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

Re: The actual running Archie paper...(this is very long!)



John:

I'm no fossil bird expert -- I study sauropods, so the saurischian
connection is loose at best.  However, I am intrigued by the
bird-dinosaur hypothesis, and by the fact that there is still a lot of
heat and very little light being shed in most arguments I have heard
surrounding the hypothesis.  I am in no sense a "believer" in
ground-up or trees-down evolution of birds, but IF they did evolve
from a ground-dwelling theropod (most paleontologists I think agree
here, although Alan Feduccia, Larry Martin, and others I know continue
to hold strong objections to this) perhaps this is why the focus has
been on a ground-up model.  Then again, I know little of the vast
literature surrounding this, so I stand corrected in advance.

Something you mentioned in your last post intrigues me:
"Look at page 217 of "Pred. Dinos.World" (if you haven't
got one, get one.)  The picture clearly illustrates a *very
important
feature* (interestingly contradicted by the text):  that the
supracoracoideus muscle pulls *forward and upwards* not just in the
modern
bird shown but also in  _Ax_ and Coelophysis too!  The pulley is
*not* there
entirely to change the direction the muscle pulls in (though the
final angle
wrt horizontal of the tendon where it attaches to the humerus
varies:
_Coelo_:30 degs, _Ax_: 45, and "moderns": 60 degs) but it is there
because
the S.C. gets bigger with more evolved flying.  The picture quite
clearly
shows the tendon passing over a promontory on the _Ax_ coracoid that
changes
the direction of the muscle by something like 135 degs.  Is that not
a loop?
In order to constitute a pulley, does the line have to go through a
closed
hole?  Is your outside elbow tendon not a pulley just because it
doesn't go
through a tunnel?  Whatever word we want to call it, the triosseal
canal of
birds is a red herring.  A neat trick, but it's the other end of the
muscle
that is special and has reversed (and enlarged)."

Okay.  I am looking at pg. 217 in Greg Paul's Predatory Dinos book. 
First off, do we know for sure that the supracoracoideus muscle in
Archaeopteryx actually formed a tendon, similar to that of a living
bird, that passed through a groove on the scapulacoracoid before
insertion onto the humerus, or would have followed a pattern similar
to what was likey the predatory dinosaur condition, as shown by Paul
in the Coelophysis drawing?

Second, I'm not sure I understand your pulley argument.  Are you
trying to argue that Archaeopteryx could fly because it had a powerful
supracoracoideus even if it did not pass through a triosseal
canal-like structure?  Or that the pulley mechanism is a "red-herring"
because even if Archaeopteryx possessed one (if it did) its
supracoracoideus would be too small to have made it useful as a flight
muscle?

Also, to my knowledge, the triceps tendon (is that what you mean by
"outside elbow tendon") is NOT a pulley.  It acts on the olecranon
process of the ulna in most tetrapods as an in-force to a lever
system.  A pulley-system would require the tendon to wrap around and
insert on the posterior surface of the ulna, perhaps with a sesamoid
bone between.  Instead, the triceps tendon inserts usually directly
onto the superior surface of the olecranon without wrapping around to
the other side.  The fibers of the tendon may wrap to various degrees
about the olecranon process, but its main function assists in the
arm's lever system, not as a pulley.

The quadraceps tendon (formed of four muscles: rectus femoris, vastus
lateralis, vastus intermedius, and vastus medialis) in mammals (and of
similar muscles in birds) inserts onto the patella, where its fibers
pass and become the patellar ligament, which then inserts on the
cnemial crest of tibia.  The patella acts as a pulley, or rather,
transmits the quadraceps's tendon through an angle as if it were a
pulley -- to my knowledge, the patella doesn't roll. =)

I bring this up because you are correct, a tendon need not pass
through a tunnel to be a pulley, but pulleys have special conditions,
one being that they change the angle at which a muscle inserts to gain
some sort of mechanical advantage, so you see that tricpes tendon
cannot be a pulley system.  This is why (as I assume you know) the
supracoracoideus passes through the triosseal canal to insert on the
deltopectoral crest of the humerus -- it provides a better angle of
insertion and mechanical advantage: it's a pulley.

So you know, I took the "dare" and went to your web site.  I do not
wish to debate with you point by point your argument against the
ground-up hypothesis on bird origins, but I am curious as to why you
suggest we "disbelieve" cladistics.  While some cladistical studies no
doubt abuse the power of the hypothesis generator, surely you don't
suggest we toss out cladistics entirely?  Many folks I know spend so
much time and energy to make sure their characters are meaningful,
non-repetitive, and testable that it is hardly the
dump-everything-in-and-see-what-we-get approach you seem to think it
is.

