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Re: giant birds



Jaime A. Headden wrote:
> 
> Kendall Clement writes, in response to Chroc Brochu
> (whose smug superiority of croc-specialists over
> dino-specialists is even more evidence in person than
> across the internet,


There are several reasons why crocs are simply better than dinosaurs. 
We don't have to rely on special effects to see a croc eat someone, for
example.  And remember, "superiority" is a croc synapomorphy.  (I know
because I built a tree for them myself.  Ask Paul Willis - he also knows
this to be true.  Working with tyrannosaurids is showing me that a trend
toward superiority exists in at least one non-crocodyliform lineage, but
they became extinct before the full transition could occur.)




 but whose talk at SVP was
> probably the best I attended [how did a _log_ get in
> there?] :)):

Not sure.  But you're kind to say this - after having the computer crash
on me, I wanted to hide for the rest of the meeting.  


[shortened]


>   The fact is, as George has said before, BCF requires
> no miracles. It requires only that there being the
> precedent dinosaurian taxa having been arboreal to
> aquire many of the features that led to birds.

I'm not sure any tree would ever require a "miracle," but the kinds of
changes George proposes are still not optimal on any tree currently
accepted by the community.  The simplest set of transformations on any
of them would resemble the ground-up hypothesis; I know not everyone
buys it, but it's simplest.  The trees-down hypothesis is less simple,
and BCF less simple than that.  DOes that mean they're "falsified?"  No
- only that the best-supported trees we currently have prefer a
different model.  

Whether or not a tree "falsifies" anything will depend heavily on the
question being asked.  Examples:

1.  "Taxon A and Taxon B last shared a common ancestor in the Miocene." 
Discovery of a fossil phylogenetically closer to one or the other in the
Cretaceous would falsify this statement.  This is about as strong a
level of falsification as you will find in a phylogenetic framework.

2.  "Special defenses against nasal mite infestation arose twice
independently within clade X."  If a tree optimizes these special
defenses as a synapomorphy of Clade X, this statement is poorly
supported.  But strictly speaking, the tree *only requires one
transformation at a minimum* - one could still argue that they arose
independently and that the tree simply isn't showing it.  

3.  "Feathers must have been present in the last common ancestor of
ornithischians and saurischians."  At present, the least-inclusive group
for which we can regard feathers (or something like them) a synapomorphy
is within Dinosauria, not Dinosauria itself.  Is the statement
falsified?  No - but neither is it supported by physical, phylogenetic
evidence.  One could predict the discovery of a feathered ceratosaur or
basal ornithischian, but this would not be based on the tree itself, as
(for example) feathered tyrannosaurids would be.

(The painting showing feathered baby tyrannosaurids in the Nat. Geo.
article is precisely that - a prediction.  We would predict that any
theropod descended from the last common ancestor of Sinosauropteryx and
living birds to have feathers.  Of course it's possible they were lost
in one or more lineages - that's why we cannot *conclude* that
tyrannosaurids had feathers, only predict that tyrannosaurid skin
impressions, should they be found, will bear evidence of feathers.  This
same method would predict full pelts on African elephants and all
whales, so it isn't really foolproof.)



-- 
----------------------
Christopher A. Brochu
Department of Geology
Field Museum of Natural History
Roosevelt Road at Lake Shore Drive
Chicago, IL 60605

voice: 312-665-7633  (NEW)
fax: 312-665-7641 (NEW)
electronic:  cbrochu@fmppr.fmnh.org