Re: "Flight theory has legs"

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

Scott Hartman <DinoBoyGraphics@aol.com> wrote:

>      Ok, so the results are preliminary and need to be confirmed, perhaps 
> further study will show otherwise, but common [sic] guys, you are really 
> special pleading to shoe-horn the current results into the trees-down (or 
> trees-sideways) idea of flight origins.

I would say that the morphologies of _Microraptor_ and _Cryptovolans_ 
require very little "special pleading" to be shoehorned into (a) an arboreal 
or (b) a gliding ecomorphology.  What's the difficulty?  the fit appears to 
be very comfortable.

> Dinobirds leaping from branch to branch would be in dire need of pedal 
> claws for grasping the branches the landed on.

Excuse me, but never once did I say that arboreal/scansorial maniraptorans 
leaped from tree to tree, or from branch to branch.  The evidence favors 
descents from trees to the ground on the part of small-bodied, 
long-feathered maniraptorans.

> The condition is made worse because their long columnar limbs put their 
> center of gravity further away from the branch, decreasing mechanical 
> advantage for stopping their momentum.

Please show me your calculations for estimating the center of mass (CM) in 
Mesozoic maniraptorans.

I would also add that, as demonstrated by Gatesy and others, the most basal 
birds retain primitively long hindlimbs that are strikingly 
dromaeosaurid-like in their proportions.  This includes _Sinornis_, which 
had a highly specialized perching pes.

> Archaeopteryx and other dinobirds may not have even had a fully retroverted 
> hallux, and they certainly did not have the large flexor tubercles and 
> curved pedal phlanges needed for such scansoriality.

Well, perhaps the fact that they had arms and hands to help them climb 
alleviated the need for a specialized grasping pes.  _Archaeopteryx_ (at 
least) does appear to have had a fully retroverted hallux.

>     I know that a trees-down origin is more intuitive,

Intuitive, schmuintitive.  A "trees-down" origin of birds is corroborated by 
anatomical and phylogenetic evidence, and is at least as strong as the 
"ground-up" scenario.  BTW, as has been argued many times on this very list 
(by myself, among others), there's no reason why these two scenarios 
("ground-up" vs "trees-down") should be considered mutually exclusive.

> and I've convinced that both bats and pterosaurs evolved flight in an 
> arboreal context,

Intuiton is all we have in the case of pterosaurs and bats.  There are no 
known (or recognized) fossils of non-volant sister taxa to either the 
Chiroptera and Pterosauria.

> but dinobirds are very different.

Yes, they have a fossil record - which includes scansorial, aerial taxa such 
as _Microraptor_.




Tim

_________________________________________________________________
MSN 8 with e-mail virus protection service: 2 months FREE*  
http://join.msn.com/?page=features/virus