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Re: New dinobird "Sinovenator"



Tim Williams wrote-

> The skeleton of _Archaeopteryx_ is actually one of the most conservative
and
> least specialized of all maniraptorans.  Identifying autapomorphies in the
> skeleton of _Archaeopteryx_ has actually proven to be very challenging
(and
> may simply be because they're just not there).  The integument of
> _Archaeopteryx_ appears to be a totally different matter - although here
the
> frame of reference is much narrower, since comparatively few Mesozoic
> maniraptorans have integument preserved along with the skeleton.

Identifying apomorphies in Archaeopteryx seems to be "challenging" more
because people want it to be the perfect bird ancestor (a metataxon if you
will).  Two very obvious apomorphies are the distally bifurcated ischium and
the lack of caudal neural spines.

> Just because a species is "closer to Neornithes" DOES NOT automatically
mean
> that it is "more derived" (or "more advanced)".  This assumption goes
> hand-in-hand with the outmoded notion that birds should get their own
> "Class" because the darn things can flap and fly.

I too refer to theropod taxa closer to birds as being more derived.  Derived
not referring to superiority or specialness, just amount of difference from
a basal taxon.  We don't have any standard scale of "apomorphic development"
to compare taxa with.  Do segnosaurs, oviraptorids, troodontids or
Microraptor have the most apomorphies compared to each other?  It's not
answerable currently.  Eventually, we'll tease every bit of information out
of the specimens, but not any time soon.  It might be possible to determine
the amount of genetic change from something close to the common ancestor of
life in the future, once we've sequenced more organisms, but this won't work
for most fossils.  It seems to me taxa are deemed to be "derived" based on a
combination of qualifications-
1. The taxon is more closely related to us.  Thus, deuterostomes, tetrapods,
amnoites, mammals, primates, etc. all get to be thought of as more derived
compared to their counterparts (protostomes, actinopterygians, amphibians,
reptiles, uh... dermopterans?).  This doesn't work for a lot of taxa though,
so we depend on....
2. The taxon is living.  Hence, neornithine birds are the most derived clade
of theropods and crocodylians are the most derived crocodylomorphs.
3. The taxon is most speciose.  Often a corollary of reason #2, the most
speciose clade will always be organized around the "main line" when looking
at phylogenies and always on the right side of traditionally made
cladograms.
This is the best way to determine how derived a taxon is in my opinion.
It's always going to have the most taxa between it and the most basal
organism in our phylogenies, so will presumedly develop the most differences
along the way.  I'm aware that evolution is not that steady in all
probability, and that missing species in our phylogenies will have a small
effect.  Still, I feel this method of polarizing derivity(?) is more stable
and easier than trying to quantify apomorphies.

Mickey Mortimer