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

Re: BADD BCF



In a message dated 95-12-06 18:32:13 EST, ornstn@inforamp.net (Ronald
Orenstein) writes:

>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.

In the forelimb (various modifications for a wing), in the foot (retroverted
hallux), in the skeleton (pneumatization), and probably in the soft anatomy
as well. Not to mention a number of less important but noteworthy cranial
modifications, and flight feathers.

>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.

When those characters _all_ involve modifications that significantly improve
an animal's capacity for flight, I begin to suspect that they arose in that
context.

>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.

Sad to say, all too true. So everyone's speculations are just as wild as
mine.

>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.

I'm dealing in likelihoods here--and in the unified picture of theropod/avian
evolution that BCF presents. If theropods started out as cursorial animals, I
would expect to see evolutionary modifications that improved their
cursoriality, or their ability to use their forelimbs better for grasping and
holding prey, and so forth. It is true that evolution sometimes hands us the
unexpected, and to that extent I'm willing to entertain the idea that small,
bipedal cursorial theropods acquired feathers (somehow), lost two or three
manual digits (who needs 'em?), enlarged their grasping hands into grasping
wings (for some reason), retroverted the hallux (well, why not?), etc., etc.
Beyond the fact that my lame mind cannot imagine how the appearance of any of
these characters would have conferred any selective advantages on a cursorial
animal that used its forelimbs for snatching prey, it is the ad hoc
accumulation of these features all in a single lineage that I find so
unlikely.

>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.

It's not a single functional complex, it's virtually every part of the
skeleton.

>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.

Actually, it does--Archaeopterygiformes. _Archaeopteryx_, _Deinonychus_,
_Velociraptor_, and other dromaeosaurids share enough apomorphies that even
cladists will soon admit that they belong to a single clade. The big problem,
of course, is that only one (maybe two or three, if _Archaeopteryx_ comprises
more than one species) of the known archaeopterygiform taxa was volant. But
soon the Korean archaeopterygiform will be described, and there will
undoubtedly be others.

>>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.

It was, but you may have to go back into Diapsida to find it.

>You have merely restated
>your conclusion, not produced evidence refuting my statement.

Was I trying to refute your statement--or just restating it?

>>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 probably did. So--?

>>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.

The modifications aren't "massive"--just some irreversible vestigialization.

>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.

This will never happen, because the BADD cladists weasel out of presenting
_any_ taxa as ancestral.