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On Fri, 5 Jul 2002, Williams, Tim wrote:
> Differential extinction is an observation: some groups went extinct,
> others didn't. Your use of the term 'targeting' (and 'surgical precision'
> in early posts) implies that the mechanism was highly selective in which
> groups it erased.
Yes. I believe it was.
> Sure, some groups survived, others didn't. Certain
> groups were certainly more vulnerable, just as others had traits that might
> have improved their chances of survival (small body size; lack of dependence
> on photosynthetic substrates; ability to descend into torpor; ability to
> endure protracted periods with little or no food).
And yet no such traits have been suggested for marsupials or
> But for those groups
> that did survive it might have been a close-run thing. Don't confuse
> survival (which is an observation of the fossil record) with invulnerability
> (which is an inference).
I agree. But the strength of an inference is dependent upon the strength
of the data. And this case, I claim, _no_ inference can be made. One
cannot say the extinctions were random, nor that they were
selected. Wouldn't you agree that if we found a trove of neornithine
ancestors of modern orders in pre K/T that this would affect
conclusions. What if we also found a decrease in enantiornithine
diversity pre K/T? My _feeling_ is that this will eventuate. And if it
does, it is not a healthy finding for the power of the bolide. What makes
me feel this? Simply: I don't buy global segregation of opposite and new
birds; and, if both clades enjoyed pre K/T diversity, I don't see a
weakness in enantiornithines such that it would predispose them to higher
extinction levels; and, if molecular estimates are right and modern orders
were represented pre K/T, the probability of zero to several by random
processes is small.
> ...your terminology implies (and
> occasionally says outright) that the K/T extinction was hit-and-miss: either
> you were affected or you weren't; and that some clades crashed to
> extinction, while others were immune.
I am just skeptical of estimates of the power global devastation on one
hand, and the inability of some clades relative to others to withstand
them. Clearly, when Lillegraven, Eberle, Currie, examine these horizons
and can hardly tell when one starts and the other stops...well...and, in
other locations, Horner looks and finds changes afoot earlier than dogma
suggests, and when Ir turns up in inappropriate levels...these may all be
justified in the end and brought in line with the bolide idea (like
Darwin's hypothesis for the formation of coral reefs!)...we'll see.
> Because lepidosaurs, eusuchians,
> champsosaurs, chelonians, mammals and neornithines all survived across the
> K/T doesn't mean they were immune, and therefore not 'targeted'. The entire
> 'targeting' thing (or 'blind selection') is beside the point.
Well, not really. Is Archibald's claim that placental extinction in NA
were almost all pseudoextinctions still supportable? When some creatures
in NA enjoy nearly 100% at the species level and other buy it
completely...then one has to begin to think of reasons, to make inferences
from this pattern.
> And the iridium spike at the K/T boundary? And the Chixculub Crater?
> Circumstantial, perhaps? Maybe something collided with the earth about the
> time that placentals decided to kick some marsupial ass, and the
> neornithines elbowed out the enantiornithines, and a whole bunch of marine
> invertebrates suddenly went to meet their Maker. This, if I understand you
> correctly, is your hypothesis.
Paleontological conclusions should be drawn from the totality of the
evidence. I'm just saying that I don't find the terrestrial vertebrate
evidence convincing. Attempts to make it so seem shoe-horning and resort
to circularity. Specifically, however, we have seen marsupial ass kicked
before...and we are witnessing marsupial ass being currently whupped in
Australia as we speak; and, we currently have no hypotheses for the
elimination of enantiornithines should it turn out that they were
selected. Indeed, other hypotheses enjoy better evidenciary
support: pterosaurs were practically gone by then and we need a non-bolide
targeting killer for them. It should have wings (because flying things
can take refuge from non-flying things), and it should be a new species
(otherwise it would have happened before). And, so I ask: would it change
your mind if it were KNOWN that neornithines played the leading role in