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