Phylogenetic vs. Hypothetical Arguments

["Jerry D. Harris" <102354.2222@compuserve.com>]

Message text written by INTERNET:Dinogeorge@aol.com
>Unfortunately, this cannot be the case. There is no >phylogenetic<
evidence
for the cursorial origin of avian flight, nor, for that matter, is there
any
>phylogenetic< evidence for the arboreal origin of avian flight.
Phylogeny
simply presents hypotheses about relationships based on the supposed
distribution of characters; it says nothing about what the organisms that
had
those characters may or may not have been doing with them. Theories of
the
origin of flight are constrained by physics and mechanics, not by
phylogeny.
Bock's arguments hold water.<

        In the absolute _strictest_ sense, yes, there is no actual,
tangible, physical evidence either for cursorial or arboreal origins of
flight -- such evidence would include something akin to film of the first
bird ancestor attempting flight, which of course we'll never have.
However, if the phylogeny shows that the closest common ancestor of birds
(and even moreso if several more distant ancestral nodes) are all
cursors, then the evidence points very strongly towards a cursorial
origin of flight for birds (not necessarily for pterosaurs or bats,
though).  

        Is it possible that, between the closest _known_ ancestor of birds
and the earliest known birds that there remains a huge gap in the fossil
record (though not necessarily a huge gap in time!) in which the cursors
became arboreal and evolved flight from there?  Yes, there is...but this
is negative evidence; hinging a hypothesis about avian flight origins on
negative evidence is less parsimonious than hinging it on positive,
phylogenetic evidence.  As always, new discoveries can change this!

        Is it possible that the interpretation of the closest known
ancestors of birds as cursors is incorrect -- that they were actually
arboreal?  Again, yes...but most of the closest known avian ancestor
(_Caudipteryx_, per Ji, Currie, Norell, and Ji) seems to be well adapted
for cursoriality (for example, possessing much longer hindlimbs than
forelimbs) -- even the next most recent ancestors (other eumaniraptorans)
also make excellent cursors (the arboreal capacity of some dromaeosaurids
could is debatable).  

        I agree with George that Bock's (and others who have elaborated on
it) arboreal theory still has a great deal going for it; as I said
before, I perceive that originating flight from the trees down is
infinitely simpler than from the ground up, and of course am wide open to
the idea that future discoveries of fossils phylogenetically situated
between _Caudipteryx_ and _Archaeopteryx_ will clarify the issue further.
All I'm saying is that, as of this moment, the phylogenetic evidence
points to a cursorial origin of flight, while hypotheses of arboreal
avian flight origins remain unbolstered by actual fossils.



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                     Jerry D. Harris
                 Fossil Preparation Lab
          New Mexico Museum of Natural History
                   1801 Mountain Rd NW
               Albuquerque  NM  87104-1375
                 Phone:  (505) 899-2809
                  Fax: ; (505) 841-2866
               102354.2222@compuserve.com