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Re: polarity of bipedality in dinosaurs (real long--TOO long)



At 01:39 PM 9/26/96 -0500, Dinogeorge wrote in response to Jeff Martz:

>_Archaeopteryx_) would be true dinosaurs, having diverged later than the
>ancestral phytodinosaurs from the central lineage leading from the common
>archosaur ancestor to modern birds. And the less advanced dino-birds

        I still have yet to see a well reasoned, detailed argument for the
monophyly of the "Phytodinosauria".  Bakker did not, as far as I could tell,
provide one.

>The current BCF reason for manual digital loss is to accommodate the
>increasing size of the wing feathers, which extended backward across digits
>IV and V. The presence of the extra digits hampers the airfoil and, as you
>note below, adds unnecessary weight to the wing.

        Excess digits don't seem to hamper bats, and pterosaurs just shoved
theirs up front, without losing them.  Seems to me it would make more sense
to increase the outermost digit (ala pterosaurs and like the hindlimbs of
many clmbing quadrapeds), first for climbing, then for strengthening the
wing.  Primitive birds which required grasping hands retained the minimum
number of fingers needed to preserve a grasping function, but an ancestor
had shed the rest, serially, for whatever reason.
        The third finger was already reduced before flight began, possibly
on the way to being lost as it is in Tyrannosaurs, and so could not form the
center of the outer portion of the wing, and was fused into the
carpometacarpus to strengthen the manus.  If they were reducing the number
of fingers to get out of the way of the feathers, why not just eliminate
that finger and expand digit II?  Why not just build you wing from digit V
and move the other digits up front to form a grasping battery as in
pterosaurs?  I'm afraid that it makes more sense that three fingers were
what primitive birds started out with.
        And I know some people automatically dismiss bats and pterosaurs as
a basis of comparison.  I think that this is a good idea some times, but in
this case the comparison is valid.  

>To me the >serial nature< of the loss is important, and suggests
>aerodynamic factors in addition to weight reduction[...]

        Actually, many, if not most, adaptions involving changing numbers of
serial elements in vertebrates (eg. vertebrae, fingers, phalanges, etc)
occur in serial (length changes do too, with notable exceptions: I believe
Walrus flippers are way weird this way...).  Serial loss of digits suggests
that birds are vertebrates.
        As for the out-to-in direction, well, I recall that pterosaurs lost
their manual digit V, no?  Why?  To improve the efficiency of the airfoil?
No, because it was the outermost digit, and they were looking to lose one.
Of course this doesn't always work completely in series.  Many animals loose
digits from both sides, ungulates being a very good example.  And you can
see that the dinosaurs also did this, as mc I is much shorter than the others.

>what a terrestrial biped might find handy about fewer, longer, more awkward
>hands.

        They only needed three fingers for a grasping function, and those
with less flexible wrists had smaller hands and arms and probably didn't
worry too much about them anyway (why do you think they were losing
fingers?)  By the time somebody figured out that them hands was useful, they
were down to three fingers.

>The awkward manus is something theropods were stuck with after
>becoming ground-dwellers.

        That's funny, we all know Deinonychus could open doors... :)

> Of course, the problems created by the much more awkward manus of
> hesperornithiforms, phorusrhacids, kiwis, moas, and so forth, which
> vestigialized from true wings, were evolutionarily overcome in time.

        What problems?  These forms were using their mani for one thing
before they became flightless, and that was flight.  If I move to an
apartment, the only akward thing about my lawnmower is the space I use to
store it in (which is why I "vestigilize" it).

>considering island endemism. Island endemism usually acts to reduce >giant<
>forms, such as the dinosaurs of Hateg, Romania and the mammoths of various
>islands, but on small forms island endemism often acts as a stimulus to
>evolving larger forms.

        When there are niches for large animals to exploit, and no large
animals to exploit them!  When there are niches for small animals, and no
small animals to exploit them, the reverse can happen.
        So, do you think that each smaller species of hummingbird had to go
out to an island and get isolated in order to get smaller?
        However, islands are not neccessarily a precondition for size
reduction.  There are other possiblities which have not been explored as
fully as that, but the island pheonomena make a keen example of evolution,so
they are commonly cited.  If islands were necessary for size reduction, we'd
probably have a lot less variety on this planet.
        Put simply, what you are looking for is directional selection
pressure aimed at body mass, perhaps coupled with reproductive isolation,
perhaps not (I don't want to get into an in-depth debate on this, so let's
keep it simple).  What could this be?  Why are so many of the Mongolian
Cretaceous forms smaller than North American ones?  There could be many
reasons, but I think differing environmental pressures is likely. How does
this work, you ask, oh skeptic of evolutionary theory?  Given selection
pressure for smaller animals, the runts of the litter will be selected for.
It's that simple.  Ok, it isn't, but you get the point.

> Large size would generally be selected against in habitually
> arboreal forms because of the falling problem, at least until the
> animals acquired a reliable solution to this problem (such as really
> good hand-eye coordination, as in monkeys and apes).

        Staying low to the ground until you developed better co-ordination
sounds like a much simpler method...

>now?) skeletal specimens of _Archaeopteryx_, and the first specimen found is
>>nothing but< a feather. Indeed, it was seeing the feather impressions in the

        To my knowledge, this has never been *proven* to be Archy.

> I can imagine many reasons why a flightless dinosaurian
> bird-descendant might >lose< its feathers (too hot under them,

        Yeah, I always hear roadrunners complaining...  

[Did you just check the December archives, by any chance?  -- MR ]

 too awkward for running,
        See above...

>too awkward when grasping

        A problem you suppose Archy wouldn't have had?!?

>but very few why such a form might gain them or enlarge them if it
>had them plesiomorphically. (Maybe for display? Shade? Possible but pretty
>flimsy and not compelling, I think.)

        Uh... *I*N*SU*L*A*T*I*O*N* comes to mind...

        Wagner
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