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Re: Quo vadis, T. rex (WARNING: LONG)



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

> >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?
> 

As I recall, it's in Holtz's 1994 paper.
Semilunate carpals appear to be a primitive tetanuran or neotetanuran 
feature.  Allosaurs have them, and apparently _Afrovenator_ does, too.


> 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, 

I would have to agree that the hand is too poorly preserved give it *too* 
much taxonomic weight.


> 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_.  

I'm sorry, but I'm not up on my vertebral anatomy.  What about birds?  
What do their cervicals show?


> three more characters are reversed, including another unambiguous 
> synapomorphy of maniraptorans--posterodorsal margin of ilium curved 
> ventrally in lateral view.  

I'm not all that thrilled with this particular character.  Some tyrannos 
and ornithomimes have dorsally curved ilia, While others show a double 
curve:  ventrally in back; dorsally in front.

Troodonts are almost universally considered quite advanced, yet 
_Sinornithoides_, the earliest known troodont, has a dorsally curved ilium!

_Archaeopteryx_, _Sinornis_, and _Confuciusornis_, at least, have dorsally 
curved ilia, too.


> 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.  

I don't know much about numerical taxonomy, and I do not consider 
cladistics to be the last word on phylogeny.  You still have to choose 
which characters to evaluate and how to weight them.


> But I thought we should be suspicious 
> of reversals.  Am I wrong on that?  

IMHO, we ought not shut out the possibility of reversals 
entirely.  Troodonts look manifestly more birdlike than dromaeosaurs to 
me, yet they have a propubic pelvis.  I think the most 
logical explanation is that they are birds more derived than dromaeosaurs 
that reevolved a propubic configuration for some reason or another.


> 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.

They weren't more derived than maniraptors; they WERE maniraptors.

It all depends on what you look at.  In the pelvis and wrist, tyrannos 
and ornithomimes look fairly primitive; but if one looks at the detailed 
anatomy of the foot and braincase, they are clearly rather derived birds.
 

> >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???

Sure looks that way.  The feet appear to be a little more birdlike, but 
it's hard to tell about Archie's feet.  

I sure wish someone would CAT-scan an Archie 
and look at the neural pathways, assuming they would be preserved at all.

In almost all theropods (and almost all tetrapods, as far as I can tell), 
nerve V1 exits the braincase out the side, along with a bunch of other 
nerves.  In birds, it exits out the front of the braincase, though its 
own hole.  Dromaeosaurs, allosaurs, and most other theropods show the 
primitive pattern; tyrannosaurs, troodonts, and ornithomimosaurs show the 
bird pattern.  I have no idea whether this feature has ever been checked 
out in _Compsognathus_ or _Archaeopteryx_, but if not, it's certainly 
high time.

This unusual modification, apparently of little functional significance, 
is what convinced me firmly of the avian status of the arctomets.  The 
idea that the position of the V1 would persist through almost the entire 
history of the Tetrapoda and then simultaneously and spontaneously shift 
in several separate Cretaceous theropod lineages seemed pretty untenable.


> About primitive ornithomimosaurs lacking arctometatarsalian feet:
> 
> >_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!  ;-)

I didn't mean to imply that; the ornithomimosaur status of _Harpymimus_ 
just sounds fishy to me.  _Pelecanimimus_ had *far* more teeth (more 
primitive) than _Harpymimus_, but its thumb metacarpal was nearly as 
long as the others (apparently more advanced).  _Harpymimus_ just doesn't 
fit easily into ornithomimosaur evolution the way it used to.  It still may 
be an ornithomimosaur, but it would be very much off the evolutionary mainline.

If _Harpymimus_ is, in 
fact, an ornithomimosaur (quite possible), it probably lost its teeth 
independently of the rest, and it most likely secondarily rebroadened 
its feet.  I haven't seen most of the bones, and 
as I said they are apparently poorly preserved, so I can't say anything 
definite on the subject.

To put it simply, _Pelecanimimus_ seems to consist of better material 
than _Harpymimus_, and its ornithomimosaurian affinities seem more secure 
than those of the latter genus.  Therefore I will look to _Pelecanimimus_ 
for primitive ornithomimosaur characters before I look to _Harpymimus_.

I just wish we had a _Pelecanimimus_ foot!

Troodonts and ornithomimosaurs seem to be pretty obviously closely 
related and very birdlike.  I'm not quite as sure about a special 
relationship between these and tyrannosaurs, but tyrannosaurs are 
certainly at least as birdlike as the other two groups.

All three groups most likely arose from one or two advanced Jurassic or 
earliest Cretaceous birds.  And I'm not talking birds in the vague 
Olshevskian sense.  I mean birds more advanced than _Archaeopteryx_.  The 
propubic pelvis and arctometatarsalian foot may indicate that they are 
one group, but the tyrannosaurs would evidently have split off early.


> Norman R. King                                       tel:  (812) 464-1794
> Department of Geosciences                            fax:  (812) 464-1960

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