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Re: OCCAM'S RAZOR & THERIZINOSAURS



In a message dated 98-08-02 16:57:37 EDT, cbrochu@fmppr.fmnh.org writes:

<< Actually, it has more to do with the fact that it requires a whole slew of
 reversals.  I seem to recall a recent post of yours arguing that we should
 prefer convergence over reversal every time, and yet strict interpretation
 of BCF requires multiple reversals to a nonflying condition.  And the
 number of required reversals is immense - this, by itself, renders it
 nonparsimonious. >>

The morphospace consisting of only the characters "flying" and "nonflying" is
one-dimensional (indeed, it has only two points). As I noted in that post, in
such a simple one-dimensional morphospace, reversals may happen frequently;
given enough time, they're inevitable, since the only possible change from
"flying" is to "nonflying," and vice versa. Indeed, today we have >dozens< of
living examples of secondarily flightless birds, which shows just how easily
this particular change happens. Depending on your definition of "flying," the
opposite change from "nonflying" to "flying" might be much more difficult to
accomplish than the change from "flying" to "nonflying." Most vertebrate
lineages remained at the "nonflying" state throughout their evolutionary
history, but, every so often, a lineage did accomplish that more difficult
change to "flying."

But this simple picture is by no means a true description of what happens in
secondary flightlessness. When you start looking at these so-called
"reversals" in detail, examining many more characters than just "flying" and
"nonflying," you see that these turn out not to be reversals at all: Each
lineage loses its flying ability in a different way. No two such "reversals"
are exactly alike, and of course, in no instance do these "reversals" return
the animals to the original states of all the relevant characters, for example
to bring back the small, arboreal, sprawling, quadrupedal ur-archosaur. In
fact, these so-called "reversals" are actually >convergences<; flightless
birds always converge on the morphology of a large, cursorial biped. Each
loses/acquires a different suite of characters from the others, which makes
for quite a free-for-all when attempting to analyze their relationships. This
is why the known theropod lineages have been so maddeningly intractable to
phylogenetic analysis.

Morphological convergence is much easier to accomplish than a reversal,
because in convergence all you need do is come within the ballpark of a
particular morphology, not exactly retrace your steps in morphospace.
Secondary flightlessness is not a reversal but simply a derived state like any
other.

Finally, BCF suggests that >most< Mesozoic avian lineages did >not< leave
flightless descendants. There must have been a profusion of avian species,
much as there is a profusion of avian species today. Just as there are dozens
of Galapagos finches and hundreds worldwide, so must there have been dozens or
scores of archaeopterygids in the Jurassic, exhibiting all kinds of variation
among themselves, occupying all kinds of aerial niches. (Indeed, it has been
argued that just the seven Solnhofen archaeopterygid specimens fall into three
or four species, even two or three genera.) One or two of these species may
have adopted secondarily flightlessness and gave rise to the dromaeosaurids
(for example), but most probably remained small and lightweight and continued
to fly around in the trees. The so-called "reversal" to flightlessness may not
have been a very frequent occurrence at any time, but because some of the
descendants of these flightless lineages became gigantic and ranged across
significant spans of territory, their remains have turned up with some
frequency in the fossil record, skewing our conceptions of these events.