Re: Phytodinosauria status

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

David Marjanovic wrote (in response to George):

> > The basalmost dinosaur could well have
> > been an obligate semisprawling quadruped (and arboreal into the 
> bargain).
> > If,
> > as I think, the forelimb evolved gradually into a functioning wing
> > (progressively losing its outermost digits in the process) in a lineage 
> of
> > arboreal dinosaurs,
>
> Here you IMHO run into the same problems as Feduccia -- why should the
> forelimbs (and only they) evolve into wings in a quadruped, and why should
> they lose digits in an arboreal animal???


IMHO too.  Modern gliders are arboreal and quadrupedal, and all are likely 
to have evolved from arboreal and quadrupedal ancestors: "flying" frogs, 
"flying" squirrels, "flying" possums, "flying" lemur (colugo)...

Most gliding mammals have the gliding membrane (patagium) strung between the 
forelimbs and hindlimbs.  This gives the largest possible extent for the 
gliding surface.  This is important for generating lift.

Among gliding mammals, the only exception are "flying" primates, such as 
sifakas.  Primates are primitively quadrupedal, and most primates move 
around on the ground using all four limbs.  However, they progress from tree 
to tree by bipedal leaps -
most lemurs (and relatives) are "vertical clingers and leapers".  The 
purpose of the sifaka's gliding surface (small "armpit" patagium and 
brachial "mat" of hair) is as much maneverability as lift.

There is no evidence that the ancestors of birds were anything other than 
bipedal - unless George is advocating a "Tetrapteryx"-like stage in the 
evolution of birds.


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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