Re: Conodont elements

[Andrew MacRae <macrae@xxxxxxxxxxxxxxxxxxxxxxx>]

[I wrote]
>One hypothesis I (and I am sure others) have considered is the  

>possibility the bedding plane assemblages represent the "rest" or 

>some other position of the apparatus rather than the "functional" 

>position --  i.e. the geometry that has been reconstructed is *not* the 

>operational geometry. 

[Paul Smith wrote]
|As one of the authors of the Aldridge et al. 1987 paper, I can confirm 

|that this idea has been previously considered - in that paper (p.73) we 

|clearly stated that the reconstruction presented, and the attitude of 

|most bedding plane assemblages, is the resting position.  We were 

|not the first, the idea had previously been forwarded by, at least, 

|Lennart Jeppsson and Stefan Bengtson.

	<Blush> I knew I had seen it somewhere before :-)  I should have  
known where.  Thanks.

	So *if* that interpretation is correct, I guess the obvious  
question is -- what was the operational geometry, and is there any  
evidence in the bedding plane assemblages that could help determine it?

	One of the things I have noticed is *slight* variations in the  
bedding plane assemblages that can not be accounted for by a single 3D  
model.  To a first approximation, the proposed model works really well,  
but sometimes the M elements are positioned a bit differently, and  
sometimes (and this should be no suprise) the Pa elements are rotated in  
the opposed "scissor-like" fashion, with the blades overlapping more and  
the platforms opened (e.g., as described in Purnell and von Bitter, 1992).   
When the Pa elements do this, the Pb elements are often overlapped to a  
greater degree as well (coordination?  Or just similar compression-related  
effects?).  If enough bedding-plane assemblages were examined, I wonder  
whether different positions for the elements could be resolved despite the  
variable orientation of the compressions?  Could this indicate how the  
elements could move and/or which moved independently when operating?



	-Andrew
	macrae@geo.ucalgary.ca
	home page: "http://geo.ucalgary.ca/~macrae/current_projects.html";