Re: Bird arms

[twilliams@canr1.cag.uconn.edu]

> All this talk about secondarily flightless dinosaurs has got me wondering...
> How come, when flying dinosaurs decide to return to the ground, they usually
> reduce the arms, in some cases making them mere vestiges or practically
> nonexistent?  Wouldn't it make more sense to evolve "normal" arms again, since
> you're not using them for flight anymore?  

Something to do with Dollo's Theory / Rule / Tendency, which states that 
organisms cannot re-evolve adaptations once they are lost.

Alvarezsaurids have truncated, coossified forelimb bones because they 
evolved from volant birds which had a fused carpometacarpus.  A fused 
carpometacarpus is (presumably) useless for predation, but it may have been 
co-apted as a digging instrument by alvarezsaurids.

However, Greg Paul (in Predatory Dinosaurs of the World) suggested that
dromaeosaurs are the flightless descendents of archaeopterygids.  And 
if Andre Elzanowski is correct, then the ground-dwelling 
oviraptorosaurs may be secondarily flightless avialans.  If either 
scenario is correct, then certain ground-dwelling coelurosaurs did 
retain (rather than regain) the use of their hands and forelimbs for 
procuring food.  Most recent cladistic analyses (Holtz, Sereno, Norell,
and a whole host of others) suggest that dromaeosaurids and 
viraptorosaurs are outgroups that evolved before the Avialae (=birds, 
including Archaeopteryx).

And then there's BCF...