The problem of growth rate and life span of Nummulites foraminifers, attaining giant sizes during the Eocene, has been addressed by analysing their Sr/Ca ratio across the Eocene/ Oligocene Boundary (EOB) of Kutch, western India. The Eocene ratio (∼ 1) rapidly decreases during the Oligocene (∼ 0.5) and is coincident with the extinction of most of the Eocene Nummulites species, a rapid enrichment of δ18O and decrease in both test size and species diversity across the boundary. The high Sr/Ca ratio in Eocene foraminifers can be explained by their rapid growth under a favourable climatic condition. The climatic deterioration (e.g. δ18O cooling) across the boundary and during the early Oligocene possibly forced the Nummulites to adopt a slower growth rate (and stunted growth). The rapid growth of the Eocene Nummulites indicates that the giant sizes of these protists need not necessarily involve a large life span.
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