Re: Conodont elements

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Mark Purnell <map2@leicester.ac.uk> wrote:
...
|>There are many other ideas about what the elements are - as people 

|>who have read around must know - the main problem being that they 

|>were tissue covered

|There is no evidence that conodonts were tissue covered during use.  

|Sure, they must have been embedded in tissue or tissue covered at 

|some time (during their growth) but this has no bearing on their 

|function (toothlike or otherwise).  Irrespective of conodont 


	I agree and disagree :-)  I agree that they must have been  
"covered by tissue" at some time in order to account for their external  
accretionary growth.  I also agree that the fact they were covered does  
not exclude the likelihood (e.g., as demonstrated by wear patterns and the  
allometric factors) they functioned as teeth.  However, I disagree that  
the presence of covering tissues has "no bearing on their function".  One  
of the features that really puzzled me when I looked more closely at the  
reconstructions of Aldredge et al., 1987 for the ozarkodinid apparatus is  
how close together the individual S and P elements are positioned in  
bedding plane assemblages.  For example, the S elements do not seem  
further apart than their height, and are probably much closer.  Adding a  
basal plate in the bottom, musclature to move the elements, and sufficient  
tissue to cover the elements is tricky given the amount of space.  Then it  
becomes even more challenging to think of how they could be extruded from  
the tissues in order to function as teeth.  I do *not* think this in any  
way questions the likelihood the elements (or at least some of the  
elements (e.g., P) in some groups) functioned as "teeth" -- the  
independent evidence is now fairly convincing (in my opinion) -- but the  
geometrical limitations on space are really interesting.
	
	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.  I will say again, just to make it clear: I think Mark is right  
that a "tooth" hypothesis is best supported by the current evidence  
(especially the information on wear).  The basic problem, in my opinion,  
is how to come up with a plausible hypothesis to both "grow" the elements  
and still have a plausible hypothesis about how they were used as "teeth".   
I.e. I think the issue of the growth of the elements *is* relevant to  
function.




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