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Re: fore limbs of dinosaurs



At 06:06 PM 9/13/96 -0500, Aravind.L wrote:

>In the Mesozoic archosaur evolution has been characterized by
>experiments with the forelimbs

        This seems true.  I would argue that this has more to do with the
move to bipedality rather than some intrinsic plasticity in the genetic code
for the archosauran arm.  This may seem counter to my earlier statements on
a similar vein (coelophysoid head crests), but the two are related.  Once
the arm was freed from its locomotor role, it could use whatever tendancy it
may have had to go nutty with adaptions (George Olshevsky, for one, has some
good points about how unlikely it is for a locomotor organ to start wildly
"experimenting" in a genetic sense).

>One trend which is very prevalent is that they have undergone
>shortening in a progressive fashion along some lines. This was first
>seen in the rauisuchians and the ornithosuchians.

        I believe that this is strongly correlated with a move to
bipedality, in which case, perhaps the trend towards bipedality ought to be
examined first.

>Subsequently in the theropod lines like the ceratosaurs,  allosaurids
>and maniraptorans it occured independently. In many cases like the
>tyrannosaurs and abelisaurs no apparent selective advantage is visible.

        Look at it in either of these two ways:
        1) If the arms _were_ nonfunctional, why should it eliminated them
entirely?  Common sense, which doesn't always apply to nature, would dictate
that it might be more advantageous in the long haul for organisms to retain
vestigial organs, perhaps because a tendancy to do so eventually resulted in
a selective advantage at a past point in the history of the genome (my
terminology may be inaccurate there).  In any case:
        2)  The arms of these critters were reduced to the point where there
was no selective advantage in reducing them any more.  Evolution is a
refining process, not a constructive one (again, my terminology may be weak
here).  It is possible that the sort of consideration like the one pointed
about above may have come into play, or perhaps there was a point below
which having the arms sank below the rest of the "genetic noise" and were
lost to selective pressure.

>This may mean that the trait even though not selective remained in place
>because the orgainsms derived huge advantages from other innovations
>like teeth which over rode any dis advantage due to  the fore limbs.

        What are the disadvantages of having tiny forelimbs, I wonder?
Raptors seem to do fine with no grasping forelimbs (a point which Horner
seems to have forgotten when he came out with "scavenger T. rex").  I am not
sure what you mean by "innovations like teeth", since most tetrapods have
teeth already, and I haven't yet heard of a situation when teeth and
forelimb design were directly "coupled" evoltionarily.  I think you are
right though that these animals were far more busy evolving other parts of
their body.

>[...]To a molecular evolutionist it becomes apparent that the
>presence of instability like - repetitive elements in the FGF/
>hegdgehog response elements of the hox gene of archosaurs could
>provide a predispositon for limb changes[...]

        I think I get the idea here.  What you think is that maybe
archosaurs were predisposed to innovation in limb design.  Listen, friend,
I'd be very careful bringing up advanced stuff like that around here, some
people don't like them new-fangled ideas ... ;)  Personally, I am interested
in this idea.  However, I think it needs to be addressed first in relation
to the issue of archosaur bipedality.

>[...]This may also explain the unusal evolution of the
>alvarezsaurid.

        To what aspect ,exactly, of Alvarezsarus are you referring?  Or do
you refer to Mononykus' arms?  I think the only way you can explain M.
olekranus' [sic?] arms is that it had a durned good use for them.

>[...]The avian and pterosaurian flight may also be some thing to do
>with this.

        See again bipedality, also look a arborality, and compare with bats.
The origin of flight amongst archosaurs was probably helped by a degree of
flexibility in archosaur forlimb design, but that certainly wasn't the
smoking gun.  Sad as it is, when you talk about this sort of thing, people
want to see the gun, not the spent cartridge.

>Another possibility which I am serious considering is the Hox D13
>genes poly A region. Deletions of some such regions causes
>tridactyly in humans.[...]

        I'm not sure exactly what you are getting at...

>[...]What do the paleotologists have to suggest?

        About tridactyl babies?  Seriously, this does seem to be an
important line of quesitoning, I just couldn't resist.  :)

>Cann you think of any advantage in Carnotaurus or tyrranosaurid fore
>limbs.  

        Numerous people have wondered:
        Nick Longrich(?): The were used to push against the prey during
feeding to increase the cutting efficiency of the teeth.
        Myself:  Used to carry food back to the nest, if there was one.  Or,
perhaps, to carry the little ones around.
        Other ideas:  Positioning during sex, pushing up off the ground, etc.
        I think it comes down to "what _couldn't_ one of these animals do
with it's forelimbs".  It probably did whatever it physically could do
because they were there.  Maybe just having them around conferred a tiny
selective advantage through many different uses.  What do you do with your
forelimbs?

>Aravind
        :) Wagner
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