Re: Longisquama "feathers"

[Tim Williams <twilliams_alpha@hotmail.com>]

David Peters wrote:

> Such an exotic growth would seem to have to have had some sort of smaller, 
> less exotic and more common precedent. The only apparent precedent that I 
> can imagine is in the dorsal frill of Sphendon, Iguana, Huehuecuetzpalli, 
> Macrocnemus and Cosesaurus ? in that order.

You raise an interesting point.  What ever these things of _Longisquama_ 
were (and I doubt that they were feathers, or even feather homologs) they 
certainly show a predilection for being detached.  Sharov found several of 
these appendages scattered and unattached - leading to speculation that they 
might have come from plants, and that the _Longisquama_ specimen happened to 
be lying on top of a bunch of leaves!  I think the consensus nowadays is 
that the appendages found as part of the specimen were actually attached to 
the body, and so are of animal (_Longisquama_) rather than plant origin.  At 
any rate, it seems that more of these appendages have been found in 
Kirghizstan, detached from the body of _Longisquama_.  If these things do 
fall off easily in life, it weighs against the idea that they had a purpose 
in aerofoil generation, or even parachuting.  Sure, birds lose or shed 
feathers all the time; but they have plenty more where they came from.

Illustrators (following the respective leads of individual scientists) have 
differed over whether to reconstruct _Longisquama_' appendages as a single 
dorsal frill, or as paired 'wings' used in gliding.  As Jaime mentioned 
there does not seem to be any skeletal contribution to these appendages, 
unlike _Coelurosauravus_ or kuehneosaurids.  Interestingly, Senter (2004) 
found _Longisquama_ to be the sister taxon to _Coelurosauravus_ 
(=_Daedalosaurus_), which does have paired appendages that appear to 
specialized for gliding (or at least parachuting).  Every time I see a 
picture of those weird appendages sticking out of _Longisquama_ I'm reminded 
of the methods used by trees for seed dispersal by wind - but I don't want 
to carry that comparison too far.

As Jaime said, because _Longisquama_ is preserved in 2D ('steamrolled') we 
cannot know the 3D orientation of the appendages (collectively or 
individually), or whether two or more rows were originally present.  And 
because the specimen only has its front half preserved, we don't know how 
far back these appendages extended.

Phylogenetically, _Longisquama_ can be removed from the debate over the 
origin of birds and/or avian flight.  There's no compelling evidence that it 
is even an archosauromorph; most recently, Senter found it to be a basal 
diapsid, and close to drepanosaurids.  I know Senter's study will not be the 
last word when it comes to the affinities of _Longisquama_; but at the 
moment an onerous burden of proof is on those who want to tie _Longisquama_ 
to the origin of birds.

Cheers

Tim