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

["Dewey M. McLean" <dmclean@vt.edu>]

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...."

Cordially,
Dewey McLean