Benthic invertebrate communities as a function of sea-level fluctuations and hydrodynamics: A case from the Cenomanian-Turonian of Wadi Tarfa (Eastern Desert, Egypt)

Abstract

Understanding the response of the benthic invertebrates to long-term sea-level changes, which is possible only from the fossil record, is crucial for conservation biology. Based on integrated sedimentological and biostratigraphical data from the Cenomanian-Turonian succession of Wadi Tarfa (Eastern Desert, Egypt), a cyclic pattern, comparable to those of 2nd order sequence stratigraphy, between the community structure of the invertebrates and the sea-level fluctuations was revealed. Quantitative analysis of 1756 specimens representing 154 invertebrate genera encompassing mollusks and echinoid revealed that invertebrate communities can be classified into three different types. The first associated with the initiation of transgression (TST) and dominated mainly by opportunistic, epifaunal suspension-feeders, including gastropods and bivalves. This community is characterized by lower diversity values. The second community belonged to the maximum flooding zone (MFZ) and mainly dominated by infaunal deposit-feeders of irregular echinoids and bivalves and characterized by higher diversity. The last community is less diverse, dominated by few epifaunal-suspension feeders and occupying the late highstand system tract (HST). The lower diversity values during the early TST/late HST may result from the persistence of waves/the higher sedimentation rates. Consequently, only opportunistic taxa flourished. In contrast, diversity increases with flooding and is at a maximum and dominance at a minimum at the transgression peak (MFZ). Here, the high diversity, which is commonly augmented by time-averaging, most likely contributed also to the environmental stability, which increases with flooding. Accumulation of biogenic hardparts in combination with low rates of sedimentation may increase substrate heterogeneity and thus increases beta diversity (niche differentiation). The cyclic pattern of the community attributes results from the hydrodynamic setting accompanying the sea-level changes. The role of ecoenvironmental and taphonomic mechanisms in shell concentrations are also discussed.