Archaeopterygid bird from China

[Karl Magnacca]

On 31 Mar 2005 at 19:53, Barry Roth wrote:
> In order to be a hypothesis, which is what everyone seems to want, a
> proposition must be testable.  What kind of evidence can test (and potentially
> falsify) the proposition that a certain fossil at a certain geologic time and
> geographic place documents, roughly, the time and place of the origin of its
> clade?

Well, the hypothesis (at least the one I think we've been discussing;
maybe I've been interpreting things differently) isn't that a particular
fossil documents the origin.  I think it's taken as a given that we're
not going to find *the* common ancestor of the clade.  The hypothesis
here (as I see it) is that the archaeopterygids common ancestor lived
somewhere around China and the group had its first round of
diversification there.  The evidence to test that is how many
archaeopterygid fossils you find, where, and how old they are.

I suppose on a philosophical level it is necessary to assume that the
oldest fossil on hand is actually the oldest one - even though it's
almost certainly not true - because to do otherwise would mean you'd
have to throw your hands up in the air and stop thinking about it until
you know you have the oldest.

> Not more fossils, if, as per John Grehan, the chrono-geographic
> occurrence of fossils is irrelevant to detecting origins.

That "if" is the crucial thing.  As I see it, every fossil from China
adds a little more weight to the hypothesis' support.  Finding equally
old fossils somewhere else would throw and wrench into the works, and
we'd have to start thinking about it all over.

> I suppose if there were many fossils of a taxon, all at about the same
> age horizon, and widely spread geographically, most observers would
> balk at declaring one or the other to definitely represent _the_ place
> of origin of the taxon.  Either they all are (per the school of Grehan)
> or one is (but we don't know which one) and the taxon spread quickly
> over a large area soon after its origination: so rapidly that
> stratigraphic resolution is not precise enough to sort it out.

It also depends on your concept of a center of origin: as the
distribution of the ancestral species, or the broader, more diffuse area
(spatial and temporal) where early diversification took place.  It
sounds like you're referring more to the former; I've been thinking of
it as the latter.  So I would call it a third alternative: the combined
distribution of these similar-age fossils is the origin (of course,
depending on semantics, you might not really consider a whole continent
to be "center" of origin).

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