Re: conodont growth etc.

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|Andrew MacRae makes some sensible points, but perhaps 

|overemphasises the apparent uniqueness of conodonts.  It is true 

|that no other vertebrates (or chordates) have a feeding apparatus 

|the same as that of conodonts, but the key to understanding the 

|conodont apparatus is the recognition of its bilateral operation 

|across the axis of the animal.  Such bilateral "biting" also occurs 

|in Hagfish and Lampreys, but they don't have mineralised feeding 

|elements.

	There are certainly some analogies, but the scale seems  
dramatically different for most conodonts (the large _Promissum_ apparatus  
being a notable exception), and the apparatus is significantly more  
elaborate in some groups.  It is also elongated along the axis of the  
animal by comparison, and it seems to be more differentiated into units  
with different functionality (e.g., "grasping" or "biting" for the  
ramiforms, "cutting" and "crushing" for the P elements), at least in some  
conodont groups.

	There seem to be at least two basic plans for the more  
differentiated conodont groups -- one with the P elements posterior and  
slightly ventral to the ramiforms (e.g., the ozarkodinids), and one with  
the P elements anterior and dorsal to the ramiforms (e.g., _Promissum_).   
These differences are fairly fundamental, and could be an indication there  
is more variation in the geometry of the conodont apparatus waiting to be  
discovered.  Perhaps a better analogy with hagfish and lamprey only awaits  
the discovery of other apparatus structures.
	
	<Shrug>  To be honest, I have only a basic familiarity with the  
way hagfish and lampreys use their teeth.  Have any detailed studies been  
published that go beyond morphology and into the details of the mechanics  
of the teeth usage?  You mention they have "bilateral biting" -- where is  
this documented?

|Secondly, the way in which conodont elements grew is not as simple 

|as much of the literature might suggest.  Continuous external 

|apposition of growth lamellae over the entire surface of the 

|elements throughout life has long been part of the classic 

|arguments for why elements could not have been teeth and conodonts 

|could not have been vertebrates.  These arguments and traditional, 

|oversimplified models of conodont element ontogeny are no longer 

|tenable.

	I agree that the style of growth of the elements does not exclude  
tooth-like functionality, now that there is independent evidence (e.g.,  
wear and limitations on filter-feeding habits due to allometric scaling).   
I find the evidence for a tooth-like functionality to be quite compelling.   
I am not questioning it.  But the fact remains that "continuous" (okay,  
throughout life) external growth of teeth is, as far as *I* know, unusual,  
if not unique in chordates (and I would honestly like to hear otherwise.   
Information about this style of growth in teeth from any organism would be  
almost as interesting.)

	Can you add any newer information to the current understanding of  
the growth pattern in conodont elements?  I know you have proposed the  
discordant growth lamellae commonly interpreted as resorption features  
could actually be due to wear (and I think this is a great  hypothesis  
that should be investigated further), but is there anything else that  
questions the conventional understanding of the growth pattern?

|So, may be conodont were not that bizarre after all.

	It is certainly possible, but conodonts do possess some   
differences from other known early chordates: the growth pattern of the  
elements and the differentiation and complicated geometry of the apparatus  
in some groups.  I think these are significant, fundamental differences,  
but that assessment is purely subjective.  Identifying distinct  
characteristics is one thing, deciding their significance is another :-)  
<grin>.  It is difficult to tell how much significance to attach to these  
characters with so few modern representatives of the earliest vertebrates  
to compare to and with the fossil record for the invertebrate chordates  
and early vertebrates so tantalizingly incomplete because of limited  
mineralization of tissues.
	
	I think conodonts are fairly unique, but I am interested in any  
information that could change my mind :-)

	Thanks for your comments, Mark.



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