Re: Conodont elements

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|>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.

[Stefan Bengtson wrote]

|I couldn't agree with you more, Andrew: There is something unfishy about

	Well, "different", anyway :-) <grin>

|the mode of growth of conodont elements, even though the fishy evidence
|for conodont affinity now looks pretty good.

|In the late 1970s, Lennart Jeppsson and I started waving our respective
|arms about conodont elements as teeth, in opposition to the then
|prevailing model that the elements served to support soft tissue such as
|feeding tentacles. The tentacle-support model was based on the correct
|observation that conodont elements grew through their life by all-around
|accretion and on the incorrect inference that they must therefore have
|been embedded in living tissue from beginning to end and from head to
|sole. We pointed out that conodont elements are morphologically identical
|to various structures with known tooth functions and that the growth
|paradox could be solved by assuming alternating periods of growth and
|function.

	Yes, I think the presentation of you and your colleagues,  
including the illustrations of hypothetical elements surrounded by tissue  
that were extruded for use, convincingly demonstrated that "all-around"  
growth does not preclude tooth functionality.  It may make it trickier to  
envision only because there are few (are there any?) modern analogues.

|A key observation was for me the lack of a crown in Cambrian conodonts:
|the so-called paraconodonts  are morphologically and histologically
|comparable to the 'basal body' of euconodonts, and the first euconodonts
|to appear (Proconodontus et al.) have an exceedingly thin crown. (The
|crown is the part that forms the oral surfaces of euconodonts.)

	And the interesting part is the implication for the way the  
paraconodonts grew -- basally.  I.e. the same way hagfish teeth grow, if I  
remember correctly (although they are not mineralized at all).

|I envisaged the conodont
|elements to have been derived from epithelial tissue that in the
|euconodonts formed epithelial pockets, from which the elements were
|extruded only when in function. When the first Granton conodont animal
|was found, I saw the tooth model as vindicated and interpreted the animal
|as a kind of chaetognath, with the teeth situated in a dilating pharynx.

|Later developments have partly falsified this interpretation: The
|amassing amazing evidence for chordate characters in conodonts and for
|protostome affinities of chaetognaths currently make it difficult (not
|impossible, but that would be special pleading that I don't want to
|indulge in here) to maintain a chaetognath nature of conodonts. My
|interpretation of the various types of bedding-plane assemblages as
|representing various stages of extrusion of the conodont apparatus was
|shown to be rubbish through the nice study by Aldridge, Smith, Norby &
|Briggs (1987, in 'Palaeobiology of Conodonts') on the effects of
|compaction of a properly configured 3-dimensional apparatus model. In
|some other respects, however, the model is still viable. Mark Purnell's
|work on allometry and microwear pattern supports it. Szaniawski &
|Bengtson (1993, Journal of Paleontology 67) have now documented the
|paraconodont-euconodont histological transition in some
|detail, and the late evolutionary origin of crown tissue seems well
|established.

	Hmmm... could this imply the euconodonts occur "crownward" of  
hagfish and lampreys, which possess the "basal" growth also characteristic  
of paraconodonts?  I can not remember if this character was used in the  
recent analyses of conodont affinities.

|I'm not impressed by the much-quoted histological evidence of Sansom et
|al. (1992, Science 256; 1994, Nature 368) for vertebrate hard-tissue in
|conodonts; it looks like a case of histological shoe-horning. Conodont
|histology differs from that of vertebrates and may well not be homologus
|with the latter. I fully agree with you that formation is relevant to
|function; any intepretation of the formation and function of conodont
|elements must take into account the curious combination of
|circumferential growth and tooth function.

	Well, I do not think the identification of a "tooth function" for  
conodont elements necessarily requires addressing the growth pattern.  I  
think a "tooth function" can be recognized independently, and Mark Purnell  
and many others have made a compelling (if not convincing) case for this.   
However, I think the growth pattern does bear on exactly *how* conodonts  
functioned as teeth -- i.e. the *mechanism* by which they used them for a  
"tooth function".  That is what I am now curious about.

|It would be nice to see physical

	Tricky, I would suspect.

|or virtual apparatus models that attempt transformations between possible
|resting and functioning positions. Can you do that with the model you
|placed on your http site?

	Theoretically, yes, and it is something I plan to eventually do --  
both testing hypotheses about the way they could move, and about what sort  
of structure is necessary to support them.  However, this *is* going to be  
speculative, and I am not sure it will be fruitful if people simply say  
"wow" (the "Jurassic Park effect" :-)) and are persuaded to believe an  
interpretation because of the way it looks.  This is one of the reasons I  
am interested in more information on the musclature and the motion of the  
teeth in hagfish and lampreys -- in an attempt to better understand what  
the possibilities are.  For that matter, information on the feeding of  
chaetognaths may also be interesting to try.  Perhaps the functional  
styles of these organisms can be attempted with the apparatus models, and  
it might be possible to eliminate some possibilities on that basis.

	However, the first item to fix is the details of the shape of the  
elements.  Although they are accurate in size, their shape is  
oversimplified to flat slabs, some with additional pieces.  This is  
unacceptable to really test the geometrical limitations more accurately,  
and it is something I plan to fix before attempting anything more  
elaborate.  One thing is for certain -- there are currently no  
technological barriers to accomplishing 3D models of the apparatus in as  
much detail as a person wants.



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