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Mike Taylor wrote:
ALL RIGHT! We GET IT! We ALL UNDERSTAND that birds are dinosaurs.
Now can we please -- just occasionally -- have a conversation where, when
someone uses the word "dinosaur" in a context where is very, very obviously
means "non-avian dinosaur", they are allowed to do sowithout half a dozen
people leaping down their throats waving their big cladistic banners?
But Mike (Taylor), Mike (Keesey) makes an excellent point. Why would you
exclude birds from this particular context? This has nothing to do with
cladistics. Birds are, and always have been, dinosaurs. Phylogenetic
nomenclature only presents this hypothesis more explicitly. Saying that
dinosaurs never became aquatic, with the caveat that by 'dinosaurs' we
really mean 'non-avian dinosaurs', strikes me as unfair to the scope of
dinosaur diversity. Because yours is essentially an evolutionary question,
the pool of taxa relevant to the discussion should include *all* descendents
of a common dinosaurian ancestor. IMHO.
Still, I agree with the semantic aspect of your grievance. In retrospect,
it might have been better if the term 'dinosaur' was limited to non-avian
dinosaurs for vernacular usage, and a new term was erected for
dinosaurs-including-birds (like Avedinosauria or something, analogous to
Cetartiodactyla). Even better, the word 'dinosaur' could be used in the
vernacular sense, and 'dinosaurian' (noun) used in the scientific sense
(i.e., including birds). Although there is no paradigm shift in an
objective biological sense in having birds-as-dinosaurs, there is certainly
a paradigm shift in a more subjective ideological sense that is rooted in
two centuries of Linnaean taxonomy.
Personally, I'm happy with Aves as a subset of Dinosauria; if evolution put
them there, who am I to argue? But I can see the confusion this
nomenclature can cause.
Mike Keesey wrote:
Good points. Another question to ask: why is it that only ornithurans
(sensu stricto), out of all pan-
avians/avemetatarsalians*, were able to return to the water (and repeatedly
so)? [snip] And it isn't "birds" (whichever sense) that are the only ones
to have become aquatic; it's specifically (as far as we know) ornithurans
(node-based sense: hesperornitheans + carinates sensu stricto).
I've wondered this too. My guess is that the ornithurans/euornitheans got
to the water first. Their early evolution is dominated by water-loving
forms. This part of avian phylogeny is chock-full of species that spent
most of their time in or around water (_Gansus_, _Archaeorhynchus_,
_Yanornis_, _Ichthyornis_, etc). At least one of these lineages committed
itself to a fully aquatic ecology (Hesperornithes) in the Early Cretaceous,
and many fully aquatic neornithean lineages may have evolved in the Late
Cretaceous (like penguins).
Although there are enantiornitheans that probably lived near water (e.g.,
_Longirostravis_), overall the Enantiornithes seemed to have preferred dry
land, especially trees.
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