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Re: Bird arms

In a message dated 98-06-24 14:31:57 EDT, RaptorRKC@aol.com writes:

<< All this talk about secondarily flightless dinosaurs has got me
 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,
 you're not using them for flight anymore?  Boy, evolution is full of weird
 quirks... >>

It all depends on how far along in the evolution of wings the flying ancestral
form got, before giving rise to the flightless form. If the wings were just
slightly modified grasping organs, then the flightless descendant could well
retain the grasping function after the flying function was abandoned. But if
the wings had become more like the highly modified forelimbs of modern flying
birds, then they probably would retain little or no function in the flightless
descendants, and would vestigialize.

A big cladistic problem with vestigialization is that there can be many
different kinds of precursors of a vestigial organ, all of which arrive at the
same derived vestigial state. Lots of phyletic information can become lost
through vestigialization, potentially confusing cladistic analysis. This kind
of thing has happened, for example, with _Hupehsuchus_, an aquatic animal so
derived for its lifestyle that its relationships are presently unanalyzable.