Re: New Jeholornis specimen

["Tim Williams" <twilliams_alpha@hotmail.com>]

Gregory S. Paul wrote:

> Yet another example that cladistics is at best hard pressed to handle the 
> degree of reversal, parallalism, and convergence going on in the mosaic 
> development of basal birds that had not yet evolved far from the 
> dinosaurian grade, and were often losing and perhaps even reevolving 
> flight.

I see what you're driving at - and having read both PDW and DA, I'm 
sympathetic.  It is conceptually difficult to distinguish the incipient 
flight apparatus of "pre-flight" theropods, with the "post-flight" apparatus 
of secondarily flightless birds close to the origin of flight.  Some of the 
latter may be unfairly pigeonholed into the former under the invocation of 
parsimony.  However...

> Cladistics is one important tool for restoring paleophylogentics, but not 
> the only one, and in this case is inferior to assessing the relative 
> derived degree of flight adaptations that are present.

I sense a subjectiveness in this method, which relies too heavily on 
assessing the "flightworthiness" of individual osteological characters.  
Take the more lateral orientation of the glenoid (for more anterolateral 
excursion of the humerus) or the broader second manual digit (for attachment 
of primaries).  Both *might* be directly related to the inception of powered 
flight, specifically the flight stroke.

However, these and other maniraptoran / avepectoran characters may initially 
have evolved for other purposes.  For example, the lateral glenoid allowed 
the arms to sweep forward to an improved degree - an advantage when grabbing 
large prey with both hands, or snatching flying prey out of the air with one 
or both hands.  The improved attachment of primaries to the hand may not 
necessarily be an adaptation for resisting torsional forces during flapping 
flight, but for resisting the rigors of other behaviors that involved the 
hands.  Grappling prey, for example.  Or more rudimentary forms of aerial 
locomotion than flapping flight - ground-to-air leaps, or tree-to-tree (or 
tree-to-ground) glides.

Long story short, I think your model understates the role of exaptation in 
the assembly of the avian flight apparatus.  As I said before, I don't 
dismiss the idea that many non-flighted maniraptorans / avepectorans were 
secondarily flightless.  But proposing a volant pedigree for all these taxa 
is throwing the baby out with the bathwater, IMHO.  Personally, I think 
maniraptorans underwent a long period of experimentation with aerial and 
arboreal behavior prior to the advent of powered flight; and that 
oviraptorosaurs, therizinosaurs, and deinonychosaurs are relicts of this 
process.  But only birds took this process to the next (and most demanding) 
level of aerial locomotion: powered flight.

Fire away!




Tim

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