Archaeopterygid bird from China

[Karl Magnacca]

On 1 Apr 2005 at 9:14, John Grehan wrote:
> 'Restricted' is a relative term. I was using in relation to the total
> range. To postulate that archaeopterygids originated in one part of the
> total range is to invoke a restricted center of origin. I argue that it
> is not an empirical necessity.

No it's not an empirical necessity.  But here it's what the evidence
suggests.

> I don't think I am that stupid (although one never knows). In our
> panbiogeography book I and my colleagues did point out that there was a
> broad adoption of a vicariance mode of reasoning. However, much of the
> vicariance reasoning is still Darwinian in that they often appeal to a
> center of origin followed by dispersal before the appearance of a
> barrier so its just dispersalism before the barrier as opposed to
> dispersalism after.

Well look, the ancestral taxon had to get out to a wider range somehow.
If you start with a widespread taxon and speciation proceeds mostly by
vicariance, as time goes on you end up with a lot of species with
smaller and smaller ranges.  Sooner or later at least one of them will
have to disperse over the wider range again to start the process over
again.

> Please show me how the fossils are empirical evidence of a center of
> origin and that I am wrong to assert that the distribution of fossils
> are simply the distribution of fossils.

Because the distribution of fossils = former distribution of species.
Yes there are plenty of caveats about fossilization probability in
different locales, but when you find lots in one place and none in other
fossil-bearing areas from the same time period, then it supports the
hypothesis that the distribution of the fossils does more or less
reflect the true distribution of the group at the time.

> > If this [vicariance from a widespread ancestor] happens, then you
> > will find all of them distributed across the entire area from the
> > start.
>
> Not true.

Why not?  There are all the caveats about fossilization and extinction,
but you would expect that at least a few would be found across the whole
range.

> > Finding a diverse array of fossils in one area, and few to
> > none elsewhere (and those being later), seems to me to be empirical
> > evidence that the group originated in that area and had its first
> > round of diversification there.
>
> This appears to be the nitty gritty. If it were empirical I would agree
> with you, but it's a theoretical interpretation of the distribution of
> fossils that leads one to reason a restricted center of origin - not the
> other way around.

Just what would you call "empirical"?  Empirical evidence is just
objects and numbers.  As you said above, the distribution of fossils is
just the distribution of fossils.  A morphological character matrix is
just descriptions of species.  Changes in species abundance are just
numbers on a graph.  The whole point of science is to use the empirical
evidence to determine the viability of different hypotheses.  Otherwise
you're just wasting your time collecting mounds of data.

So, with the empircal evidence of one fossil from Europe and many older
ones from China, and absent any theoretical considerations, let's look
at the three competing hypotheses for early archaeopterygids:

Originated and diversified in Europe, spread across Asia: Unlikely.
Would require a diverse array of forms in Europe that have not been
found.

First or very early ancestor lived across Eurasia from the start,
diversified vicariantly: Possible, but somewhat unlikely.  Requires
heavy bias towards fossilization in China compared to elsewhere in
Eurasia.  Discovery of older fossils from Europe or western Asia would
shift support to this.

Originated and diversified in east Asia, spread to Europe: Most likely.
If the fossils we know about reflect what really happened (i.e. no
archaeopterygids outside east Asia for some time after origin of the
clade), then it would be definitely true.  More definitive confirmation
requires more fossils, and early fossils found elsewhere would overturn
it, but this is where the state of knowledge stands.

> There's no theory involved; whatever Darwin
> > would think is irrelevant.
>
> Perhaps, but his thinking has influenced a lot of biogeographers.

SO WHAT?  That helps your case against the theory, but it has no bearing
on this argument!  If a person had never heard of Darwin's biogeographic
theories, or current ones, they would still come to the same conclusion.
If you think otherwise, what conclusion do you think they *would* come
to?

> > No it's not conclusive proof, but you're
> > never going to get that (especially with fossils).  It's a matter of
> > the evidence at hand.
>
> As argued above, it is not evidence at hand - its theory.

As I argued above, no it isn't.  Fossils are empirical, by definition.
If you think they don't support the hypothesis that's another matter,
but so far your only argument has been that fossils aren't really
evidence at all and biogeographers are trapped in the world of Darwin.

Karl
=====================
Karl Magnacca, USGS-BRD
PO Box 11, Hawaii Natl. Park, HI 96718
"Democracy used to be a good thing, but now it has
gotten into the wrong hands."   --Sen. Jesse Helms