Re: Gliding vs Non-gliding

["Tim Williams" <twilliams_alpha@hotmail.com>]

Marcel Bertolucci wrote:


>     I think that, at least, we've to compare _Icarosaurus_ or 
> >_Longisquama insignis_ to modern (co-)relatives, like the "flying 
> >dragon"(_Draco volans_), a reptile with some sort of membrane beside >his 
> chest that allows him to glide. Not that we cannot take mammals, >for 
> instance, but it would be better taking reptiles to compare to >extinct 
> reptiles.


No.

The most important factor to consider here is biomechanics, not phylogeny.

_Icarosaurus_ (and other kuehneosaurids), _Coelurosauravus_ and the modern 
_Draco_ glide(d) by a membrane supported by rib-like extensions of the axial 
skeleton.  (In _Coelurosauravus_ these rib-like struts are of dermal origin; 
in kuehneosaurids and _Draco_ they derive from the thoracolumbar vertebral 
column).  An analogous behavior has been hypothesized for the dermal 
appendages of _Longisquama_; the evidence is not as convincing.

As such, all these reptiles are pleural gliders.  Their gliding membranes do 
not involve the limbs.

It has been forcefully argued (by Ruben and Feduccia, among others) that 
_Megalancosaurus_ possessed a gliding membrane suported by the forelimbs, 
hindlimbs and possibly the tail.  This is a patagium.  Modern gliding 
mammals glide by way of a patagium.  So (possibly) did _Sharovipteryx_; the 
fossil specimen shows an impression of what appears to be a uropatagium.

As such, modern mammalian gliders (such as flying squirrels and phalangers) 
are more relevant to inferred patagial gliding in _Megalancosaurus_ than the 
"rib-wings" of _Icarosaurus_ and _Draco_.


Tim


------------------------------------------------------------

Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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