Re: Re: Quo vadis, T. rex

[Thomas_R_HOLTZ@umail.umd.edu (th81)]

>
>About tyrannosaurs having a degenerate semi-lunate carpal:
>
>>There is only one really well-preserved tyrannosaurid carpus that I know 
>
>>of, and that is from _Albertosaurus_ (?).  It shows five carpals, one of 
>
>>which is evidently a degenerate semilunate.
>
>Does "evidently" mean that one really looks like a degenerate semi-lunate 
>carpal, or is it evidently so because it must be to fit the 
>classification?

There is a big flat degenerate distal carpal, which covers the base of mcI &
mcII.  It could either be a degenerate semilunate carpal block, or a
degenerate fusion of the distal carpals in the homologous position in other
tetanurines (Allosaurus, for example).
>
>About tyrannosaur carpals coming straight from _Compsognathus_:
>
>>Why are you suggesting this?  Does it have anything to do with the 
>>didactyl manus in that genus? (which, incidentally, has a reconsturcted 
>>phalangial formula of 2,3,3,0,0, more "derived" than any tyrannosaur).  
>>Didactyly is pretty easy to develop in parallel.
>
>No.  Actually, I have seen the claim that the manus of _Compsognathus_ is 
>too poorly preserved (I don't have a reference handy for that one) to say 
>it is unquestionably didactyl, although I realize that Ostrom has looked 
>at it recently and pronounced it so.

Having seen the casts of the specimens, it looks like someone took a shotgun
to its hands and blew them up!  Gauthier, who has seen the real things, is
equally unconvinced of the didactyly.  I think it is a distinct possibility,
but unresolved.

> I was concerned about reversals of
>characters reported by Holtz.  The Maniraptora developed flexed cervical 
>zygapophyses, but this was reversed at Arctometatarsalia (so, that 
>character considered alone [ok, I know!], we go back to the next step 
>before Maniraptora, which "picks up" -Compsognathus_.  Then, at the next 
>node beyond Arctometatarsalia, but before we get to the tyrannosaurs, 
>three more characters are reversed, including another unambiguous 
>synapomorphy of maniraptorans--posterodorsal margin of ilium curved 
>ventrally in lateral view.  One of the other reversals at the node beyond 
>Arctometatarsalia is for a synapomorphy at the next node beyond 
>Maniraptora.  I didn't "grow up" with cladistics, but rather with 
>numerical taxonomy, back in the late 60's, so maybe I still don't have 
>command of cladistic principles.  But I thought we should be suspicious 
>of reversals.  Am I wrong on that?  If I'm right, why aren't we 
>suspicious of these?  It seems that to make tyrannosaurs and 
>ornithomimosaurs more derived than maniraptors, we have to undo a lot. 

Suspicious about reversals?  I've got to get that "reversals happen" bumper
sticker :-)

Incidentally, with elmisaurs and oviraptorids over on the Maniraptora vera
side, many of the previous "reversals" disappear.

>
>About arctometatarsaian feet:
>
>>> According to Holtz, tyrannosaurs are grouped with troodonts and 
>>> ornithomimosaurs due to their arctometatarsalian feet (due to this 
>>> largely or entirely[?], 
>
>>Nope.  Tyrannosaurs also show a good many more maniraptoran 
>>autapomorphies, particularly in the skull.
>
>If we undo all of those characters--like assume they never developed in 
>the first place--the arctometatarsalian feet certainly become much more 
>important (ok, I've left out one group of synapomorphies--at the node 
>just before Arctometatarsalia); "entirely" was too strong a word.

If you run the dataset without those cranial characters, tyrannosaurs fall
back to basal coelurosaurs.  Same with the arctomet feet.  "Entirely" is too
strong a word.
>
>>In fact, tyrannos show innovations in foot structure and neural pathways 
>
>>strongly indicating that they are closer to birds than dromaeosaurs are, 
>
>>and maybe even closer than _Archaeopteryx_!
>
>You mean Bakker was right???

I remain unconvinced of such a position.  I am among the last of the people
on this net, it seems, who believes that dromaeosaurids are the sister group
to the Archie + later bird clade...
>
>About primitive ornithomimosaurs lacking arctometatarsalian feet:
>
>>_Garudimimus_ has been shown to be fully arctometatarsalian, and 
>>_Harpymimus_ is apparently very poorly preserved.  It is also more 
>>primitive in the hand than the much earlier _Pelecanimimus_, and this, 
>>along with some other features, including the enlarged preacetabular 
>>blade of the ilium, suggests to me that this genus may not be an 
>>ornithomimosaur at all, but possibly a relative of the oviraptorosaurs.
>
>Wait a minute!  Why is this counter example invalid?  If it's more 
>primitive in hand, why can't it also be more primitive of foot, and 
>legitimately so?  Sounds like Johnny Cochran--if it doesn't fit, it's not 
>legit!  ;-)

There are questions as to the ornithomimosaur nature of the really scrappy
Harpymimus type.  I still think it is an ornithomimosaur, myself.
>


Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661
Phone:301-405-4084