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Re: extinction (gradual K-T transition biological turnover)

Dear Colleagues,

As we all know, considerable controversy exists on whether the K-T transition biological record is one of generalized gradual turnover over a long Late Cretaceous-Early Tertiary time interval, or abrupt extinctions of much of earth's life precisely at the K-T boundary. A paper by Norman MacLeod et al. (with 21 other authors) titled "The Cretaceous-Tertiary biotic transition" (Journal of the Geological Society, London, 1997, v. 154, pp. 265-292) demonstrates clearly that the record is one of gradual long-duration K-T transition turnover. This paper is the most excellent overview that I have yet seen on this topic.

The MacLeod et al. review spans a vast array of types of organisms: "calcareous nannoplankton, dinoflagellates, diatoms, radiolaria, foraminifera, ostracodes, scleractinian corals, bryozoans, brachiopods, molluscs, echinoderms, fish, amphibians, reptiles and terrestrial plants (macrofossils and palynomorphs)." It includes a wealth of detail on the history of individual groups--so much so that I cannot possibly abstract it. For those interested in the K-T extinctions, I highly recommend this paper.

I will include quotations from the paper's Conclusions:

"First, global events at the K-T boundary occurred within a longer period of sustained biotic change. This longer episode affected different groups at different times, but most often manifested itself as a progressive reduction in biotic diversity throughout the Maastrichtian."

"Second, a much shorter global biotic event appears to have taken place close to the K-T boundary. This event is most prominent among a few groups of marine microfossils (e.g. calcareous nannoplankton, planktonic foraminifera), which seem to have remained relatively unaffected by the long-term Masstrichtian decline....The extent to which this short-term, near-boundary event was influenced (or precipitated) by a bolide impact is uncertain. In most microfossil lineages, with the possible exception of calcareous nannoplankton, decline in species numbers begins prior to the occurrence of impact debris in various K-T boundary successions...."

Dewey McLean