Re: Feather Flap

[dinoboygraphics@aol.com]

Tim Williams wrote: >>>But colugos and dracos evolved within lineages 
that were already specialized for arboreality. In these groups gliding 
evolved as a means of commuting from tree to tree. The same is true for 
most gliding tetrapods, including gliding marsupials and rodents. I'm 
not sure this holds for the first gliding theropods, who may have 
evolved gliding ability as means of getting from trees to the ground. 
In this case, it would make sense to retain a morphology that was 
suited for terrestrial bipedal locomotion. Of course, this scenario is 
a "just-so" story, and not evidence - but I don't see any reason to 
assume that you have to be specialized for arboreality in order to be 
an effective glider.<<<

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, 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.  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.

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.  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.

> > > This is not true for all arboreal gliders. Some gliding amphibians 
have flaps on the distal limbs, such as "flying frogs" (_Rhacophorus_ 
spp.) which have huge webbed feet for gliding. The "flying geckoes" 
(_Ptychozoon_ spp.) have gliding surfaces on the abdomen and feet that 
are not confluent. <<<

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 ).  The smaller distal 
membranes are used for control...the exact mechanism I'm invoking for 
maniraptorans, except not for tree to tree locomotion, since clearly 
maniraptorans (including early birds...under any definiton) are not 
using their stomachs as a significant lift-inducing mechanism.

I think Mike made several good points about tertial preservation, and 
that's all I have to say for now...

> > > Nevertheless, certain maniraptorans do show some traits that might 
be associated with arboreality - or at least scansoriality. There's 
small body size, for one (yeah, I know this one's a stretch). There's 
possible scansorial/arboreal traits in the manus and pes, including 
(for example) an enlarged hallux that has shifted distally on the foot 
compared to other theropods. Still not a reversed hallux, just a longer 
and lower one.<<<

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.


Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333

www.skeletaldrawing.com


-----Original Message-----
From: Tim Williams <twilliams_alpha@hotmail.com>
To: dinosaur@usc.edu
Sent: Mon, 4 Jun 2007 6:20 pm
Subject: Re: Feather Flap


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Nevertheless, certain maniraptorans do show some traits that might be 
associated with arboreality - or at least scansoriality. There's small 
body size, for one (yeah, I know this one's a stretch). There's 
possible scansorial/arboreal traits in the manus and pes, including 
(for example) an enlarged hallux that has shifted distally on the foot 
compared to other theropods. Still not a reversed hallux, just a longer 
and lower one.Â
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CheersÂ
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TimÂ
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