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----- Original Message -----
Sent: Sunday, January 14, 2001 11:19 AM
Subject: Re: avian flight
Can I send this to the list?
you wrote
<<: Bird ancestors, early tetanurans,
theropods, dinosaurs or ornithodirans in general are commonly thought to have
been insectivorous>>
when saying "insectivorous" you refer to ALL the
groups you've mentioned ?
what is the evidence for insectivorous
early , dinosaurs theropods or tetanurans?
Herrerasaurians' and other basal
dinosaurs' (or theropods depending on what phylogeny you
follow) dentition seem hardly indicative of insectivorous
diets;
I once read something about the
multicuspidated teeth of some little pterosaur that were considered sign of an
insectivorous diet and I found that interpretation very convincing;
However Herrerasaurus is a pretty
big-toothed animal and I'd rather
think of it as of an active meat-eating
animal.
Other basal dinos don't
show either dentitions adapted in particular for an insectivorous
diet, although I think it would still be possible for them to hunt insects .
The oldest known(and perhaps basalmost, but I
don't know) tetanuran ( not considering another one, form SA that may even be
older ) is an eight meter-long predator from N Italy and it surely wasn't an
insect-eater neither, I think, were the other basal tetanurans, all being
pretty big beasts.
Basal ornithodirans were little animals and an
insectivorous diet is probable, but I don't know enough about them to comment
on this possibility.
OK: "Bird ancestors, early tetanurans, early
theropods, early dinosaurs or early ornithodirans in general are commonly
thought to have been insectivorous to some
degree." Evidence? The size of lagosuchids
etc.. I'm not speaking of *strict* insectivory (anteater-style) here, just the
fact that today all small non-herbivores eat insects, and not of the size range
of herrerasaurids.
So I think that the solphur argument would
fit pretty well when talking about these animals, but not so much if talking
about the other groups; so i think you should consider the presence of
feathers >from the earliest ornithodirans( and i think this is what you've
proposed somewhere in the post).
I do.
The other thing that made me think a bit was the
possible explanation for the origin of avian flight.
The arguments you use to exclude tail and feet as
propellers in the water are good, but i find the idea of a heavily feathered
long arm( nearly a wing , right?) as a "fin"(althought it would be better
to say "acquatic wing") a bit hard to be accepted.
That is why:
first, a long heavily feathered( with fully developed feathers I
mean) wing would be really non-idrodynamic, because of the nature of
feathers.
They would be really mobile and their movement
would prevent water flowing easily around them(this in
the case of a non active movement, with wings distended laterally or
latero-posteriorly or even kept folded ), in the case of an active stroke, the
problem of feathers would appear much bigger.Feathers wouldn't form a
single unit, like a paddle or a fin, but instead a non-uniform
highly instable surface thus of little help as propelling
surface.
It would be really expensive in terms of
energetic needs because a predatory lifestyle would need a continuous
movement, trying to catch really fast little fishes.
The rest of the body would seem a big obstacle
too: the legs couldn't be kept in line with the rest of the body, for simple
mechanical reasons( I don't think the hip joint can allow such an
extreme movement in an animal that has unreduced hindlimbs like
penguins have ) and would therefore be kept postero-ventrally inclined to
the body axis; this would lead to two main problems: the idrodynamic one and
the one concerning the damages that would occur to the animal's hip joint
after such a prolonged position.
You know, most or all of the above is true,
and yet it works in today's dippers (Cinclus), as I have mentioned, who
are passeriforms (songbirds; they are called "water blackbirds" in German) and
have, count them, ONE adaptation to their lifestyle, they can close their
nostrils with flaps of skin that are unlikely to fossilise. Obviously
coelurosaurs never got very specialised in this lifestyle before the evolution
of penguins, auks and plotopterids (extinct pelicans which converged on
penguins).
The tail would be useless at least but very
probably of some obstacle too.
I should have added (or have I? I think I
have) that Ebel proposes that the stiff, feathered tail of Archaeopteryx was for
steering underwater (changing directions laterally would have been possible by
rotating the tail, he writes; pelicans use their remarkably stiff tail feathers
for steering when they're diving).
Another problem:"flying "underwater could need a
different movement than that needed for an active flight in the air.
Ebel, who seems to understand the physics
(at least better than I) and includes force diagrams, says no.
And again, why should a diver, and active
underwater hunter, try to fly in the air after having cought its
fish?
Trying to escape from predators? Some kind of
pressure is needed for such an expensive behaviour to evolve , and i think you
would need the same explanation given by the ground-up
supporters.
<<Thus, after having got a fish, it’s
easier to reach the shore by flying in the air than underwater.
>>
I think it would be an adaptation of the use of
the flight ability more than the selective force that selected
it.
Well, as you have mentioned, underwater
flight is expensive with such a body shape, and flying in air is easier (after a
few adaptations to underwater flight have evolved). Flying offers the advantage
of being able to get to new fishing grounds fast and to look for fish from
above, which tends to be easier.
I think you provided some
evidence for some changes( well, i've not the knowledge to be
able to say this, but this is my impression) in the philogenies(although i
find it revolutionary), but i think , from what you've written(I have to read
the paper, though) that the base behavioural explanation (may
I say ecological?) is weak and and difficult to be tested.
Biomecanichal studies have been done about the
possibilities of climbing trees, running keeping the arms laterally distended,
changing direction and getting balanced using the tail, and this studies have
been used to support one theory or the other , so i think some work should be
needed to try to test you ipothesis.
Of course it must be tested. It's supposed
to be science. Much hasn't done yet because the Ebel paper is from December
1996, and I have hardly anywhere seen a citation of it. (Though Chatterjee in
his 1997 book tries to combine parachuting out of trees with landing in water
and then underwater flight; I think he must have read the paper, just he doesn't
cite it).
I find it unlikely to have happened, for the
reasons i've written, but some others may find it useful to explain the origin
of avian flight based on some reasons I've missed or by demonstrating
that what I've said is incorrect.
True, it doesn't sound extremely likely,
but more likely than ground-up and trees-down (and the rather silly speculation
of "ground-down", which involves a coelurosaur living on slippery ground and
evolving feathers for parachuting instead of walking away).
Thank you for your comments, you're the
first one to comment at all!
PS Have you received any attachments with
my post? I didn't send any, but Mary Kirkaldy has received one and
complained, probably her computer doesn't understand HTML
mail.
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