Re: No more paedomorphosis

["Matthew Troutman" <m_troutman@hotmail.com>]

<<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.>>

     I was using the point of the ischium as an example of how it is 
that Rahonavis was a bird. Rahonavis also has a laterally facing 
glenoid, more closely appressed metatarsals, and a slimmer tibiotarsus 
than most dromaeosaurs. 

The ischium of Sinornis is preserved two different ways in the slab: one 
way it is short and blade-like, the other way it is long and ventrally 
facing. Posterodorsal processes maybe were there ( L. Martin seems to 
think so ), but given that the ischium was rather incomplete and the 
evidence that can be gleaned from it is somewhat ambiguous, I guess we 
will have to wait for Sereno's upcoming paper on Sinornis. 

<<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.>>

     Therizinosaurs also have a first metatarsal that does not contact 
the tarsus and a large ascending process of the astralagus. I didn't 
treat the therizinosaur metatarsus as a reversal, but rather as a 
rebroadening. But before we get tangled in another 
therizinosaur/segnosaur mess, let's move on.

The ischium can well change and perhaps it did, but this still doesn't 
deal with the other characters that say that Rahonavis was a bird and 
not in the ancestral stock of the dromaeosaurs. 

Based on the evidence we have at the time, it seems unlikely that the 
dromaeosaur ischium was ever like a bird ischium ( I think that every 
dromaeosaur ischium known does not have a hint of these processes). 

MattTroutman

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