Re: Back-evolution of limbs.

[John Bois <jbois@umd5.umd.edu>]

On Sun, 28 Jun 1998, Larry Dunn wrote:

> Aren't we discussing a monophyletic group here, or, more precisely,
> whether this group ("dinosaurs" + "birds") is monophyletic or not?  It
> seemed to me that an analysis encompassing the entire phylum is more a
> discussion of ancestral rather than derived characters.  If I'm wrong,
> could someone please explain why? Thanks.

I think you are right.  I wanted to exclude the original move to land for
consideration inasmuch as back then there was both no competition and lots
of time.  

I hope this will simplify the idea:

Move from flight to non-flight is easy in birds because rear limbs are not
involved and are therefore available for terrestrial locomotiom.

Move from land to water in quadripeds is easy because of relatively simple
structural change needed, eg., skin between phalanges.  (But why didn't
fish competition prevent it?)

Move from water back to land difficult in birds and mammals because
of pre-existing competition and lack of time to develop complex support
structures.

And, to the point, move from secondary flightlessness to flightedness
difficult because of pre-existing competition and lack of time to achieve
such elaborate structures.  In other words, once a bird comes out of the
air it is unlikely to make it back again.