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Re: Digits



In a message dated 97-09-08 14:25:16 EDT, jrhutch@socrates.berkeley.edu
writes:

<< Yes, it does seem that inheritance of digital atavisms is uncommon, and I
 agree that it is a general trend that digits are not regained once lost.
 But it is not an inviolate law; no developmentally minded evolutionist
 would hazard to make such an assumption a priori. >>

So as not to be further misunderstood, let me state here and now that I agree
without question that it is "not an inviolate law." My point is that when
one's cladogram starts to show a plenitude of serious reversals--which the
cladogram of therizinosauroids among the theropods assuredly does--then it
might just be time to have another go at the cladogram. I'm perfectly willing
to accept reversals here and there, every so often; but I think I've counted
more than a dozen reversals required to accommodate segnosaurs among the
theropods, reversals that, strangely enough, take a character state into
Prosauropoda or Sauropodomorpha >every time.< Remember, a reversal is a
derived state of a derived state, and there's no _a priori_ reason for a
derived^2 state to become the same as the primitive state, or to become
characteristic of the same clade as another reversal. So, when one takes the
segnosaurs out of Theropoda and redoes the analysis, the reversals >vanish<,
and instead there appear some convergences with Theropoda. I consider
convergence much more likely than reversal, given the exclusive choice,
particularly since segnosaur-theropod convergences occur in characters
randomly dispersed throughout the anatomy: a few in the skull, a few in the
limbs, and so forth. Sort of like neutral mutations. Indeed, the convergences
of segnosaurs show no particular preference for any theropod subgroup; some
are ornithomimid-like, some dromaeosaurid-like, some oviraptorsaurian, and so
forth. Just what you'd expect from random convergences.