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giant birds



Chris Brochu wrote:

>>"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.)">>

Hear, hear! Thanks Chris! To those who bemoan the inconstancy of 
phylogenetic arrangements under cladistics, what's your alternative? 
Tree topologies are hypotheses, and hypotheses are improved as new data 
come to light. To those who would like a user-friendly but 
authoritative account of cladistic theory and methods, check out "The 
age of dinosaurs" CD (Timothy Rowe, Kyoko Kishi, John W. Merck, Jr, and 
Matthew Colbert) or the book "The Mistaken Extinction" (Lowell Dingus 
and Timothy Rowe). Both are excellent.

Kendall

----------------------
Kendall Clements
k.clements@auckland.ac.nz