Re: bird evolution

> There were always two theories as of from what type of vertebrates did
birds
> arose: from dinosaur or from other reptiles. Now we know that: a) birds
> probably arose from theropod dinosaurs; and b) that classic Reptilia is so
> really polifiletic, and that all classis as Dinosauria or Synapsida should
> be seperate types in Vertebrata or should divided into two types:
Synopsida
> and Sauropsida.

Reptilia is a polyphyletic taxon ? Well: that would imply that the amniotic
egg appeared several times independently during the evolution of tetrapods,
which seems (to me) very unlikely... Reptilia is undoubtely a paraphyletic
clade, but not polyphyletic.

Another question. I have a huge "theoretical" problem with paraphyletic taxa
and their treatment within the cladistic paradigm. Can anyone here give to
me at least one (non-circular) reason to methodologically eliminate
paraphyletic groups from our systematic *and* phylogenetic classifications?
Why paraphyletic taxa should be estimated to be useless for our
understanding of evolution? For me, taxa such as Amphibia, Reptilia, or
Condylarthra, are very usefull systematic *and* phylogenetic concepts. Of
course they are "evolutionnary grades", but everything -- including a
monophyletic taxon -- living in past and present nature is an "evolutionnary
grade"!... On the other hand, most of time a monophyletic group is nothing
but a forthcoming paraphyletic group: a monophyletic taxon probably will
become a paraphyletic taxon in the future, but a paraphyletic taxon will
never become a monophyletic taxon... Therefore, I think there is no
"essential" (i.e., from an evolutionnary-process point of view) difference
between a mono- and a paraphyletic taxa, and I really don't understand why
the latter should be eliminated from our phylogenetic classifications -- as
most cladisticians claim.

Gilles.