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On Wed, 3 Jul 2002, Michael Habib wrote:
> I still do not see how competition from placentals, predation from
> neornithines, and the like are viable explanations (in a direct sense,
> at least).
What I meant was: to make the case for mass extinction caused by a diffuse
agent, i.e., dust, sulfates, whatever, one must rule out alternative
hypotheses. At issue are two extinctions, local marsupials and
enantiornithines. Both of these clades are a big problem for the bolide
inasmuch as neither presents a reason why they were targeted and not their
ecological equivalents, placentals and neornithines respectively. We
understand a possible mechanism for dinos--lack of bulk food for bulk
animals--but we don't have one for our diminuitive niche holders. And so,
the bolide is forced to propose mechanisms which I find
very far-fetched: e.g., a hermetical seal seperating enantiornithines in
the North and neornithines in the south.
> Reason: these seem very unlikely to act as global events, nor are they
> patterns to which the dinosaurian (and other) taxa had not been
> exposed. LOCAL extinctions from competition are common. A GLOBAL
> mass extinction requires a NOVEL effect across the entire range of
> several groups severe enough to reduce ALL viable populations.
I hope I clarified above. However, I _do_ believe the new species
evolving had a crucial and global role. For example, the most popular
hypothesis for pterosaur extinction _is_ competition/predation from
birds! This is a novel effect, right? And it is also global.
> Perhaps large-bodied species would do
> worse, perhaps hibernation would be important, etc.
Yes on the first. But, on the second I'm not so sure--and, in any case,
extinctions in "equable" climates--equatorial, at least--are unlikely to
involve hibernation (just a side note, hibernation cannot be induced in
naturally non-hibernating creatures). Also, birds are not great
hibernators--indeed, the most notable example of metabolic shutdown is the
large emu. Presumably in order to not advertize the presence of his nest,
the male emu goes without food and water for 40 days (from memory) or so.
> In a sense, this IS a competition model, and this point is (as far as
> I can tell) often overlooked (...) So the bolide could make new
> predation/competition factors
> important very rapidly. These are global, novel effects AND they are
> targeted based on biology/ecology: some groups do better because they
> compete well under post-bolide conditions.
I couldn't agree more. As Leigh Van Valen has said: causes of extinction
are often a matter of taste. Ultimately, it is our dichotomous brain that
forces us to choose alternaives that don't exist in reality (I said that,