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

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



What is biomechanics? (or, The Truth About Flying Snakes - Was: Re: science
and philosophy)
****************************************************************************
*********************************
Just a warning - I'm afraid I've done it again folks, and written a monster.
This post started off as a brief clarification of Jaime's description of
biomechanics, to be followed by a challenge to his statements on Longiquama
and flying snakes.  The biomechanics sort of took over a bit.

For those who are bored senseless by verbose explanations of biomechanical
methodolgy, skip to the middle bit, which has a bit about dinobirds, or the
end, in which I bravely stand up to the Headden monster and teach him a
thing or two about flying snakes....

***************
Dear all,

I've been really enjoying this thread on the philosophy of science, and have
even been moved to start composing a contribution (and most of the threads
I've contributed to lately have died soon after my posting, so be warned!).
Having gotten has far as just pasting in the relevant comments that people
have made, the message is already a few thousand words long, and I haven't
written a thing yet!

An interesting thing that I have noticed is that I agree with about half of
what everyone has said, and disagree with the other half of what those same
people of said.  In fact, if you line it up on one page, it's funny just how
much people contradict themselves as much as each other.  So I am forced to
conclude that we
disagree with ourselves 50% of the time.  Brings to mind Mickey's pearl of
wisdom (as quoted by Tim a couple of days ago) - the easiest person to fool
is yourself.

Anyway, to business.  Since I may never get around to finishing my discourse
on the philosophy of palaeontology (not that I advise anyone to hold their
breath) I just couldn't let this one go...


***************************
Jaime said:
>  No, two people have explicitly explained how one can test volancy: Tom
>Holtz and myself. The answer being biomechanical tests. Comparative
>anatomy and the Extant Phylogenetic Bracket are procedures within which to
>operate in which these tests can be determined ...

And comparative anatomy tells us what, exactly?  That dromeosaur forelimbs
worked liked those of preying mantids, judging by some of the posts to this
list recently.  And as for the 'Extant Phylogenetic Bracket', I thought very
little of this logic when I first saw it, and capatilising it hasn't changed
my mind.   'Hey, the 'EPB' of archosaurs tells us that the specific aerobic
capacity of dinosaurs was somewhere between that of sparrows and
crocodiles..'  Well, we've just described the largest range of
aerobic capacity that you can find in living amniotes. That really narrows
it down....


WHAT IS BIOMECHANICS?
Perhaps we need to remind ourselves of what biomechanics really is (or at
least, how I understand it).  Biomechanics is often talked about as if it is
an inherent property of the animal, like the overall length or some such
(Not sure how the forearm works?  Hang on, we'll just dig out the
biomechanics of this beast, I know I left it here somewhere...).   But of
course it is  not some waiting-to-be-discovered property of the animal under
study.  It is a
conceptual framework for investigating the envelope of possible behaviours
of an organism, as is constrained by that organsim's anatomy.  It explicitly
models the skeletal anatomy of the organism as a mechanism, and then
investigates that model using the theoretical framework of mechanical
engineering.  Note that a biomechanical model is required to consider the
entire skeletal anatomy of an animal (or at least be very confident that the
rest of the skeleton will have no bearing on the final result if the
analysis is restricted to a part of the skeleton, such as the forelimb).
Putting 'functional morphology' in the title of a paper doesn't necessarily
mean that you are necessarily performing a biomechanical analysis.

Comparative anatomy  and 'the Extant Phylogenetic Bracket' are but (small)
components of what a biomechanical analysis should include.  There seems to
be an tendanncy to use 'comparative anatomy' in the reductionist sense (see,
for example, Tony Thulborn's post on
Life Beyond the Cladogram -
http://www.cmnh.org/dinoarch/1998Dec/msg00591.html ), where comparative
anatomy is the comparison of part with part.   This can often be useful in
the initial
stages of constructing a question which might be tackled using biomechanics
(why does a penguin's flipper look like a turtle's?) but does not constitute
the whole (or even half) of a biomechanical anaylsis.  It's really for
identifying animals (or rather, biomechanical models thereof) which might be
appropriate for making informative comparisons with your study animal, and
for assisting you to construct your model.  Sometimes it helps to interpret
your results after the anaylsis (often in the form of constructing the next
set of questions for the second generation of the model).

As for phylogenetic brackets...well, I'll concede that they can be useful,
but they've been horribly overemphasised of late. The construction of a
biomechanical model should not require any phylogenetic information about
the animal under study - it should be a model of the animal's form, not its
evolutionary history.  The only time were phylogenetics creeps into the
construction of the model is were the anatomy is incompletely known and
anatomical information from related _and_ similar taxa is used to fill in
the blanks.  Preferably, the relationship and similarity of the auxillary
taxon should be pretty obvious - the sort of relationship that even a
Linnean taxonomist could get right ;-).  Extending anatomical
reconstructions beyond family / superfamily level is likely to get you into
trouble.  And if the anatomical point of contention is variable enough
within the family that you need a detailed phylogeny to try to resolve it,
then you should probabaly be including a range of the possible anatomical
configurations within your model.

