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Re: Arctometatarsalian



Tom Holtz wrote (02/13/96; 10:28a):

>The arctometatarsalian condition is the pinched third metatarsal 
condition
>seen in tyrannosaurids, ornithomimids, troodontids,
>elmisaurids/caenagnathids, Avimimus, and Mononykus (but NOT in
>Archaeopteryx, ornithurine or enantiornithine birds, etc.). [snip]

>Tony Thulborn and a certain poster to the net from out in Washington 
State
>have found the position you describe, but (important emphasis) NO 
computer
>generated cladogram has found the same.

>In the various phylogenies I've cooked up, true Arctometatarsalia 
(including
>at least Tyrannosauridae, Troodontidae, and Ornithomimosauria) is less
>closely related to birds than are Archaeopteryx or the Dromaeosauridae.  
In
>my most recent studies, oviraptorosaurs and therizinosauroids were also
>closer to birds than are the arctomets.

[snip]

>Currie finds birds within Arctometatarsalia (maybe closer to troodonts 
than
>to ornithomimosaurs or tyrannosaurids), but his most recent coelurosaur
>cladograms are still not publically presented (i.e., they were not shown 
at
>the SVP theropod meeting).

Although earlier discussion considered, without resolution, possible 
adaptive value for the artomet condition, that character is easy for me 
to rationalize as adaptive for certain lifestyles--I infer that it is 
mechanically different from non-arctomet feet.  As an adaptive trait, it 
might appear convergently in different lineages, or be secondarily lost.  
The semi-lunate carpal is similar--the semi-lunate wrist it is 
mechanically different from non-semi-lunate wrists.

Nick Pharris mentioned a character that caught my attention in a big way, 
but not a sole (soul?) has commented on it (are Nick and I out in left 
field?).
Nick said (02/01/96; 9:49p):

>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.

I have a harder time rationalizing the routing of nerves through the 
brain case as changing for mechanical advantage, and therefore as being 
an adapatation for a particular lifestyle.  There is adequate precedent 
for giving great weight to the routing of nerves--it's how we determined 
the origin of jaws in fish.  If this is just one character state in the 
cladistic matrix, with no special weight, the computerized cladogram may 
not show it uniting birds and arctomets.  I don't know if Nick's 
information about V1 is right, but if it is, I think this is an area that 
might be profitably pursued to help work out the theropod/bird 
phylogenetic mess.


*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu