Abstract

Well preserved nannofossil assemblages in upper Albian and lower Cenomanian hemipelagic sections from Ocean Drilling Program (ODP) Leg 171B preserve a record of the early history and adaptive radiation of the genus Eiffellithus. Seven distinct taxa are recognized and differentiated, with one previously named taxon (E. monechiae) emended and four new species described: E. praestigium, E. vonsalisiae, E. equibiramus, and E. parvus. Sample census data indicate that most species evolved, rose to dominance, and then rapidly declined to extinction, to be replaced by other members of the genus. Newly evolved species tended to remain at low abundance levels until a significant disruption in the pelagic realm resulted in the precipitous decline of the dominant species. This decline provided open niche space in the pelagic realm into which the new species could rapidly rise to dominance. These major disruptions correspond to significant changes or shifts in the sedimentological and carbon isotopic records associated with the late Albian Oceanic Anoxic Event (OAE-1d), suggesting that major changes in the strength of deep mixing and the structure of the surface water mass drove the early adaptive radiation of Eiffellithus.

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