theropod hands

["Nicholas J. Pharris" <pharrinj@PLU.edu>]

Some of you (particularly Norm King) were interested in knowing whether 
there are maniraptoran autapomorphies present in the hands of tyrannosaurs.

Well, I seem to have found a pretty good one.

Much has been made of the fact that the proximal phalanges of the third 
finger are hypershortened in maniraptorans.  Less well-known is the fact 
that the same is true of the proximal phalanx of digit two.

Among the theropods whose manus I could get a clear look at, 
_Allosaurus_, _Sinraptor_, _Ornitholestes_, _Chirostenotes_, _Elmisaurus_, 
and _Alxasaurus_ have phalanges II-1 and II-2 of roughly equal length.

_Deinonychus_, _Oviraptor_, and _Archaeopteryx_ have phalanx II-1 about 
two-thirds the length of II-2, although _Ingenia_, in typically weird 
fashion, has phalanx II-1 slightly *longer* than II-2.

Finally, _Tyrannosaurus bataar_, _Gorgosaurus_, _Harpymimus_, 
_Struthiomimus_, _Ornithomimus_, _Gallimimus_, and _Sinornithoides_ have 
phalanx II-1 from slightly under half to somewhat over half the length of 
II-2.

So tyrannosaurs share a derived form of manual digit II, along with many 
other characters, with ornithomimosaurs and troodonts.  

How much, BTW, is known of the manus of _Baryonyx_?

(I also checked out the palate of _T. rex_ and looked for maniraptoran 
characters.  A lot of the palate looks much like that of _Dromaeosaurus_, 
particularly the vomers and ectopterygoids--certainly much more like a 
maniraptor than like _Allosaurus_.)

This brings me to my thoughts on the reconstructed manus of 
_Compsognathus_, which I scrutinized for a while last night.  My thoughts 
are this:  pretty screwy.  As reconstructed, metacarpal I is the longest 
and most robust of the three.  The *only* theropod in which this is 
definitely the case is _Ornithomimus velox_.  Not impossible, but suspicious.

"Metacarpal I" is also shown with a wide base and narrowing from the 
medial side.  I can find no other theropod manus structured in this 
manner, but this bone *is* quite reminiscent of metacarpal *II* in 
tyrannosaurs.

Finally, as reconstructed, phalanx I-1 is about half the length of 
metacarpal I.  No theropod thumb, none, is structured in this manner.

My hypothesis is that Ostrom got manual digits I and II confused:  that 
the digit reconstructed as II is, in fact, the thumb; and that the digit 
reconstructed as I consists of the metacarpal and proximal phalanx of 
digit II.  The medial phalanx of digit II is, apparently, missing.

More detailed analysis of the specimen will be necessary to confirm or 
refute this hypothesis.

The presence of such a short phalanx in the manus of _Compsognathus_ 
strongly suggests that this little animal had the hypershortened phalanx 
II-1 characteristic of the Arctometatarsalia; and this may, along with 
cranial, pedal, and pelvic characters, lend support to the hypothesis that this 
animal is a basal arctometatarsalian.

The ankle of the type looks pretty beat up.  Anyone who has actually 
examined the specimen:  is it possible that metatarsals II and IV do 
touch on the dorsal surface of the pes?

Just a thought.

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