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----- Original Message -----
From: calzola
Sent: Sunday, January 14, 2001 1:16 AM
Subject: avian flight David,
I've just read your post to the list about your
paper and have something to comment on
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.
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).
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.
The tail would be useless at least but very
probably of some obstacle too.
Another problem:"flying "underwater could need a
different movement than that needed for an active flight in the air.
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.
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.
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.
Filippo Calzolari
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