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More Therizinosaurs






Dinogeoorge wrote:
>People on this list at various times have noted (I'm paraphrasing here) 
that
>segnosaurs have the lower jaw of a derived ornithomimid, the skull of a
>derived troodontid, the pelvis of a derived dromaeosaurid, and a random
>assortment of derived oviraptorid features. The hind feet are unlike 
those of
>any theropod. But one can find a similar lower jaw among prosauropods, 
a
>similar skull among prosauropods, and a similar pelvis among the 
prosauropods
>(although there's nothing like the ilium of a segnosaur in any other 
dinosaur
>group; it's a segnosaur autapomorphy)--as well as similar hind limbs 
and
>feet. (Here I'm using the word "similar" to mean "sharing several 
characters
>with," not just "looking like.") I can show you lots of cranial 
characters of
>_Erlikosaurus_ that also occur in _Plateosaurus_, for example; the 
point
>being that in segnosaurs we can find suites of prosauropod characters 
but not
>suites of theropod characters. So why shouldn't I think that segnosaurs 
are
>derived prosauropods, or at least close to that lineage?

 First of all therizinosaurs do not have a prosauropod mandible. They 
have a more theropod-like mandible. One of the linking features among 
prosauropods is the increased prominence of the coronoid process which 
therizinosaurs * lack*. Also in ALL prosauropods the articular is 
depressed below the tooth row. Quite unlike the condition in 
therizinosaurs where the articular is at the tooth row level. 

 The troodontid skull is more like a derived ornithomimid. The 
proportions fit together, the orbit is proportionally like that of an 
ornithomimid, the parasphenoid is bulbous and pnematizied ( sp?), and 
the teeth are like that of a derived troodontids or basal ornithomimid. 
Actually it can be said that an ornithomimid skull is a derived 
troodontid skull. I did a lot of thinking on the therizinosaur phylogeny 
yesterday and I decided to compare the derived features of the 
prosauropod skull to the therizinosaur skull. Here's what I found:
 1) Loss of caudal contact between the premaxxila and nasal. 
 Plesiomorphic.
 2) Decrease in size of the antorbital fenestra by development of 
lateral lamella on maxilla ( from Galton.) Strangely ( unless I am 
misinterpreting this) this is the same condition seen in ornithomimids. 
Right? In both therizinosaurs and ornthomimids the development is 
rostral and extends and tapers dorsally. 
 3) Incresed prefrontal contributation to the orbital rim. And the 
consequencesal decease in contributation of the frontal. This is *not* 
seen in therizinosaurs. In fact this is the direct opposite. 
Therizinosaurs have large contributation with the frontal. And the 
prefrontal is reduced . This is opposite that of prosauropods. But not 
unlike that of maniraptors and ornithomimids.
 4) Depressed articular. As I expressed above therizinosaurs have the 
direct opposite of this. The jaw joint and the articular are not 
depressed below the tooth rows and this is directly opposite that of 
prosauropods.  
 5) Increased depth of the coronoid. Then again this is not seen in 
therizinosaurs. In prosauropods the coronoid process is very prominent 
and it is visible in both medial and lateral views. In therizinosaurs by 
contrast, the coronoidr is reduced and is gone. 
 
 Well even though these are just some of the prosauropod features in the 
skull that are considerd synapomorphic for that group and many are a 
*reversal* from the typical dinosaur condition. Therizinosaurs lack all 
of these characteristics or if they have them they are also seen in 
other groups. 
 
 And about the pelvis of therzinosaurs. It can be said that you cannot 
find characteristics like that in any group of dinosaur. Therizinosaurs 
no matter where they are are definitely abberent. But certain 
similiarities between some maniraptors can be found among the 
therizinosaur pelvis. Now I made the half mistake of saying that the 
therizinosaur pelvis is like that of a dromaeosaurid. Well it is obvious 
that they are not very very close to dromaeosaurids, but they do have 
some characteristics that line up nicely to dromaeosaurids. The degree 
of opisthopuby, the reduction of the preacetabular blade. a hook-like 
process extending from the preacetabular blade, a block-like ilium that 
is narrow dorsally, and a large, flaring pubic foot. I hold the stance 
that these two pelves were acquired convergently. The similiarities can 
be said that since they evolved the same pelvis from a typical 
maniraptor pelvis which has some peculiariaties that can be overly 
expressed if the pubis goes to the opisthopubic pubis. I am not good at 
describing things like this but my basic point is that it is possible 
that if the two groups evovled the same type of pelvis from the same 
type of animal, of COURSE they would look similiar! 

 It is not impossible to 'reevolve' a certain characteristic such as a 
toe. But as I can see it the reversal is just a rebroadening of the pes. 
This is not SO hard to swallow. Is it?
 
 WMattTroutman 

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