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Re: What is biomechanics? (or, The Truth About Flying Snakes - Was: Re: science and philosophy)



Quoting Colin McHenry <cmchenry@westserv.net.au>:


> Jaime said:
>
> >to provide the most useful
> >data and be provided and likelier to be true than not (Occam's Razor).
> 
> OCCAM WAS A LOUSY BIOMECHANIST
> And as for parsimony, I still don't understand how this is of any relevance
> to biomechanics.  

Well, the assumption that _Tyrannosaurus_ did not have supermuscles capable of 
propelling it over a tall building in a single bound would be one way.

The fundamental assumption of paleobiomechanics--that the hard and soft tissues 
of extinct animals worked in certain fundamental ways like those of living 
animals--is a sort of invocation of parsimony.


> Anyway, are you (Jaime) talking about using parsimony to choose between
> different operational possibilites outlined as the result of performing the
> biomechanical analysis upon your model?

Well, if several alternatives are equally parsimonious, then this is, by 
definition, impossible.


> If you're talking about using parsimony to help reconstruct the evolution of
> a certain property of the animals (such as flight), which seems more likely,
> then that is a whole new ball game.  Look, I hate to keep being so negative
> about cladistic methodology, but I do find this unquestioning reliance upon
> parsimony to be a huge problem.  And it's not as if it's the only option
> available to cladists, either.

At least parsimony is an objectively definable criterion that all workers 
could, in principle agree upon.  Would you really rather have a system that 
allowed or encouraged the introduction of all sorts of extraneous assumptions?


> RECONSTRUCTING EVOLUTIONARY TRANSFORMATIONS USING BIOMECHANICS
> An important part of Kronstruktionsmorphologie is the mapping of
> biomechanically viable transformation pathways across a series of different
> 'Konstruktions' (organisms with differing bodyplans).  What this means is,
> instead of constructing a cladogram to make your hypothesis about the
> evolution of a feature, you construct biomechanical models of each taxon
> included in the analysis.  Each distinct biomechanical model (which may
> include a number of different taxa, if you conclude from the scope of your
> model that they operated in a fundamentally similar way) is called a
> Kronstruktion.  You take your Kronstruktions and then you work out which
> transformations from one Kronstruktion to another are biomechanically viable
> without compromising the integrity of the organism.  

Would that we all knew in advance, as the Konstruktionsmorphologisten 
apparently do, what sort of transformations are "biomechanically viable without 
compromising the integrity of the organism".  It seems to me that this is 
liable to lead us down the sticky and ultimately intractable path of relying on 
whether or not some worker is *inspired* to see how the change might have 
occurred and how well he/she can convince others of the correctness of his/her 
vision.

Plus, this sort of methodology would seem to suffer just as much as cladistics 
from the problem of missing data.  For instance, say you have Konstruktion A 
and Konstruktion B and you can't for the life of you figure out how B derives 
from A, so you conclude that they are not related.  Then the next day someone 
publishes a specimen with a new morphology, from which you abstract 
Konstruktion X, and lo and behold, X perfectly bridges the gap between A and B.

Would people have been able to bridge the gap between a pelycosaur-type 
Konstruktion and a mammal-type Konstruktion without the aid of a 
_Diarthrognathus_-type Konstruktion?


> There appears to be some profound disagreement over the identity
> of the group of terrestrial reptiles which gave rise to brids - most likely
> one of the various groups of 'advanced' theropod dinosaurs, but if there is
> currently  any consensus over which family of dinosaur birds evolved from
> I've missed it.  

As mentioned by a previous poster, there is considerable consensus today that 
Dromaeosauridae+Troodontidae form the sister group to _Archaeopteryx_+modern 
birds.


> There is an as yet unresolved debate as to how flight might
> have evolved from any type of theropod dinosaur - some say ground [up],
> others say trees down, 

Forgive me for shouting, but THIS ARGUMENT HAS NOTHING TO DO WITH CLADISTICS.  
Cladistics is about reconstructing the evolutionary relationships between a 
certain set of terminal taxa.  It is not, in and of itself, about describing 
the characteristics and lifestyles of the various (hypothetical) organisms 
arrayed along the branches leading to the terminal taxa, except possibly in 
relation to those particular characteristics used in constructing the cladogram.

Thus, if we find that Dromaeosauridae and Troodontidae are closely related, and 
that an enlarged second pedal ungual is one of the characters supporting this 
relationship, we would expect the common ancestor of _Troodon_ and 
_Deinonychus_ to have possessed an enlarged claw on the second digit of its 
pes.  However, it would not in and of itself contradict anything the parsimony 
analysis said if this common ancestor were 40 feet long and consisted entirely 
on ginkgo fruits, if body length and diet were not explicitly involved in the 
construction of the phylogenetic hypothesis in question.


> but amidst claim and counter-claim there doesn't seem
> to have been any attempt to show how, biomechanically, a theropod dinosaur
> Konstruktion can be transformed into a proto-bird Konstruktion.

Actually, it seems to me that the problem has been an overabundance of more or 
less successful attempts to do so, resulting in a number of plausible 
hypotheses.


> In fact I'd go as far as to say that until those palaeontologists who do
> support a theropod origin for birds demonstrate that their cladogram of
> choice shows a permissable sequence of biomechanical transformations, the
> work has only been half done.  I am hapy to admit a large amount of
> ignorance on this particular issue, but I'm not aware of anyone having done
> this.  Until such time as I see such a study backing up a cladistic
> hypothesis of dinobird origins, alternative hypotheses of bird origins, such
> as George's 'Birds Came First', remain just as plausible.

Again, "Birds Came First", as I understand it, is not necessarily a hypothesis 
of phylogenetic relationships, nor is it necessarily incompatible with the 
phylogenetic hypotheses that have been published by "main-stream" 
paleontologists.  BCF is a hypothesis about the appearance and lifestyle of the 
taxa lying in the nodes and along the branches of the cladogram.


> If cladistics was the be-all-and-end-all of phylogenetic
> analysis then why hasn't there been a definitive answer to the question of
> what animals birds evolved from.

Uh, because the evidence keeps changing?  Because people come up with new 
characters to use in their analyses?  Because people keep digging up new 
fossils with new and unexpected combinations of characters?  Because we don't 
know, and will surely never know, every feature of the anatomy of every 
relevant organism?


> Surely no-one on this list
> believes evolution is parsimonious?

Well, now, that's a tricky question.  No, I don't necessarily believe that 
parsimony will find The One True Phylogeny of the various organisms represented 
by fossils in our possession as of 1:42 Eastern Daylight Time on September 20, 
2002.  But yes, I do believe, in my heart of hearts, that if the anatomy of 
every creature that had ever lived were completely known, the most parsimonious 
phylogeny and The One True Phylogeny would be one and the same.

Anyhoo, that's my take.

Nick Pharris
(note new address)