RE: The iguanodont paper

[Tim Williams <twilliams_alpha@hotmail.com>]

Greg Paul wrote: 

> The above arguments can be flipped to show why they are simplistic. For
> example, it remain extraordinary that among all the dinosaur groups only one, 
> the
> flying birds, have developed marine forms, unlike mammals which have spawned a
> series of marine forms from nonflying types.  

Here's the thing.  You're saying that birds are "only one" group because you're 
drawing a line under Aves and treating birds and non-avian dinosaurs 
separately.  Certain members of one group (birds) evolved a marine ecology, 
whereas the other group never did (non-dinosaurs).  But my question is: why 
treat them separately?  If you are saying that some quality about being a bird 
facilitated their evolution of an aquatic/marine ecology, then I completely 
agree.  This is an ecomorphological statement, and there have been studies to 
back this up.  

But if you say that "Dinosaurs never evolved an aquatic/marine ecology", this 
is both a phylogenetic and ecomorphological statement.  And the phylogenetic 
statement is incorrect - because Aves is nested inside Dinosauria.

All I'm trying to say is that context is important here.  You claim that it is 
extraordinary that non-avian dinosaurs never became marine; but in making such 
a claim you are arbitrarily excluding the one group of dinosaurs (the birds) 
that did take up a marine lifestyle.  Therefore, the claim is not really 
extraordinary at all.  It's circular reasoning at work.

> This important truth is obscured
> by the silly notion that saying that just because one group of dinosaurs
> evolved marine forms that this is typical of the entire clade. 

No, I'm not saying (or even implying) this at all.  What I'm saying is that 
when you're discussing the morphological diversity (or morphological disparity) 
of a given clade, you cannot arbitrarily exclude certain members of the clade.  
There has to be a reason for excluding certain members (such as birds vs 
non-avian dinosaurs), and that reason has to be more than just "because they're 
different".  

Hadrosaurs have a tooth battery.  Hadrosaurs are dinosaurs.  Yet, this does not 
mean that a tooth battery is "typical" for dinosaurs.  Similarly, some birds 
are marine.  Birds are dinosaurs.  Again, this does not mean that a marine 
lifestyle is "typical" for dinosaurs.  Why do the rules of engagement suddenly 
change when we're discussing avian vs non-avian dinosaurs, compared to (say) 
hadrosaurian vs non-hadrosaurian dinosaurs?  Why can't we treat Aves the same 
as any other subset of Dinosauria?  Why the special pleading, just because the 
thing has feathers and can fly (maybe)?

> In any case the old
> exclusion of birds from Dinosauria was not a taxonomic issue, but a
> phylogenetic one due to the mistaken consensus that birds evolved from early 
> archosaurs
> independently of theropods.

This is only true if you regard taxonomy as separate from phylogeny, rather 
than the former being predicated upon the latter.  Even with the wide 
recognition that birds evolved from within the Dinosauria, many people still 
discuss the relationship between birds and dinosaurs in Linnaean terms.  Under 
this neo-Linnaean view, birds are regarded as evolving from dinosaurs, but 
still remain as a "class" separate from dinosaurs.  Sure, the concept of "Class 
Aves" may persist simply for taxonomic convenience.  But in maintaining "Class 
Aves" we carry a whole lot of typological assumptions along for the ride, and 
those assumptions are just plain unhelpful.

> Hadrosaurs were not a mere subset of iguanodonts.  

Yes they are.  Hadrosaurs are indeed a subset of iguanodonts.  
Phylogenetically, anyway.  The "mere" part is just an unnecessarily subjective 
qualifier.  I don't subscribe to the "mere" part; but I'd say the rest is true.

I'm really trying to hammer this point home: Phylogeny does not make judgements 
about relative success (or otherwise) of individual taxa.  Phylogeny confines 
itself to making statements about how taxa are related to one another.  

> They had more complex
> dental batteries -- the most sophisticated among dinosaurs - a tendency to 
> develop
> nasal passage crests, modified ilia and the like.  

All very impressive, and I agree with you 100%.  But you're missing the 
context.  I'm not claiming that hadrosaurs were "superior" or "better" than 
other iguanodonts, because such discussions lay outside phylogenetics.  I'm 
merely saying that, phylogenetically speaking, hadrosaurs are nested inside 
Iguanodontia.  This in no way detracts from how successful and interesting 
hadrosaurs are, or how many novel characters they accrued since they split off 
from "iguanodontids". 

> Lumping them together is
> rather like lumping bison and cattle. Of course, we could call bison noncattle
> bovids. Or are cattle nonbison bovids? 

I detect a note of facetiousness here.  ;-)  But there are examples of when 
phylogenetic nomenclature is helpful.  "Non-avian dinosaur" is one example .  
So is "non-dinosauromorph archosauriform" is another.  "Non-primate mammal" is 
another.  Of course, these are all horribly long-winded and cumbersome terms.  
But these terms are also explicit, and therefore extremely useful.  

> Is suppose will have to check a cladogram
> and see.

Cladograms are not the be-all-and-end-all of phylogeny.  But cladograms do 
offer a clear and explicit statement (or hypothesis) about the relationships 
among taxa.  On the other hand, a return to warm-and-fuzzy nebulous terms like 
"thecodont" does nobody any good, because they allow people to be vague (and 
even evasive) about their evolutionary hypotheses.  Statements like "dinosaurs 
evolved from thecodonts" are more notable for how much they *avoid* telling us 
rather than for what they actually *do* tell us about the ancestry of dinosaurs.

Cheers

Tim

_________________________________________________________________
Connect and share in new ways with Windows Live.
http://www.windowslive.com/connect.html?ocid=TXT_TAGLM_Wave2_newways_112007