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Re: Viva Neornithine Birds!
> Not at all. All I want are hypotheses for why neornithines. Previously,
> this request was resisted on grounds that extant bird diversity descended
> from a single surviving species. Under this conceptual regime, it made
> sense to invoke luck. However, with more surviving species it is sensible
> to delve a little deeper. Your hypothetical example of arboreal species
> suffering higher extinction rates might be testable...at least it is
> attempt to _explain_.
I like your sentiment in this regard. I do think we need to be careful about
how far we take it, however. While equal extinction makes a nice null
hypothesis on paper, it is so unlikely during a global catastrophe that it
isn't as useful as it appears. There are some possible explanations worth
testing: habitat differences and geographic range differences are two
reasonable ones (assuming that the true habitat preferences during life are
represented by the fossil individuals). However, I don't think a failure to
find evidence for an enantiornithine/neornithine difference in those parameters
would be evidence against the bolide explanation. It's just uninformative.
We may never know exactly what the critical differences were (heck, it's hard
enough working out extinction risk correlates in *living* taxa). If evidence
were to accrue for an additional extinction mechanism specifically related to
differential avian survival at the K/T then that's great. Marking the bolide
as being too indiscriminate out of hand is, however, not a strong case. There
just isn't a good way to demonstrate expected *lack* of discrimination for
particular groups in a global manner. Specific differential mechanisms are
testable, but we pretty much have to hit them one at a time, and there's no way
to confirm that we've covered all those bases.
> Two things: 1. Evidence of an impact and mass extinction is not necessarily
> evidence for differential survival of neos vs. enantis--unless you can
> propose a testable hypothesis for why.
I disagree slightly. I think it is still decent evidence for general causal
mechanism, it just isn't as strong as we might prefer because it lacks a
specific mechanism. To have an impact, a mass extinction, and differential
survival of neos and enantis...and then say that the bolide cause of the latter
is poorly supported because we haven't worked out the differentiating mechanism
is a bit of a weak argument. Still, I agree that a testable hypothesis of why
that differential extinction occurred is badly needed, and I wouldn't be
shocked if other factors are demonstrated to have contributed.
> The tyranny of this
> bolide-explains-everything is weakened by increased survival and especially,
> differential survival of similar species!
But that depends on how similar they really are. The
neornithine/enantiornithine problem is particularly tricky because we don't
really have any phylogenetic degrees of freedom. With living taxa, we can
either predict risk based on similar phylogenetic position (works pretty well
most of the time) or we can remove the phylogenetic effects and look at
functional trends. With the avian survival trends, we're trying to explain the
phylogenetic position effect in the first place, without really knowing the
functional signals. How similar are they really?
One thing though that I will through out there: an awful lot of the
neornithines crossing the boundary seem to be pelagic or near-shore critters.
We may not have the single "seabird lineage" survival case once proposed, but
there does seem to be some kind of habitat signal there. Shoreline to pelagic
species tend to be fast-flying, high AR species with efficient flight over long
distances. They do well in open habitats and can easily cross most barriers.
Such taxa seem to produce smaller clades than the easily fragmented arboreal
groups, but they are difficult to kill with certain extinction threats because
they a) can escape and b) are less likely to have their entire range heavily
affected. If enantiornithines really were a primarily continental group, with
an emphasis on arboreal habitats, then we might expect most of them to be
short-distance dispersers (with all of the associated locomotor traits; low
wing loadings, low AR). Some continental forms also disperse ea
sily. The inland convective soarers are the long-distance champions of
continental forms. However, I note the apparent lack of continental,
convective soaring forms among enantiornithines (or Mesozoic neornithines).
Of course, the long-distance pelagic flyer mode as an advantage has its limits.
The best dispersers are the true marine soaring specialists (such as
albatrosses and some pelicaniforms), and I'd wager that the K/T impact did
nasty things to soaring conditions for a while. That's fine for many pelagic
birds that aren't really reliant on extended soaring. Soaring specialists
would get hammered. Of course, there weren't any albatrosses or gannets around
at the K/T, so it's a moot point for birds. The pterosaurs probably didn't
appreciate the conditions so much...