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Re: Conodont elements



Andrew MacRae 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.  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.
>

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

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.) 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.

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. It would be nice to see physical 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?



Stefan Bengtson                      _/        _/ _/_/_/    _/        _/
Department of Palaeozoology         _/_/      _/ _/    _/  _/_/    _/_/
Swedish Museum of Natural History  _/  _/    _/ _/    _/  _/  _/ _/ _/
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e-mail Stefan.Bengtson@nrm.se




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