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Trying to falsify the bolide
I also agree with Dinogeorge that this thread has gone on long enough. I
beg the list to take seriously this final (from me) tangent.
Chris and Toby provided what I believe are the best-case terrestrial
scenarios for the bolide-as-sufficient hypothesis (albeit coupled with some
well considered ecological contribution). Both are dependent on an
assumption which is perhaps universally held, i.e., that suspended particles
caused the cessation or severe retardation of photosynthesis. An important
and obligatory corollary to this is that dust clouds also block heat. Under
these scenarios mammals survive because they are small and insectivorous and
insects are in continuous supply because detritus remains abundant. I will
now attack these assumptions.
Detritivory was not, in my view, sufficient to keep the global ecosystem
fueled. An important fact of detrivore life is that detritus rarely
accumulates (ref. if necessary). It is as valuable to organisms who eat it
as prey items are to us. There is competition for it, etc. One must assume
then that it must be constantly replenished. The stocks of detritus were at
regular levels when the bolide hit. And under cool conditions, decomposers
being completely at the mercy of temperature, no new detritus would be
Perhaps the main thrust of these scenarios is the valid idea that small
things need less absolute quantities of food and this would enable better
survival rates. But this also deserves scrutiny.
Many (most) communities are structured by their vegetational environment.
Significant plant death would result in defoliation. Without cover, mammals
would be at the mercy of larger predators. Yes, they might take cover in
burrows, except that, as yet, there is no evidence of fossoriality in K/T
mammals. This defoliation, if it occurred, would have much more serious
effects on bird populations--many of today's species, at least, depend on it
for predation protection. I believe the phylogeny of cross K/T birds is a
crucial test of the bolide hypothesis. I would predict, simply, that if
modern orders of birds made it across the boundary that the bolide didn't
do much of anything. Is this fair?
Another problem with bolide extinction hypotheses is that it they
underestimate the complexity of mammalian trophic relationships. There were
likely herbivores, almost certainly frugivores, omnivores, and possible
carnivory on larger animals (ref. if needed). No one is suggesting, I
think, that with all the fruit, leaves, and prey gone there will now be a
bumper crop of insects so that all can get by. Indeed, the opposite is
likely the case. Claims that insect populations would be maintained at high
levels are unsupported. Firstly, as noted above, detritus is limiting and
not replenished. Second, many (most) insects are herbivorous. Third, low
temps retard or stop insect activity, particularly fecundity.
This does not bode well for an entire ecosystem _more_ dependent on
insectivory than at other times!
Birds, too, have diverse diets (unless Feduccia?s sole K/T shore-bird
survivor cracking crabs proves true).
Bailing out altogether, hibernating, may work for a couple of mammal
species. There is, however, no evidence of hibernation and it is unlikely
to be seen in most mammals (if extant species are any guide at all).
Hibernation is not a general strategy for birds inasmuch as they usually
prefer to "fly South". One might argue that a dust cloud might be focused
around the tropics thus leaving a sub-arctic refugia for birds. Perhaps
some bird phyla could survive in this way. But not all (again, this is
dependent upon establishment of pre-K/T bird phylogenies which may never be
Extinction by bolide invoking animal _size_ requires very small tolerances.
A bolide may have effects as disparate as complete sterilization to pretty
sunsets. I have difficulty believing that the terrestrial effects of the
bolide, whatever they were, had such a precise cutoff point--exactly at the
intersection of large pre-K/T mammal-size and small non-avian dinosaur size.
I mean what are the chances of such precision? Surely non-avians
must be the unluckiest taxa ever. I argue photosynthesis cessation would
have effects that would reach into mammal communities as well; that we
should see less discrimination. I realize this is arguable.
Finally, Chris, at least, resorts to the well established trends (not
rules!) in business-as-usual extinction patterns, i.e., small, more numerous
animals are less likely to go extinct than large animals. Even he qualifies
this rule with "all other things being equal". But the bolide is not (it is
claimed) an ordinary extinction event. As such, the immediate application
of physical and biological forces of such an event is likely to be
drastically different from the forces acting at regular times and producing
"regular" extinctions! For example, a world-wide cold snap of short
duration might well affect small creatures more than large. And the
processes mentioned above (primarily the twin hits taken by detritivores and
insectivores) may well be more influential than forces producing "regular"