Extant Phylogenetic Brackets are obviously most useful when attempting to
incorporate the soft tissue components of the anatomy into your model, but
if there is any ambiguity then it quickly loses its power.  For example, if
you are trying to reconstruct a muscle system in dinosaurs, and the
morphology of that muscle group is highly conservative within crocs and
brids (and perhaps even other reptiles as well), then you might choose to
simply incorporate that configuration into your model without further
deliberation.  But what if the morphology does differ between crocs and
birds?  You then have to resort to comparative anatomy, and find some sort
of anatomical correlate in the fossil for the soft tissue.  Depending upon
the nature of the structure you are fretting about, you could also look at
trace fossils (prints, nests, gut contents, etc) for inspiration.

Confusingly, the 'Comparative Method' of Harvey and Pagel is a dressed up
version of the 'Extant Phylogenetic Bracket', as far as I can work out.  It
doesn't have anything to do with the rich tradition of comparative biology
in palaeontology.

Of course your model is inevitably going to include some anatomical
assumptions, even with the best preserved fossils, and it is important that
when presenting your model you explicitly state which components of your
model are revealled by direct evidence, and which are assumptions.


Okay, after you've done all this you're ready to perform your biomechanical
analysis.....


>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.  Your model should show you what range of operation
(behavioural envelope, if you like) is possible for your creature, and what
set isn't.  That is to say, if you do it properly, biomechanics can show you
what your animal could have done.  It can't show you what it _did_ do - only
trace fossils (or direct observation, if your creature's still alive) can
show you that.  Incidentally, much of the discussion on this thread seems to
be preoccuppied with 'proving' what the animals did, and Tracy is quite
right about the difficulties of this.  If we spent more time working out
what they could do, and less time worrying about what they did, then I think
vertebrate palaeonotology what be a far more rigorous discipline.

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?   I must be misunderstanding you,
because I can't see how that would work.  If your results fail to decide
between a number of different biomechanical possibilities, then surely
you've got what Jere was talking about in his first post (and I agree with
him on this point, but not the rest of his post) - you've generated a number
of competing hypotheses.  The next step is to find different lines of
evidence that might help you choose between them.  For example, in my
experience of trying to develop biomechanical models of the feeding
apparatus of marine vertebrates, the biomechanical analysis on its own will
rarely tell you which part of the food web an animal might have occupied -
usually it will eliminate some possibilities but leave a few others.  It
then needs further work - looking at trace fossils, doing some comparative
ecology, or adding some new dimension to the biomechanical model - before
you can suggest that your animal specialised on neckton as opposed to
benthic invertebrates, for example.  I just can't see how parsimony can help
this part of the analysis.

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.

RECONSTRUCTING EVOLUTIONARY TRANSFORMATIONS USING BIOMECHANICS
Take, for example, the method developed by the Senckenberg crowd,
Kronstruktionsmorpholgie.  This is based firmly within biomechanical
analysis, but it is not restricted to performing the basic biomechanical
analysis of individual organisms (as I have attempted to describe above).
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.  Usually you'll end up
with a shortlist of possible transformations - A can transform into B, and B
can transform into C, but C cannot be derived directly from A).

Now, for those enlightened folk who are interested in biomechanics this is a
sufficiently interesting result in its own right to justify the exercise.
But this approach can also assist those poor deluded souls who have become
obsessed with generating phylogenetic hypotheses....After you've chopped
your animal into unrecognisable portions - anatomically or molecularly - and
pauped your matrix you're probably going to end up with a few 'nearly
equally likely' trees that you might want to choose from.   From a
biomechanical viewpoint, some of the evolutionary transformations required
by the cladograms are going to be permissable - others will not be.  You
thus have a means of testing your tree which is derived from an analysis
that is basically independent of phylogenetics - not a bad thing, some might
think.

CAN KONSTRUKTIONSMORPHOLOGIE SAVE DINOBIRDS?
What I find hard to understand is why this is so seldom done (at least, as
far as I can tell).  Take for example the present debate on the origin of
birds - an interesting question, for sure. Here you have an attempt to
understand the evolution of flying reptiles from terrestrial ones, and are
blessed with what now appears to be an embarrassment of putative
transitional fossils.  The biomechanical implications of this transition are
huge - the ability to fly should leave biomechanical signatures all over the
anatomy of an ancestral bird as compared with non-flying forms, and yet
(judging by the discussions on this list) this is a point of immense
confusion.  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.  There is an as yet unresolved debate as to how flight might
have evolved from any type of theropod dinosaur - some say ground out,
others say trees down, 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.  It seems to
me that a decent biomechanical analysis of the different animals involved,
and a consideration of which possible transformations are actually
biomechanically permissable, should be of help in clarifying this intriguing
issue.

