RE: "The mystery of the furcula"

["Williams, Tim" <TiJaWi@agron.iastate.edu>]

As Nick very articulately pointed out, the range of shapes and flexibility
exhibited by the furcula of modern birds - and its presence in some
flightless birds and its absence from some flying ones - suggests a certain
lability of function.  Certainly, it has some role in forelimb locomotion,
particularly adduction; but whether it's a spring or a rigid brace is
another matter. 

In the context that George is using the furcula in his 
"reclassificationary motions", I think he's trying to advance the presence
of the feature as some sort of "key apomorphy".  The trouble with organizing
taxa based on a single "key apomorphy", is that you can end up painting
yourself into a corner: if the "key apomorphy" pops up in another group,
what do you do?  

For example, if the Theropoda is defined (as opposed to diagnosed) by the
presence of a furcula, what do you do if a basal dinosaur (or even a
prosauropod) turns up with one?  Re-define the Theropoda?  Or expand the
Theropoda to embrace these "furculate" members of other groups, thereby
rendering the Theropoda paraphyletic.

That's just one of the problems behind apomorphy-based classificationary
systems - as Ken is in throes of discovering.  Groups have historically been
defined on the basis of one or more "key apomorphies", under the assumption
that these apomorphies represent a fundamental advancement in form or
function (usually both).  

Which brings me to the second problem in rooting your classification in "key
apomorphies": what do you do with incipient structures?  Was the fusion of
two clavicles into one furcula such a big step?  When is a feather not a
feather, and when is a wing not a wing?  The traditional definition of Aves
drew upon the typological equation between feathers and flight.  Now, with
the discovery of fossils that show feathers (or feather-like structures),
but could not fly (and never did), this definition disintegrated.  

Ken is trying to accommodate this by looking for alternative apomorphies to
define Aves - e.g. semilunate carpals (form) and arm-folding (function).
But, as Mickey M. has discussed at length, it's just postponing the
inevitable.  The same set of problems will recur for whatever "new" array of
apomorphies you marshall in support of your new definition.

To use a classical analogy, it's like Sisyphus pushing that boulder up the
mountain; just as he's about to reach the top of the mountain, the boulder
slips from his grasp and tumbles down again, and Sisyphus has to start all
over again.  But poor old Sisyphus is fated never to push that boulder over
the cliff - not through want of trying, but because something will always
come up to frustrate his efforts.



Tim