place in the uppermost Maastrichtian marl, reducing the fauna to only four species prior to the impact layer of the boundary clay. During the deposition of the Fiskeler, the chondrichthyan diversity started to increase and the majority of the species present in the Maastrichtian Chalk is also found in the upper Fiskeler (layer V) and in the Cerithium Limestone. Only nine species went extinct (33%) which is in deep contrast to the Moroccan localities, where almost 96% of the species went extinct. The oxygen isotopes extracted from the shark teeth have revealed that the sea surface temperature fell slightly prior to the boundary and
may have declined almost four degrees centigrade across the K/T-boundary. Subsequently the temperature rose during the deposition the upper layer of Fiskeler and the Cerithium Limestone. The striking fall in shark diversity prior to the boundary cannot be explained by a bolide impact and must have another explanation. This could have been caused by a sudden change in bathymetry. The calculated temperature range between sea surface and bottom water temperatures could suggest so. The extinction event which apparently almost devastated the low latitude chondrichthyan fauna in Morocco cannot be supported at Stevns Klint. The claimed major extinction event which has been assumed to have impacted the chondrichthyans at the K/T-boundary is here assumed to be primarily a result of the analysing technic used rather than a real extinction event. With the present data it is not possible to conclude whether the extinction event was a result of an instantly event or happened gradually, but the impact on the chondrichthyans was much less severe than previously assumed. By the mid Danian the shark fauna had changed radically from the uppermost Maastrichtian – lowermost Danian fauna presumably as a consequence of a change in bathymetry.