The last-abundant-appearance datum (LAAD) of the diatom Hemidiscus karstenii occurs in the upper part of oxygen-isotope stage 7 in the sub-Antarctic region of the Pacific and Indian Oceans. Strong fluctuations in abundance of this species below the datum level and the strong correlation between abundance peaks and radiolarian-derived sea-surface temperatures strongly suggest that this datum level is not an extinction. Rather it is an abrupt isochronous drop in abundance. It is suggested that this species may have become extinct during isotopic stage 6 when its abundances were low or that it retreated to some local niche where it persists to the present.

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