Palaeoecology and survival strategies of an impoverished benthic chalk community in a mid-Danian deep shelf setting

摘要

Upper Cretaceous chalk is characterized by a rich and diverse benthic fauna followed by a poor benthic fauna in the lowermost Danian immediately after the K/Pg extinction event. In the Early to mid Danian diversity increased and rich bryozoan - and cold water coral mound ecosystems were established in localized areas. In the contemporary mid Danian Dalbyover deep shelf chalk, the benthic fauna is markedly impoverished. The Danian macrofauna from bulk samples at Dalbyover comprises 22 species and several bryozoan morphogroups, and is of low diversity and density compared to latest Maastrichtian and mid Danian shelf faunas. Average Fischer Alpha Index is 2.34 and Shannon Index 1.38 with minor stratigraphic variations. Guild analysis of the macrofauna indicates that the vast majority of species show special adaptations to survival at and in the uppermost few centimetres of the soft substrate with dominance of root-attached species attaching to small substrates and free lying suspension feeders. Further, the morphology of the serpulid tubes and the occurrence of crinoids in situ reflect a stable substrate with little overturning by the infauna. The benthic foraminiferal assemblages are dominated by low trochospiral and planispiral epifaunal morphogroups, whereas elongated forms with an infaunal mode of life are rare. The combined macrofauna and foraminiferal data indicate deposition in a relatively stable environment where food was a limiting factor. For the macrofauna, it is evident that species occurring in small numbers are long ranging and also known from the Cretaceous, whereas species occurring in high numbers are known solely from the Danian. Evidently, the Mesozoic survivors had the strategy to survive in this special chalk environment where only few Danian species were successful.