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THERIZINOSAURS AND LOGIC



George Olshevsky wrote:

<<--particulary the nearly insane notion that segnosaurs are 
bullatosaurs related to ornithomimids (!).  What next?  Frogs that are 
the sister group of cats?>>

Calm down.  You seem to be saying that since this proposal makes no 
sense to you that it is an insane notion.  Some people are just 
interpreting the evidence different from the way that you see it, that's 
it.  There is no reason to call the idea insane (and by extension the 
supporters of the idea by the way you worded it) unless you have or can 
show good evidence that it is akin to making anurans the sister group of 
felids.  

Basically what this whole debate boils down to is how people interpret 
the evidence at hand.  For G.O. and Tracy Ford therezinosaurs cannot 
possibly be theropods, much less bullatosaurs, because they have a 
"four-toed" pes (among other things).  They consider it an almost 
impossible reversal that undermines the idea that therezinosaurs are 
theropods.  For Jon Wagner et al. the characters that therezinosaurs 
share with theropods, and the characters that they do not share with 
prosauropods and ornithischians, show that therezinosaurs are theropods 
and may most possibly be as high up as the Bullatosauria or 
Oviraptorosauria.  For them (including me) the pedal characters can be 
considered a reversal or a rebroadening and lengthening of certain 
elements within the foot.  It should be noted that the dorsal surface of 
metatarsal I does not contact the distal tarsals and this makes the 
therezinosaurid pes more theropod-like.  Along with the expanded and 
inflated parabasiphenoid, the teeth,  the mandible, the shoulder girdle, 
and the preacetabular region of the pelvis it seems possible (to me, 
Peter Buchholz, and Paul Sereno) that therezinosaurs are most related to 
ornithomimosaurs, but not directly descended from them.  Of course, this 
is one way that one can look at the evidence.  

"REVERSALS"

Reversals happen in evolution frequently.  If you have a knowledge of 
groups other than archosaurs (hint, hint ;) ) you can tell that within 
the fairly well-established phylogenies of living animals that reversals 
happen quite often.  For example, the tabular bones of basal reptiles, 
they are lost in most mammals primitively (or at least near "therians").  
But two groups of mammals retain the tabulars: multituberculates and 
chrysochlorids.  Not only are these two groups true mammals, the 
possession of tabulars is considered a true reversal at least in 
chrysochlorids (golden moles).  This is a reversal that is far harder to 
complete than the elongation of the first metatarsal in therezinosaurs 
because it involves a reorganization of the braincase.  Now, using the 
logic that is used by Olshevsky and Ford, we can now conclude two things 
about mammalian phylogeny: 1) that chrysochlorids and multituberculates 
are sister groups; 2) that they either retain a primitive position in 
the Mammalia or are outside Mammalia (and possibly Mammalimorpha) 
altogether.  Of course, both of these conclusions would be ludricrous, 
chrycholorids are obviously eutherians by those features that define 
Eutheria.  Now, can you explain a reversal like this?  Can you tell the 
bearing that it has on the issue of the therezinosaurid pes?

>I think we've put in enough time for at least an honorable mention. :)

OK, Jon Wagner deserves special mention for his wrestling with George 
Olshevsky on the matter of therezinosaurs.  ;-)

Matt

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