Re: No more paedomorphosis

[Dinogeorge <Dinogeorge@aol.com>]

In a message dated 98-04-16 21:30:37 EDT, m_troutman@hotmail.com writes:

<< <<So...? What does the shape of the avian ischium have to do with the 
 shape of the dromaeosaurid ischium?>>
 
 1)  It shows that Rahonavis is a bird.
 2)  It shows that Rahonavis could not have been part of the ancestral 
 stock of the dromaeosaurs. >>

And the ischium of dromaeosauroids underwent absolutely no change whatsoever
during the 100 or so million years after their divergence from the common
ancestor of birds and dromaeosauroids?? Check out the ischium in _Sinornis_.
Looks quite different from the ischium of _Rahonavis_ to me. Looks more like
the ischium of dromaeosauroids to me. Maybe we even had a >shudder, gasp<
REVERSAL here.

The absolutely hilarious part of this argument is that it is often used
>against< me when I argue that segnosaurs are not theropods. Segnosaur feet?
Don't look like theropod feet? Bah--just a simple reversal (right--of a dozen
characters!), no problem. But now you're telling me that the ischium just
can't change, that the dromaeosauroid ischium is fixed and immutable for all
time. Why not call the dromaeosauroid ischium a >reversal<, eh? A couple of
processes lost, a minor shape change. No big deal.

I'd be more inclined to accept your argument that the avian ischium couldn't
change if everyone else accepted my argument that theropod feet didn't change,
and that segnosaurs aren't theropods. Not to mention, of course, that the
avian ischium >did< change, quite frequently, including reversals, in many
lineages. And that theropod feet >didn't< change very much--certainly not back
to the prosauropod/segnosaur foot, in any (other) known lineage.