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.

To me, as a clado-skeptic, the confusion about the origin of birds strongly
supports the contention that cladistics on its own can tell you very little
of consequence.  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.  It's not as if you're short of fossils, is
it?

DON"T FORGET BODY SIZE AS A BIOMECHANICAL PROPERTY
As for 'ground-up' vs. 'tree-down', well, from an outsider's point of view
(I usually skip these posts, so if I've got the wrong end of the stick I
apologise) I am midly surprised that this is still an issue.  Whatever the
protobirds evolved from, and whenever they did it, it seems to me far easier
to derive a bird from a small, aboreal animal than from a medium sized
cursorial one.  My hunch would be that the immediate ancestor would have to
be around 1kg body max - are any of the Chinese animals getting down to that
size, I know they're pretty small by dinosaur standards?  Perhaps I lack the
imagination, but I just can't see how a Velociraptor or Dromeosaurus sized
animal is going to start flying.  And for those who might support an aboreal
origin of birds, I would suggest (for what it's worth) that a small animal
is going to be able to be aboreal without necessarily showing any of the
specialist anatomical features required by an larger climbing beast, so any
anatomical indicators related to climbing ability may be very subtle indeed.

A FINAL WORD (FOR NOW) ON PARSIMONY
The other observation I wanted to make on parsimony is that cladistics seesm
to me to be the only branch of the Life Sciences that uses it in a strong
sense.  I've sat through dozens of seminars on ecology, physiology, and
behaviour (some of these analysing very complex datasets indeed) and never
heard the word mentioned in any of them.  Surely no-one on this list
believes evolution is parsimonious?  If you don't believe in it, why use it?
Okay, you could use it in a weak sense to help indentify those hypotheses
most worthy of testing, but I just feel that if you've got a few different
candidate scenarios then you just have to find different lines of evidence
to distinguish between them.


******************************

THE AWFULL TRUTH ABOUT FLYING SNAKES

Okay, Jaime then said;
>Feathers, as
>comparative tests show, do not make an animal fly, even a bird.

But no-one said that feathers alone make the foul.  Did they?

> The arms
>provide the power, feathers the lift,

Any structure can provide lift under certain circumstances.  I can make a
brick fly if I propel it fast enough at the right angle of attack.  A
facetious point, but worth remembering.

In birds, flight feathers can generate lift at relative air-speeds (i.e. the
speed of the airstream over the surface of the feather - not the total
airspeed of the animal) that are low enough so as to be within the range
that a muscle powered vertebrate can induce by flapping its wings.  Even
flight feathers are not, on their own, a magic ingredient for producing
lift - they do so within a certain range of relative air-speeds and angles
of attack.  All feathers - and any other integumental structures - produce
drag.

It might sound like an obvious and silly point to make (and Jim, kick me if
I've got it wrong), but it is worth remembering because if you forget it
then it can lead to some faulty logic, as in...

> but feathers will not make the
>animal fly on their own, contra proposals about *Longisquama*'s supposed
>aeronautical ability.

Well, we've established that feathers on their own don't make any animal
fly, so to use this logic to make statements about _Longisquama_'s possible
flight performance seems a little disingenuous.  In fact, the presence of
feather-like structures on _Longisquama_ might suggest that it is worth
doing some basic analysis to see if _Longisquama_ might have been able to
fly on its own.  I guarantee that I could make it fly (by shooting it out of
a cannon, for example - it worked for Rocky in Chicken Run, after all), but
a better question would be whether it could 'fly' - either by powered
flight, or by gliding/parachuting from an elevated surface - in a way that
might have been realistic biologically (not to mention non-fatal).   It
seems to me, just from the appearance of the beast, that some sort of flight
might have been within the boundaries of biological achievability for
_Longisquama_.

>Studies in flying snakes show that the skin flabs
>allone (integument) do virtually nothing but impose drag on the snake,


???!!!  But surely imposing drag - and lots of it - is exactly what a
parachuting animal needs its integument to do?  And this is a parachuting
animal, right?  I mean, just because it's called a flying snake, surely
no-one was seriously suggesting that it moved through the air by lift-based
powered flight?

>it's the way the snake moves that provides the ability to glide,
>essentially a directionally-controlled descent.


