Re: BIRDY CROCS?

["Matthew Troutman" <m_troutman@hotmail.com>]

<<On this, I disagree with Walker.  The croc wrist is comprised of two 
very long proximal elements followed by small, stubby distal elements.  
The motion is very unlike that of a bird, with a great deal of 
up-and-down motion rather than the side-to-side movement emphasized in 
maniraptorans. The morphology in basal crocodyliforms (e.g. 
Sphenosuchus) more closely approximates that of something trying to add 
an extra segment for cursoriality.>>

Thats what I thought.  Walker's paper, like I said, was interesting, but 
I can spot some rather dubious assertions like streptostyly in 
_Sphenosuchus_, croc and bird sagittal crests are homologous, etc. 

Here's what Walker said:  

"The forearm and hand of _Sphenosuchus_ are unknown, but there is every 
reason to think, from the evidence of allied Triassic forms, that the 
radiale and ulnarae were differentially elongated in the distinctive 
crocodilian manner.  It is not generally recognised that functionally 
and anatomically, the bird and crocodilian elbow and wrist joints are 
surprisingly similiar.  Essentially, an oblique ridge on the humerus 
pushes the radius distally when the elbow is flexed, causing the 
radiale, ulnarae and hand to rotate laterally around the distal end of 
the ulna.  This is also the basic mechanism for folding the bird wing.  
Its significance in crocodiles is not entirely clear, but it may have 
arisen originally as a grasping adaptation during climbing and jumping - 
in early life crocodiles are active climbers - giving and extra joint 
and enabling the hand to extend further round cylindrical objects such 
as branches and tree-trunks.  In bird and crocodilian embryos also there 
is a striking similiarity in the lateral deflexion of the wrist-hand 
axis from that of the forearm, and in the reduction of the two outer 
digits." (Walker, 1972; 262) 

Now, reading this closer (something I should have done earlier) allows 
us to make a couple of observations that show that crocs are different 
from birds in matters of wrist arthrology: as noted below by Dr. Brochu, 
it is the semilunate (fused to the proximal end of the alular and major 
metacarpal in birds) bone that allows birds to execute their wrist 
movements.  By Walker's case (as noted by Dr. Brochu, it is rather weak) 
it is the radiale and ulnare that makes this movement.

<<Derived theropods have a very large distal element (the semilunate), 
and the emphasized movement is opposite that of crocs.>>

Agreed.

Matt Troutman 
m_troutman@hotmail.com


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