Re: Feather Flap

[Tim Williams <twilliams_alpha@hotmail.com>]

Scott Hartman wrote:

> See, this is what I'm calling special pleading: we don't even know if it's 
> possible for arboreal gliding to evolve in linneages that aren't already 
> specialized for arboreal locomotion,

We don't yet know that this is *not* possible.

> yet because we "know" that trees were involved in bird evolution you are 
> willing to jump right into arguements for how maniraptorans "could have" 
> done it.

This is not what I was saying at all.  I was not claiming that trees were 
involved in bird evolution.  What I was saying is that in certain 
maniraptorans there were changes in the morphology of the pes.  OK, these 
changes weren't particularly striking, and they mostly center on the 
proportions of the digits and the shape of the claws.  My question is: why 
these changes?  Is it pure coincidence that these characters first appear 
around the same place in the theropod tree as the first aerodynamic 
feathers?

If you want an example of "special pleading", you only have to look at the 
argument that _Archaeopteryx_ had a specialized perching pes because it 
"must have" lived in trees.  The BANDits are guilty of this one.

> Surely in the absence of specialized arborreal linneages of non-avian 
> theropods the burden of proof must be on those who think  that arboreal 
> gliding played a role in early bird evolution, yet not only is there not 
> overwhelming evidence of arborreal adaptations, the case is remarkably weak 
> and continues to get a free pass by many in the field.

The skeleton (manus and pes included) may not be specialized for 
arboreality, but the integument of microraptorans and _Archaeopteryx_ is 
certainly specialized for aerial locomotion.

> Don't get me wrong, I agree that it's easier to explain bird flight if wee 
> invoke trees, but evolution isn't necessarily arranged for our convenience.

Definitely.  I don't have much time for the argument that avian flight must 
have evolved "trees down" because it is "easier" to evolve flight this way.  
The "BANDits" are guilty of this one too.  In my book, as long as a 
"ground-up" origin is physically possible, it's a viable hypothesis that has 
yet to be refuted.

Personally, I favor a scenario for the origin of avian flight in which both 
cursoriality/terrestriality and scansoriality/arboreality each played a 
role.  The ol' "trees-down" vs "ground-up" dichotomy just doesn't work for 
me.

> The data to support such a scenario is so far weak, and the only way to get 
> around it is for more people to study the problem, not to pretend like 
> there isn't one.

I understand that.  I agree that the hypothesis of arboreal locomotion in 
theropods needs more rigor, and I applaud your efforts to play Devil's 
Advocate.

> Absolutely, which is why there is a slide of them in the presentaion I give 
> to the public.  However in both groups the main lift-generating surface is 
> still confluent with the stomach (in the case of gliding frogs, the 
> flattened ventral stomach surface has at least as much area to create lift 
> as the webbing between the toes ).

Are you certain that "flying frogs" use their bodies as lift-generating 
surfaces?

>  The smaller distal membranes are used for control...the exact mechanism 
> I'm invoking for maniraptorans, except not for tree to tree locomotion,

This sounds a little like the "Pouncing Proavis" model of Garner et al. 
(1999)...

Garner, J.P., Taylor, G.K. & Thomas, A.L.R. (1999).  On the origin of birds: 
the sequence of character acquisition in the evolution of avian flight. 
Proc. Roy. Soc. London B 266: 1259-1266

> since clearly maniraptorans (including early birds...under any definiton) 
> are not using their stomachs as a significant lift-inducing mechanism.

What about the tail?

> Indeed, _some_ maniraptorans do show traits that _may_ be associated with 
> arboreality.  I'm really not saying that it isn't possible, just that the 
> evidence is faaaaar worse than the common perception of bird origins would 
> imply.

In some quarters, yes.  I agree wholeheartedly.  But I do think that 
vertebrate paleontologists have taken a much more measured approach to this 
issue than certain (but not all) non-paleontologists.

Cheers

Tim

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