And paragliders do the same thing.  It's not gliding in the sense of a
glider 'plane, or a soaring bird, but surely most animals that get called
'gliders' - flying frogs, flying squirrels, sugargliders, for example - are
really parachutists of some description (with varying ability to control
their descent)?  The only 'glider' that might be closer to a glider 'plane
that I can think of offhand is _Draco_.

Now, I can't resist observing at this point that feathers are excellent
structures for producing drag,
and lots of it :-)...


****************
Just an interlude here, to observe that, as obvious as the hole in Jaime's
argument appears now (or at least, it does to me ;-), I didn't notice it the
first couple of times I read through his post.  The reason for reading
through the post in detail in the first place was because he had mentioned
the Extant Phylogenetic Bracket early on, so I was determined to disagree
with him (for once - I don't disagree with him that often, so it's a rare
treat when it does happen).  It was only when I was going over the post in
detail (the third time, after a couple of beers) that I realised that he was
talking nonsense about flying snakes.  I suspect that it took me so long
because, apart from being slow, I have a lot of respect for what Jaime says
in general, so I'm not usually expecting his logic to be arse over tit.
Says something about how critically we read respected sources, I think.

****************

Anyway, back to the biomechanics.  Jaime then said;
>  Without the, preferrably, 3-d manipulation that Eriksson and
>Paul/Christiansen did with ceratopsid limbs, and musucular studies of the
>first and of Hucthinson/Garcia and Carranno/Hutchinson, the latter two for
>theropod limbs, we can make a set of testable assumptions based on
>observable morphology. One can see the relationships of most bones and
>infer a relationship that permits a means of restoring the rest. As Novas
>and Puerta did with *Unenlagia* (and Paul dissented from) manipulating the
>limbs are ideal, but you can do as much with less with careful
>observation. My own biomechanical studies is favored by 3D studies, but
>often I must make inferences from flat images. Ideally, I would be doing
>muscular dissections, and because of this my study is limited to the major
>muscles of the jaws and limbs that I have been working with the last few
>years.


I confused again!  Jaime, I'd love to see your muscle studies, but I don't
understand what you mean here.  All biomechanical analyses have at their
core models which are abstractions of the real animal.  It doesn't matter in
itself matter if the fossil is preserved in 3D so you can manipulate it, or
if you're working from 2D photos - it matters only in how this affects the
confidence with which you can construct your model.  Or is this what you
were saying?

Where you are lucky enough to have an undistorted, 3D preserved fossil to
play around with (and a corruptable collection manager!) then the fossil
itself becomes part of the model - but it can't be all if it - what about
cartillage, muscles, etc.?.  But really this is just a detail in the
construction of the model.  The model can also be on paper, in a computer
program, or in 3D - each have their strengths and their weaknesses.  But I
don't see how this is fundamental to the logic of the biomechanical
analysis - which is what I think you're trying to say.

>  *Cryptovolans* defies 3D manipulation, but it can be useful to observe
>the elbow, shoulder, and wrist joints, articulation of elements, and
>determination of ability of flexion. Only then can we assess flight
>ability. After that, we can determine performance if the animal appears
>likely to fly. Feathers won't cut it by themselves, no matter how big they
>are.


After all that, I just don't think I agree with the jist of your approach.
There's just so much more to it than having a casual look at a few joints
and then pronouncing whether the animal could fly or not..  I mean, we're
using similar words, but it doesn't seem to me that what you mean by a
biomechanical approach is equivalent to how I understand it.  This worries
me (because I have to write up my 'theory and methods' chapter soon!)

*************
Okay, that's my 2c on the subject for the time being.  What I've tried to
articulate - for the first time on this list - is a philosophy of
biomechanics as I understand it.  I want to emphasise that I do not,
however, regard my unstanding as being in any way definitive!

I am hoping to get some feedback on some or all of the above, because - as I
just mentioned - the time has come for me to tackle the dreaded 'theory and
methods chapter' of the thesis.  As you might have guessed from the above,
I'm basing a fair bit of my methodolgy within Konstruktionsmorphologie - at
least how I understand it at the moment.  The problem with this is that the
relevant literature is largely in German, which I don't read.  The new
volume on 'Constructional Morphology' (Gudo et al) will be wonderful,
because it is in English (is this out yet?).  Also, there aren't that many
people with whom to discuss it where I am - hence the length of this post to
the list.

Okay, it's Friday night and I'm still sober.  Curses....

Again, apologies for the length of the post.  Perhaps one day I'll learn the
art of brevity.

Colin
"If the vertebrate fossil record of Australia tells us anything, it is this;
dinosaurs, bad; plesiosaurs, good."

Colin McHenry
56 Gaskill St
CANOWINDRA,  NSW 2804,  Australia
Ph: +61 2 6344 1009
Mobile phone: 0428 131 858
email: cmchenry@westserv.net.au