[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: More Therizinosaurs



In a message dated 97-09-06 11:40:54 EDT, m_troutman@hotmail.com (Matthew
Troutman) writes:

<<  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 latter is true only in a derived subgroup of prosauropods that I do not
consider directly ancestral to segnosaurs. In Thecodontosauridae,
Anchisauridae, Massospondylidae, Yunnanosauridae, and _Riojasaurus_ (and
perhaps all Melanorosauridae), the mandibular articulation point is >at< the
level of the dentary tooth row. I made this distinction quite clear in my
initial article for Gakken on the evolution of sauropodomorphs. See also
_Dinosauria_ page 324, fig. 15.3.

On the other hand, _Segnosaurus_ shows a definite coronoid eminence. See
_Dinosauria_ p. 411 figure 18.2A,B. The coronoid eminence is pretty much gone
in _Erlikosaurus_. If you make segnosaurs to be derived prosauropods, then
you must argue that the coronoid eminence is lost; if you make segnosaurs to
be derived theropods, you must argue that the coronoid eminence of
_Segnosaurus_ is acquired.
 
<<  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 agree completely, except that it's better to think of it as if they're both
derived from the same kind of small-theropod skull. Some kind of primitive
bullatosaur.

<< 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.>>

In my phylogeny, prosauropods are among the most primitive of all dinosaurs,
so you will find >lots< of plesiomorphic character states in that group. This
is one of the things that makes it so difficult to convince anyone of the
prosauropod nature of segnosaurs. Plesiomorphic character states are not
terribly useful in phylogenetic analysis. The way around this is to show that
it is likelier for segnosaurs to have retained the primitive characters of
prosauropods that for them to have reversed these characters from their
derived states within Theropoda back to the primitive prosauropod state. A
reversal is a derived state of a derived state, and there is no a priori
reason for reversals to >always< return to the primitive state found in the
>same< group (in this case, Prosauropoda).

Last night I posted a reply to this list concerning the characters used by
Russell & Dong in their analysis of _Alxasaurus_. They listed three derived
characters for _Plateosaurus_ (all the others were in the primitive state);
all three derived characters are also found in therizinosauroids. 

<<  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. >>

This character state is not seen in troodontids, so, in view of the
above-established relationship between troodontids and ornithomimids, I'd say
it's convergence. Or do we have here yet another reversal, this time in
troodontids?

<<  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.>>

The prefrontals of _Erlikosaurus_ are pretty large and prominent--perhaps not
quite as prominent as in prosauropods but certainly in the same neighborhood.
On the other hand, the prefrontals of ornithomimids and troodontids are
downright vestigial--so if we're going to derive therizinosauroid prefrontals
from those of their purported closest theropod relatives, we have to have
>yet another< reversal, don't we?

<<  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.  >>

See above.

<<  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. >>

See above.
  
<<  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. >>

See above, on plesiomorphic characters in prosauropods.
  
<<  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, >>

also found in ornithischians, which I have as the sister group of
segnosaurs...

<<the reduction of the preacetabular blade.>>

it's emphatically >not< reduced in therizinosauroids; it's laterally expanded

<< 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.>>

In this you're quite correct. The therizinosauroid pelvis has some characters
convergent with some theropod pelves.

<< 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! >>

Actually, in my phylogeny, therizinosauroids evolved their pelvis for the
most part independently of any other dinosaur group, and as you'd expect, it
looks quite different from the pelvis in other dinosaur groups.
 
<<  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? >>

The problem with the foot, as I have stated here on several previous
occasions, is not the re-evolution of a lost digit but the movement of the
articulation of the first digit from the distal back of metatarsal II to the
tarsus or, less likely, the proximal medial side of metatarsal II. We also
would have re-evolution of the proximal enlargement of metatarsal I (lost in
all theropods), as well as reversal of the non-overlapping nature of the
proximal metatarsals in theropods back to serial overlapping (although one
digit is out of line) as in prosauropods. The pes is not just "rebroadened,"
it is completely rearranged.

By the way, thanks for the most substantive discussion of segnosaur
relationships I've had in a long while on this list.