Understanding a Campanian Cretaceous Rocky Shore Ecology From Sweden

Rocky shore taphonomy—A comparative study of modern and Late Cretaceous analogues

Authors:

Sørensen et al

Abstract:

Rocky shores are rare in the fossil record due to erosion under both sea-level rise and fall. In contrast, modern rocky shores are well-studied, but little is known about the evolution of their ecosystems due to the rarity of ancient counterparts. Reconstruction of these ancient ecosystems is thus essential to get an insight into their evolution. A high-diversity Late Cretaceous (Campanian) rocky shore fauna is found in southern Sweden. The original composition of the shelly fauna cannot be interpreted by direct examination of the preserved fauna due to the effects of taphonomic processes. Life and death assemblages from a modern rocky shore fauna from Thailand have previously been analysed and a hypothetical fossil assemblage was reconstructed in order to attempt an interpretation of the Campanian life assemblage. This study shows a low taxonomic agreement between the original Campanian life assemblage and the fossil assemblage, due to taphonomic processes, and high environmental fidelity with only a few out-of-habitat species represented. The modern life assemblage showed in an earlier study, a high loss of species before onset of fossilisation. This suggests that the faunal composition of the Campanian life assemblage cannot be easily reconstructed, and time averaging by generations of death assemblages makes this even more difficult. The Campanian aragonitic fauna is poorly represented and the rarity of moulds after aragonitic species is interpreted as due to taphonomic processes and not to lower richness of aragonitic species in the Cretaceous. This is supported by comparison with the high richness of aragonitic species found on a Late Cretaceous rocky shore in Germany. An originally high-diversity gastropod fauna is thus interpreted to have dominated the intertidal zone in the Campanian example, and the rare moulds of each of the aragonitic species indicate a high taphonomic loss in spite of rapid burial. Calcitic species-richness is higher in the Campanian fauna than in the modern life, death, and constructed hypothetical fossil assemblages. This is interpreted as reflecting time averaging of generations of calcitic species and low loss of calcitic species by taphonomic processes in the Campanian fauna. It is thus assumed that the original Campanian fauna experienced a change in faunal composition from a gastropod-dominated life assemblage to a bivalve-dominated fossil assemblage due to dissolution of aragonite and excellent preservation of calcite. Reconstruction of ancient rocky shore shelly faunas can thus be considerably improved by comparison with analogous modern rocky shore faunas.