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Re: Quo vadis, T. rex?



On Thu, 1 Feb 1996, King, Norm wrote:

> Can someone convince me that tyrannosaurs are maniraptorans?  I know they 
> might be, but I'm still skeptical.

Here goes...

> 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.  

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.

> But isn't it also possible that tyrannosaur 
> carpals are derived directly from _Compsognathus_ carpals, which we 
> unfortunately know almost nothing about?  

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.

On other grounds, _Compsognathus_ is *way* more primitive than tyrannosaurs.

> (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).

Indeed.

> The same comments would apply to 
> ornithomiosaurs, if they also lack a semi-lunate carpal (that's right, 
> isn't it?).

Ornithomimosaurs also show a degenerate semilunate.  I advanced this as a 
potential synapomorphy (one of several) linking tyrannosaurs and 
ornithomimosaurs within the Arcotmetatarsalia, but my idea was not widely 
accepted.
 
> 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.

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_!

> Primitive 
> ornithomimosaurs (e.g., _Harpymimus_) lack the arctometatarsalian 
> condition (isn't that right?).  

_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.

> Now, if troodonts are truly maniraptorans 
> and have arctometatarsalian feet, and advanced ornithomimosaurs have 
> arctometatarsalian feet, but the primitive ornithomimosaurs did not, then 
> why is it not possible that troodonts and ornthomimosaurs developed 
> arctometatarsalian feet convergently (and, perhaps tyrannosaurs, as 
> well)?  

Because 1) primitive ornithomimosaurs *did* have arctometatarsalian feet; 
2) ornithomimosaurs, troodonts, and tyrannosaurs also share many other 
derived characters; and 3) the fine details of the foot in all three groups 
match up perfectly, bone for bone, curve for curve.

> 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.  

It was as long as metacarpal II and quite slender.

> 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.  

Another potential synapomorphy linking tyrannosaurs and ornithomimosaurs...

> Since tyrannosaurs lack these unambiguous 
> synapomorphis, 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, 

Untrue.

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

Actually, _T. rex_ cannot be a maniraptoran as the term was originally 
defined.  It was defined as "all theropods closer to birds than to 
_Ornithomimus_."  Well, ornithomimosaurs show many attributes which put 
them closer to birds than dromaeosaurs, which would limit membership in 
the Maniraptora to the birds alone.  Arctometatarsalians may well, in 
fact, belong to about the same evolutionary grade as the 
enantiornithians, reducing the membership of the Maniraptora down to the 
Ornithurae alone.

Nick Pharris
Pacific Lutheran University
Tacoma, WA 98447
(206)535-8204
PharriNJ@PLU.edu

"If you can't convince them, confuse them." -- Harry S. Truman