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

Fw: avian flight



 
----- 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