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Re: ARTHROLOGY AND FLIGHT CONTROL
Matthew Troutman wrote:
>Anyway, it seems that _Archaeopteryx_ and its theropodian outgroups lack
>the unique features of the avian wrist which allow these special
>kinematics to take place. Vasquez said it best:
I find such arguments unconvincing. It is, IMHO, very dangerous to
ascribe limited capabilities to fossil taxa which lack complex refinements
seen in modern forms. In some cases, this is because other less specialized
modern forms can perform the same task with less derived skeletal
morphology. In most cases, however, my objection centers around the simple
fact that there is more than one way to do something.
The entire point of evolution is that things change. Certainly,
there must be selection pressure before change can occur, leading to a sort
of chicken-and-the-egg situation where one is not certain whether the animal
is engaging in behaviour which does not assert influence on skeletal
behaviour until later, more derived taxa. Certainly, the wrists of modern
birds are a beneficial adaptation to the rigors of powered flight, but can
we really say it was less perfect without them?
I realize I may be pissing in the poridge [sic] of comparative
anatomy here, but we must not blind ourselves to the fact that the
adaptations we find in modern animals need not be the best solution, nor the
only solution to a problem. We see this all the time in modern animals (bats
flap without keeled sterna, as I recall). Simply because an animal is older,
or lacks some of the derived characters of later forms does not inherently
mean that the adaptations it does possess are necessarily worse than those
of later forms. there is nothing to say that older animals did not have
adaptations different from those we see in modern forms. It is quite
possible that we may not be able to identify such adaptations, possibly
because we are too busy looking only for the states we see in modern forms.
Example: the experiments where the supercoracoides muscles of
pigeons were snipped, and it was discovered that they could not elevate
their wings and could not lift off the ground, but could fly once airborne.
Goes the argument, _Archaeopteryx_ must have been under the same
constraints, as the supercoracoides is not positioned in the manner it is in
modern birds (i.e. to elevate the wing).
This is flawed for a major reason and a minor reason. The major
reason is that, while the supercoracoides may be the primary wing elevator
in modern birds, it need not be so in _Archaeopteryx_ (indeed, lacking the
morphology of later birds, it likely was not, hence the experiment). There
may be other muscles which can do the job, though (by analogy to humans, the
trapezius?). These muscles may not be used in this capacity, or may not seem
powerful enough in this capacity, in modern birds because the
supercoracoides has taken over this function.
The minor reason being the philosophical point that you should not
compare a broken machine to a fixed one. _Archaeopteryx_ did not have any
muscles cut when it flew or did not fly...
I am not saying that such claims are worthless, or that they should
not be persued. However, we should exercise skepticism when it is asserted
that a fossil animal could not do something, at least as much skepticism as
we show when it is said that it *could* do something. There is a quote on
this very topic cited in Shipman's excellent taphonomy book which I do not
have currently available. All I can remember is the part which is
contradictory to my point above, something to the effect that it is obvious
brontosaurs did not fly. :)
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"...To fight legends." - Kosh Naranek