hand rotation / prosauropods

[longrich@alumni.princeton.edu]

On Monday, August 5, 2002, at 09:19 AM, David Marjanovic wrote:

> > I had thought basal theropods could pronate their hands.  In fact, it 
> > was
> my
> > impresson that most non-avian theropods were capable of
> pronation/supination
> > of the manus - until that whopping semilunate carpal block came along 
> > and
> > turned the wrist into a swivel joint.
>
> That rotation movement doesn't depend on the wrists but on the 
> forearms &
> elbows, try it with your own, un/lock a door and you'll see you don't 
> need
> to move your wrists at all. It is already impossible in *Allosaurus*.

	While theropods lack the highly circular proximal radial condyle of 
humans (and therefore probably couldn't redirect the palm 180) the bones 
probably did leave some play. The radius and ulna generally aren't 
locked together as they are in, e.g. ungulates (ornithomimids and 
alvarezsaurids being the exception here, but even so the combined 
ulna-radius unit could probably pivot on the condyle in alvarezsaurids), 
the condyles of the humerus are of the bulbous (i.e. ball-and-socket 
joint) rather than pulley- spool-shaped trochlear type (as in for 
example ungulates), and the proximal end is fairly elliptical. So I 
think that while medially facing palms is probably a good neutral 
position, they could pronate or supinate from there a fair amount. No 
theropods weren't specialized to rotate the arms to the degree seen in 
humans or sloths, but I'm not sure I'd characterize forelimb rotation as 
being reduced either. Certainly birds can rotate the forelimb from the 
elbow although it seems to lock up on full extension.
	Another thing to keep in mind is that unlike humans, theropods had 
*two* wrist joints- the one between proximal and distal carpals, and the 
one between proximal carpals and the ulna-radius. Afrovenator and more 
derived theropods show a trochlea which would have constrained movement 
but only at one of the joints, and again this doesn't imply restricted 
hand rotation (humans have essentially no flexibility between the 
proximal and distal row of carpals for example).
	I also think that people have exaggerated the hingelike nature of 
the theropod hindlimb. I recently played with the carcasses of a 
cormorant and a hawk, and found that there was about thirty degrees of 
rotation possible at both the ankle and the knee, permitting a fair 
amount of pronation-supination of the foot. It may have been  less in 
the living animal but still I think it shows that these animals are 
reasonably flexible if not exactly gymnasts. Naturally this may have 
varied considerably between lineages.

	re: prosauropods- The figures in _Dinosauria_ show _Blikanosaurus_ 
as having a very massive metatarsus, with the metatarsals fairly similar 
to each other in length as in sauropods, and the calcaneum is greatly 
reduced. So it looks a lot more sauropod-like than the other guys. 
_Plateosaurus_ looks a lot more sauropod-like in the manus than 
_Thecodontosaurus_, in that again the metapodials are more comparable in 
length, penultimate phalanges are relatively shorter (increasingly 
specialized towards terrestrial locomotion rather than grasping),  the 
digits themselves are relatively shorter relative to the metapodials, 
and the overall build of the hand is much more robust. I think that 
people will increasingly find evidence that Prosauropoda is paraphyletic 
if they just look more, and some people are starting to do that. 
Speaking as a non-expert, the story sounds a lot like the Ceratosauria 
problem in Theropoda, where symplesiomorphies were misidentified as 
synapomorphies.
	The other thing is that if you make Prosauropoda monophyletic, 
where in the heck are the sauropods coming from? While I think Feduccia 
has abused the ghost lineage argument, the timing of appearance of 
organisms in the fossil record does provide some imperfect signal and an 
additional line of evidence against which our phylogenies can and should 
be tested. Part of the argument for the theropod evidence of birds is 
that any other phylogenetic arrangement demands an even longer ghost 
lineage. In particular, given that sauropod ancestors were likely (a) 
relatively large, and (b) relatively abundant, being herbivores, it 
seems unlikely that they would have escaped preservation for so long.

nL