[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: BADD BCF



GO writes:

>The semilunate carpal is but one feature related to flying that appears in
>_Archaeopteryx_. If the semilunate carpal were all there was, then I wouldn't
>even try to concoct a different scenario for bird origins from the orthodox
>one. But BADD expects me to swallow the conjunction of many different
>supposed "exaptations," _each_ arising for some reason unrelated to flying,
>in a single lineage where, miraculously, they produce a flying dinosaur! This
>is nuts. It makes much more sense to me to envision these "exaptations"
>arising in turn in a lineage as incremental improvements to a volant
>lifestyle of some kind (just what kind--climbing, gliding, lousy flying--is
>not yet clear).

This, however logical it may be, is not evidence.  If I understand you you
are saying that non-flying maniraptors share a suite of characters in the
forelimb that, in volant birds, serve as part of the flight mechanism, and
suggest that this involves too many coincidences to dismiss as anything
other than a secondarily flightless condition in those forms.  As I have
said before, this may be true, but is not necessarily so.  I note the
following objections:

1.  First of all, we are not talking about convergent evolution here but the
possession of common characters within a clade.  In and of itself this
proves nothing except that the characters were probably present in the stem
taxon of the clade, whatever that was.

2.  Second, not only are all members of the clade extinct except the "true"
birds, but there is nothing even remotely like the extinct members alive
today (ie obligate biped archosaurian carnivores with functional grasping
forelimbs).  Therefore we have no actual evidence as to how the
maniraptorian forelimb functioned in life in the non-flying forms; at best
we can speculate based on a functional analysis as to the degree of actions
possible, but we cannot reconstruct the actual behaviour involved for certain.

3.  Therefore it becomes extremely difficult to conclude that the suite of
characters we are talking about could not have evolved for the purposes to
which the flightless taxa put them, as we don't know what those uses were.
As pointed out in my longer, earlier, vanished message, this suite persisted
as part of a functioning complex for millenia and would hardly have done so
had it been disadvantageous.

4.  The fact that a number of characters are involved is less convincing
when these characters form part of a single functional complex that could
have evolved under a single series of selective forces.

These are cautions, not fatal objections.  But the fact remains that the
only way to settle the matter is by discovery of further transitional forms.
For example, the absolute clincher for showing that, say, Velociraptor was
secondarily flightless would be an analysis showing that it lay WITHIN a
clade of known winged forms.  We do not have the taxa to show this.

>Because "both lines" is not necessarily the best picture. There weren't
>simply two lines--avian maniraptors and non-avian maniraptors; there were
>lots of different lines of both. There were archaeopterygid maniraptors,
>dromaeosaurid maniraptors, avimimid maniraptors, mononykid maniraptors,
>troodontid maniraptors, tyrannosaurian maniraptors, enantiornithine
>maniraptors, and so forth. BCF says that the ancestral forms of all these
>lines themselves form a line, the line leading to the more modern avian
>maniraptors.

I do not see why the common ancestor of all these forms (I assume there was
one) could not have been a small terrestrial form - no matter how many
branches of the maniraptorian tree there were.  You have merely restated
your conclusion, not produced evidence refuting my statement.

>Right. Under special circumstances of territory or resource deprivation (for
>example), dwarf forms will evolve.

The examples (eg hawks) I cited are not merely cases of dwarfism, but of a
wide range in sizes within a closely related taxon.  Without examining the
figures in detail I imagine there would be a quite uninterrupted range in
sizes within hawks from the smallest to the largest (over a range of 100X).
I do not see a priori why a stem taxon of maniraptors could not have
exhibited the same range of variation.  My point was that the stem taxon
could have included quite closely related species diverging widely in size -
and, if so, could have given rise to both large and small forms.

>It is simple to envision in general terms the evolution of a large,
>flightless bird such as _Diatryma_ or an ostrich from a smaller, volant form,
>even though the details may not be available in the fossil record. What
>evolutionary steps would something like a _Diatryma_ or an ostrich have to
>undergo in order to produce a (small) flying descendant? Can you document an
>occurrence of this kind of evolution? If you can, I'm certainly interested in
>hearing about it, because then I would be much more willing to accept the
>evolution of a (small) flying descendant from a larger theropod.

This is not a true analogy because both the ostrich and Diatryma have
undergone massive modifications to the forearm beyond anything seen in
non-volant maniraptors (Mononykus possibly excepted), and except for display
function in the ostrich the forearm has become almost functionless, its
possible roles transferred to other bodily systems.  Thus although both
could have, I suppose, been ancestral to dwarf flightless forms I can hardly
see them re-evolving flight.

In the maniraptors (as you have noted) there are enough similarities in the
forearm structure to envisage a shift from (say) Velociraptor-like to
Archaeopteryx-like.  All we then have to envisage - perhaps - is the
downsizing - and remember that if such a thing happened this would almost
certainly have occurred before the evolution of flight.

But, as I said above, there is no reason to assume that BADD requires that
Archaeopteryx was derived from a LARGE theropod - only that both
Archaeopteryx and the large theropods like it evolved from a common ancestor
whose size is unknown.  Thus, unless you can produce a known ancestral taxon
acccepted as such by BADD theorists that is considerably larger than
Archaeopteryx, I think this particular argument is a straw man.
--
Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
International Wildlife Coalition              Fax/Modem: (905) 569-0116 (home)
Home: 1825 Shady Creek Court                  Messages: (416) 368-4661
Mississauga, Ontario, Canada L5L 3W2          Internet: ornstn@inforamp.net
Office: 130 Adelaide Street W., Suite 1940    
Toronto, Ontario Canada M5H 3P5