| [Thread Prev] | [Thread Next] | [Thread Index] | [Date Prev] | [Date Next] | [Date Index] |
A couple of recent papers address a number of points raised in the last couple of days. Regarding the _Oepikodus_ assemblages that Ian Stewart is working on, how closely do these resemble the arrangement of elements in _Promissum pulchrum_, Ian? This is published (3D model and all) in Philosophical Transactions of the Royal Society of London, series B, 1995, v. 347, pp. 275-291). _Promissum_ has its P elements (four pairs) above and between the S elements (in it's resting position anyway); its sounds like your _Oepikodus_ may have fewer elements, but in a similar arrangement. Would this work for your assemblages? Regarding the resting and functional positions for the elements, this is something we discuss on pp. 283-287 of the above paper. In _Promissum_ we do observe the kind of change in relative position of elements in fossil apparatuses that Andrew MacRae mentioned. The changes cannot be reconciled with simple variation in the angle of collapse of the apparatus and may well be recording the way in which the elements were able to move. However, we have looked at hundreds of bedding plane assemblages preserving apparatuses of the more usual ozarkodinid type (as dealt with in the 1987 architecture paper) and we do not see the same kind of difference in angular relationships. There are some slight variations, especially in M element position, that the authors of the original 1987 architecture paper were well aware of, but most of these can in fact be accounted for by a single 3D model in which the M elements are not paralell to the S elements. This model is made and the paper dealing with it and its mode of operation is in preparation. I addition to the Nature pic, for a sneak preview see "Endeavour" 1995 vol 19(1), pp. 20-27 (the model has been modified only slightly since) or New Scientist, April 29, p. 16. This is a scale model with accurate element morphology and accurate element spacing. By simulating collapse in a variety of orientations we can account for the arrangement of elements in bedding plane assemblages without having to resort to ad hoc taphonomic hypotheses. Unlike _Promissum_ we have not seen any marked differences in angular relationships that might be reflecting the difference between resting and functional states, but undoubtedly the apparatus must have moved to function. Regarding the link between element function and growth: I still maintain that the questions of element growth mechanisms and function are not as closely linked as has been thought. I agree with Andrew MacRae, function can be analysed independantly of element histogenesis. The whole area of element growth is a problem on which several people (who do not necessarily agree) are currently working. None of the published models of conodont element growth can account for what really happens during the life of an element; simply adding more layers onto the outside just won't do. The case for elements functioning as teeth is now very strong (I have to say that of course), and what we have to come up with now is a growth model that works within this scenario. In other words, whatever models are proposed for element growth, must be consistent with tooth function and surface wear. All this rules out, however, is permanent, complete cover of the elements by tissue of some sort. There is more than one way to skin an element! MARK Oh yes, bilateral biting in hagfish/lampreys: One of Dick Krejsa's papers on conodonts and cyclostomes (Courier Forschungsinstitut Senckenberg 1990, v. 118, pp.473-492) should get you into the relevant literture, but watch out for the sections about element shedding and replacement in conodonts! Dr Mark A. Purnell Department of Geology, University of Leicester University Road, Leicester LE1 7RH, U.K tel: 0116 2523629 fax: 0116 2523918
Partial index: