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Re: Quo vadis, T. rex? [long]

>Can someone convince me that tyrannosaurs are maniraptorans?  I know they 
>might be, but I'm still skeptical.
>A sticking point for me is that tyrannosaurs don't have (am I wrong about 
>this?) a semi-lunate carpal--at least their carpals are not shaped that 
>way--and it is only POSSIBLE that one was derived from the semi-lunate 
>condition, and apparently clear that it was not derived from an 
>allosaur-like carpal.  But isn't it also possible that tyrannosaur 
>carpals are derived directly from _Compsognathus_ carpals, which we 
>unfortunately know almost nothing about?  This MIGHT be a more 
>parsimonious hypothesis, since we don't have to assert that tyrannosaur 
>carpals passed through a semi-lunate state, for which we have no evidence 
>(I say MIGHT, because we know almost nothing about _Compsognathus_ 
>carpals, and maybe an assumption is involved here that makes this 
>hypothesis just as complicated).  The same comments would apply to 
>ornithomiosaurs, if they also lack a semi-lunate carpal (that's right, 
>isn't it?).

The carpal situation in ornithomimosaurs and tyrannosaurids is VERY
uncertain.  They are simple nubbins of bone.  The most found in any specimen
is five per hand (in one ornithomimid and in one tyrannosaurid).  Given that
they are highly reduced, it is equally parsimonious that they could be
derived from either carpus form.  This can only be resolved by the
parsimonious distribution of other characters.
>According to Holtz, tyrannosaurs are grouped with troodonts and 
>ornithomimosaurs due to their arctometatarsalian feet (due to this 
>largely or entirely[?], once it is assumed they are maniraptors).

Within the 'maniraptorans' (soon to be maniraptoriforms, but that's another
issue), yes: cursorial features such as limb proportions, arctometatarsi,
iliac blades that meet on the dorsal surface, etc. are among the more common
featuers uniting the two.  However, there are some cranial features
(d-shaped premaxillary teeth, and others listed in the 1994 paper) which
suggest an affinity of this group.

> This
>can be inferred to indicate they are highly derived maniraptorans, and 
>maybe that makes it easier to believe that they once had a semi-lunate 
>carpal, then lost it (since they're highly derived).  Primitive 
>ornithomimosaurs (e.g., _Harpymimus_) lack the arctometatarsalian 
>condition (isn't that right?).

Nope.  Harpymimus' foot is smeared out of shape: it is uncertain at best.
Garudimimus' foot IS arctometatarsalian, despite illustrations otherwise.
It was correctly illustrated in Currie & Russell's 1988 Chirostenotes paper,
under the name 'Oviraptor'!  Thanks to Halska Osmolska (for photos &
comments) and Mark Norell (pers. commun. a few years ago) fot the above.

Pelecanimimus, sadly, lacks feet (okay, the only specimen lacks feet).

>  Now, if troodonts are truly maniraptorans
>and have arctometatarsalian feet, and advanced ornithomimosaurs have 
>arctometatarsalian feet, but the primitive ornithomimosaurs did not

(see above)
>, then
>why is it not possible that troodonts and ornthomimosaurs developed 
>arctometatarsalian feet convergently (and, perhaps tyrannosaurs, as 
Certainly a possibility.

>Since ornithomimosaurs also lack a semi-lunate carpal, and developed 
>arctometatarsalian feet independently from derived (advanced) 
>maniraptorans, perhaps they are not maniraptorans at all.
Certainly a possibility.  See Gauthier 1986 for the most cogent arguement
towards this end.

However, in the most parsimonious distriubtion of derived characters in
analyses by myself, by the American Museum of Nat. Hist. paleontologists, by
Hans Sues, and Perez-Moreno et al. 1993, a
tyrannosaur-troodont-ornithomimosaur clade was found (including
therizinosauroids for the AMNH, troodonts not included in Perez-Moreno 1993).

>According to Holtz (J. Paleo., v.68, no. 5, p. 1107), an unambiguous 
>synapomorphy of maniraptors (sensu Gautier only?) is a long and slender 
>metacarpal III, yet tyrannosaurs have a very short metacarpal III.

Clearly an autapomorphy, whereever they belong!

> Also,
>in maniraptors metacarpal I is one-third or less the length of metacarpal 
>II, but that is not what is shown in illusrations of tyrannosaur manus 
>("manuses") that I've seen.  Since tyrannosaurs lack these unambiguous 

Unambiguous with the exclusion of tyrannosaurs, ambiguous with the most
parsimonious distribution of characters in the above data sets.

> and also lack the semi-lunate carpal of the Maniraptora
>sensu Holtz, and we know(?) that some coelurosaurs developed an 
>artometatarsalian condition independently of others, I am not convinced 
>that tyrannosaurs are maniraptorans.  Coelurosaurs, yes--maniraptorans, 
Fair enough.  I don't think they are maniraptorans anymore, anyway (my clade
"Maniraptora" is NOT Maniraptora).

>What do I have wrong, or don't know, that is keeping me from accepting 
>the maniraptoran status of _T. rex_?

Hell if I know.  Also STOP looking at T. rex, the most derived member of the
clade.  Alectrosaurus, Stygivenator, Shanshanosaurus, Gorgosaurus: these are
the primitive tyrannosaurs to look at.

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