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Here we go again. Which 'Keller & MacLeod paper are you referring to? The recent Paleobiology paper was MacLeod and Keller. In Fig. 4A of the MacLeod and Keller paper species 7-16 exhibit last appearances in the uppermost Maastrichtian interval, species 17-25 exhibit last appearances in Zone P0. The simultaneous disappearance of species 21-25 is due to a hiatus that has removed the top of Zone P0 and and the bottom portion of Zone P1a. Species 26-28 exhibit last appearances in the lower part of the overlying Zone P1a. I take it you want to know why these species cannot be considered K/T victims? The simple answer is that I don't see any data forcing that conclusion. Once again, your proposed interpretation is based on a perceived failure to prove a negative statement. As Una Smith pointed out, it's easy (and I think more appropriate) to turn such statements around and ask what positive evidence exists that corroborrates your hypothesis. Your comment does not provide any. If anyone out there has a specific hypothesis that would allow John's scenario to be tested and an unambigous decision on this question to be made, please post it. Until then I'll stick with my interpretation of no Brazos planktic foraminiferal extinctions being associated with bolide impact or proved beyond reasonable doubt to result from bolide impact. I agree entirely with Una on the question of post hoc rationalization of long term biotic effects with respect to any proposed impact event. The existence of such long term effects tells you nothing about the duration of the causal mechanisms. You might suspect that they were short term but the problem is proving it. If you can't do this, the rest of us are quite justified in either reserving judgement or supporting an alternative interpretation. Statements like "I have no problem with..." and "I think this is generally reasonable" are fine. They just don't convince me (and I don't think I'm being unreasonable). Nothing that I have seen in this latest discussion rules out a more progressive extinction mechanism and I think that the data I've published over the last few years makes an interesting case for it. But I'm willing to be convinced otherwise. The effects of the proposed K/T impact were not global in scale. Rosemary Askin's data, Kirk Johnson's data, and Gerta Keller's data all show increased survivorship in the high latitudes. Dave Archibald's data on lower vertebrates from middle lattitudes does not corroborrate predictions of some post-impact atmospheric models. Hypothesis testing is a two-way street and results like these should provide sufficient grounds to suspect that some of the various impact-generated extinction scenarios (they are not specific enough to be called hypotheses) should be modified. Finally, this is an instance where parsimony is in the eye of the beholder. In phylogenetic analysis parsimony is determined by the number of ad hoc hypotheses required to account for the observed data. Admittedly under a progressive extinction model a separate ad hoc hypothesis is required to account for each observed extinction. However, under the impact scenario this is also true. Saything that the impact did it (somehow) is, to my way of thinking, not specific enough to count as a sufficiently detailed explanation. Moreover, you are saddled with the additional ad hoc hypotheses required to explain the lack of association between the event horizon and the various observed LAD's. Both models are at the very least equally parsimonious, which is to say not very parsimonious at all. In my opinion we've beat this topic pretty much to death for now. We have uncovered some significant areas of agreement on both data and interpretation and that alone is much more than I had originally hoped for. I suggest that we all collect more fossils, do more tests, write some more about the K/T event, and then revisit this debate at some point in the future. If we can agree that we must take a hypothesis-testing approach to the analysis of mass extinction events then we can indeed conclude this on an amicable note. Norm MacLeod
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