Re: fore limbs of dinosaurs

["Tim Williams" <T.Williams@cclru.randwick.unsw.edu.au>]

In response to Aravind L.:

> In the Mesozoic archosaur evolution has been characterized by
> experiments with the forelimbs. One trend which is very prevalent is
> that they have undergone shortening in a progressive fashion along
> some lines. This was first seen in the rauisuchians and the
> ornithosuchians.  Subsequently in the theropod lines like the
> ceratosaurs, allosaurids and maniraptorans it occured independently.

Apart from several clades of flightless avians (hesperornithids and 
kiwis spring to mind), reduction in the size of the forelimbs has 
occurred several times in theropod evolution - abelisaurs 
(_Carnotaurus_ at least), torvosaurids, _Giganotosaurus_ (?an 
allosauroid), and tyrannosaurs.  In torvosaurids it was principally 
the lower elements that were shortened.  These bones were also thick 
and massive, and coupled with the robustness of the pectoral 
girdle, suggests that the forelimbs of these beasts were capable of 
impressive strength.

As for _Giganotosaurus_, I'm not too sure if the _entire_ forelimb 
was reduced.

I wouldn't nominate forelimb reduction as an overall trend in the 
megalosauroids, allosauroids, or maniraptorans.  It occurred 
independently in certain lineages within these groups.

> In many cases like the tyrannosaurs and abelisaurs no apparent
> selective advantage is visible.

Maybe there was no selective advantage.  Maybe the arms of abelisaurs 
and tyrannosaurs were vestigial and on track to being lost 
altogether.  

It is not known if tyrannosaur didactyly occurred before or after or 
even simultaneously with reduction of the forelimbs.  (George 
Olshevsky has some interesting ideas on the reasons on the 
development of didactyly.)  The Tyrannosauroidea is the only group to 
exhibit didactyl mani - _Compsognathus_' forelimbs are too poorly 
known to be sure of what they looked like. 

Anyway, that's my 2c worth (2.5c Australian)
Tim