But, let's say you are right -- cladistics is ultimately misleading
and poorly designed for fossil vertebrates.  Fine.  What are you going
to replace that methodology with?  Which former method of phylogenetic
hypothesis can you suggest that has the same testablility,
falsification, and predictive power of current cladistical
systematics?  Or, can you suggest to us a method that encompasses the
strong points of cladistics but blows away the parts cladistics cannot
touch?  Until you or someone else can do that, cladistics is the best
thing we have.  And I by no means suggest that it will stay forever or
never be replaced by better methodology -- all I suggest is that you
cannot dismantle cladistics without a replacement.  If you don't like
cladistics, suggest something else.

You mention Occam's razor and its supposed misuse in paleontology
because of the possibility of character state reversals.  Yes, you are
correct, we should be careful and understand the fossils we are
dealing with well.  However, you also explain that the first "flaw" of
cladistics theory is "How do you decide what feature(s) to choose when
defining you clade?' and then suggest this is all arbitrary and
ultimately very theoretical with little basis in reality.

Okay.  So let's examine the dinosaur-bird origin.  Well, what
characters should we pick?  We could pick any!  But, of course, some
characters are more valuable than others in providing information.  We
could count every bone molecule and make that a character, but that
would probably not be too informative, nor would measuring the length
of each animal and plotting them out -- characters of size.

I don't think you'd disagree that birds are archosaurs?  My
assumption is that birds are indeed archosaurs based on skull and
postcranial characters I do not wish to exhaustively list now.  Be
that as it may, most people would agree birds are archosaurs based on
characters, or morphology, or what have you.  Now, instead of taking
the normal tack of listing characters and having you or someone else
critize my choice of them, lets instead start with gross function.

Let's look at basal archosaurs and our closest living approximation
to them, alligators and crocs.  These animals are quadrupeds.  They're
fore- and hind-limbs are coupled together in locomotion, and they have
a big tail muscle called the caudofemoralis longus which runs off the
tail and inserts on the fourth trochanter of the femur.  The hindlimb
is coupled to the tail.  That is our ancestral functional condition:
quadrupeds, fore-limbs and hind-limbs coupled, and hind-limbs coupled
to tail.

Look at birds.  The arms are indepedent and de-coupled from the
hindlimbs.  The hindlimbs in turn no longer have a strong attachment
to the tail, and are decoupled from the tail.  If birds are
archosaurs, and if they descending from an archosaur, at some point
they decoupled their arms from their hind limbs and became bipedal,
and then lost their connection of their tail to their hindlimbs.

So, what we need to find in the ancestor to birds is an archosaur
group that first loses its quadrupedality -- they become bipedal. 
Next, we also need a member of that bipedal group to lose its tail
connection.  Now we look for possible candidates in the archosaurs. 
Theropod dinosaurs come up twice.  First, early in their evolution
they become bipedal, freeing their forelimbs to do other things. 
Second, we look at the fourth trochanter, the ridge where
caudofemoralis longus muscle inserts from the tail.  We would predict
that if that muscle were to become decoupled from the hind limb, the
fourth trochanter should diminish in size and ultimately disappear all
together.  And that is what we find happening in the maniraptoran
dinosaurs.

If there are other archosaurs that have these two crucial decoupling
steps, I am unaware.  This argument is not my own, it is a
conglomeration of arguments from Padian, Gatesy, Chiappie, Gauthier,
and many others.  It is only in my own words.

Now we set parsimony (Occam's razor) to work.  Could dinosaurs have
evolved from other archosaurs?  Sure.  But how?  How do you get the
bipedalily and tail decoupling happening in another group of
archosaurs?  Could birds and dinosaurs have evolved convergently? 
Sure.  But think of how many reversals and convergences that would
take.  And what would they be converging to.  And then think of this. 
Birds and theropods have a mesotarsal ankle joint.  Other archosaurs
have a different tarsus situation.  Birds and theropods have a
semi-lunate carpal bone in the wrist, something no other archosaur
has.  I could go on, but I hope you understand where this is all going
... and you can test me on this.  Find another bipedal archosaur
lineage that decouples its tail from the hind-limb, and you've got a
good starting point.  And notice -- I could have picked ANY characters
to focus on, but I choose these because of their informative value. 
And, I'm not saying you don't think birds came from dinosaurs -- this
was just a way to demonstrate a little bit of the character choosing
in cladistics.

Cladistics is a fine scientific tool.  While all scientific tools are
misused by various individuals, that doesn't mean the tool is bad. 
And if it is, a better replacement for it needs to be set forth. 
Otherwise, we're just nit-picking characters, arguing about what we
would prefer, and ultimately getting nowhere in the end.

Sorry for the long e-mail, but I wanted to get my point across.  I
hope you take my criticisms as they should be: against your
objections, not about you personally.

Sincerely and with best regards,
Matt Bonnan
Dept. Biological Sciences
Northern Illinois University