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As suggested by Norm, there follows a summary of the conodont discussion that has been going on on vrtpaleo over the last week or so. All of the postings that have a serious point to make are included. I hope this is of interest to some of you who have not already seen it. ******************************************************************* >Date: Wed, 24 May 1995 09:18:11 +1000 >From: S.Turner@mailbox.uq.oz.au (Dr Susan Turner) > Dear all, I would be interested in peoples' response to the idea >currently getting a lot of media coverage i.e. that conodonts are >vertebrates and our earliest 'ancestors', and not only that but >they exhibit features of a predatory lifestyle and have advanced >gnathostome characters including the presence of 'enamel' on their >"teeth". As I have been attempting to provide a forum for debate >on this in Ichthyolith Issues, and as one of the world's leading >authorities on early vertebrates, Dr Phil Janvier, has now come >out in favour of the theory, I think it is time for all vertebrate >workers and conodont workers to stand up and be counted and really >get to grips with the meat of the arguments being put forward by >Dick Aldridge, Mark Purnell, Ivan sansom, paul Smith and Moya >Smith. There are already several papers out and more in press >refuting their arguments and interpretations but these do not hit >the media because they are not so "sexy". So, what do the >community think. > > Unless stated, any responses may be used in the >Ichthyolith Issues Conodont Corner or sent to the Pander Society >for their use. Hope to hear and discuss, Sue ******************************************************************* >Date: Wed, 24 May 1995 07:55:13 GMT >From: henry@chiswick.demon.co.uk (Henry Gee) >Re: Sue Turner and the great conodont debate. > > Sue, if you're out there, I'd love to know about these papers >casting cold water on the latest conodont work. Frankly, if >Philippe Janvier believes it, then so do I. Whatever these >rumblings of dissent may be, they have yet to reach the offices of >Nature. Can you enlighten? > > -- Henry Gee ******************************************************************* >Date: Wed, 24 May 1995 10:49:28 +0100 (BST) >From: Mark Purnell <map2@leicester.ac.uk> >Regarding Sue's posting concerning the great conodont debate: Yes, >we are saying that conodonts are vertebrates (based on anatomy, >strongly supported by hard tissue histology); Yes, we are saying >that conodont elements functioned as teeth (based partly on >allometric arguments that they could not have been >suspension-feeding devices, but more importantly on microwear >analysis); Yes, we are saying that they were macrophagous (based >on microwear) and that they may have been predatory; No, we are >not saying that conodonts are our earliest ancestors; we are >saying that they are among the earliest vertebrates known and >therefore have a role to play in understanding what the earliest >vertebrates were like. > > Regarding "the several papers out" that refute these arguments, >we too would very much like to know about these, irrespective of >their relative "sexiness". ******************************************************************* >Date: Thu, 25 May 1995 09:17:19 +0000 >From: jac18@hermes.cam.ac.uk (Jenny Clack) >Subject: Conodonts' eyes > >Yes, I am putting conodonts into one of my vertebrate biology >courses as early vertebrates. But does anyone else, like me, not >believe in those eyes? To work eyes like that, the neuro folks >tell me, would take an elaborate brain and nervous system, of >which not only is there no evidence, but its presence seems highly >unlikely. As for the recently described 'eye muscles'. Frankly >I think they could be any old bits of tissue. Jenny Clack ******************************************************************* >Date: Thu, 25 May 1995 10:23:33 -0700 (PDT) >From: John Ruben <rubenj@ava.bcc.orst.edu> >Subject: Response to Jenny Clack re conodont "eyes" > >Jenny-- I suggest that if conodonts lacked proper vertebrate-like >eyes (and, like you, I have serious reservations about those >supposed conodont "eyes" [they could just as easily be >invertebrate-like eyespots]), then conodonts are NOT vertebrates. >Specialized chordates perhaps. As Halstead and others have quite >correctly emphasized, the overriding feature that distinguishes >the earliest vertebrates from their chordate ancestors is >cephalization (i.e., development of a tripartite brain associated >with olfactory, optic and otic structures), not just the capacity >to produce mineralized skeletal parts. The old classification of >vertebrates as the "Craniata" is just as valid today as ever. So, >perhaps you should deal with conodonts in your course as >representatives of the "protovertebrate invertebrates"(!!!). >Cheers-- John Ruben ******************************************************************* >Date: Thu, 25 May 1995 14:10:56 -0400 (EDT) >From: "James M. Clark" <jclark@gwis2.circ.gwu.edu> >Subject: conodont eyes (In response to the posting by John Ruben) > This suggestion seems to be based upon the assumption of >non-reversability in evolution (i.e., "Dollo's Law"). If >conodonts lack eyes, there is no reason to assume a priori that >this is plesiomorphic rather than a secondary loss (the question >of whether the absence of eyes can be determined to be >non-artifactual aside). The two other features shared with >gnathostomes (bone cells) and with gnathostomes and >"ostracoderms" (mineralized exoskeleton) indicates that if eyes >are absent then this is secondary. Why dismiss this evidence? ******************************************************************* >Date: Thu, 25 May 1995 15:52:21 -0700 (PDT) >From: John Ruben <rubenj@ava.bcc.orst.edu> >Subject: for J Clark & C Brochu (fwd) > Jim , Chris -- Of course skinks and moles are vertebrates: they >may lack eyes, but still retain a complex tripartite brain, >olfactory and otic structures. Clearly they are derived from >ancestors that DID have eyes, nose, ears. As to whether the >(apparent) lack of proper vertebrate eyes (and probably complex >brains) in conodonts reflects a secondary loss of these >attributes, nobody can say. But in the absence of evidence that >they have been secondarily lost, the most parsimonious assumption >is that they were never present in the conodont ancestor. >Consequently, at this point the safest conclusion is that >conodonts were neither cephalized, nor specialized forms derived >from cephalized ancestors and, therefore, weren't vertebrates. >The apparent presence of "bone cells" in conodonts hardly suggests >that the absence of eyes, and, presumably a brain was secondary. >Eyes, nose, ears are derived embryologically largely from >epidermal placodes. Bone is probably ultimately a derivative of >neural crest. It is highly unlikely that neural crest is an >ancestral derivative of epidermal placodes (or vice versa). >Consequently, the presence of a neural crest derivative (bone, >bone cells) in conodonts is hardly a compelling reason to assume >that epidermal placode derivatives (or, for that matter, brains) >were ever present in conodonts or their ancestors. Your >turn... John R ******************************************************************* >Date: Thu, 25 May 1995 18:44:35 +0100 (BST) >From: Mark Purnell <map2@leicester.ac.uk> >Subject: Re: Conodonts' eyes (In response to Jenny Clack's posting) > Interesting point, but what it is that bothers you about the >eyes? Their size? level of sophistication? Would you think it >unreasonable for conodonts to have a brain and nervous system as >complex as that of a Lamprey? They seem to be able to manage >their eyes without too much difficulty. Also, if they are not >eyes, what organs are represented by the large ring shaped >structures preserved at the anterior end of the animal? > > Dick Aldridge and Sarah Gabbott will probaly have something to >say about the extrinsic eye muscles tomorrow. MARK ******************************************************************* >Date: Fri, 26 May 1995 09:48:39 +0100 (BST) >From: "Philip C.J. Donoghue" <pcjd2@leicester.ac.uk> >Subject: Re:Them eyes is eyes (In response to posting from John Ruben) > Yes, eyes, nose and ears are derived embryologically from >ectodermal placodes, but not exclusively so. A recent paper by >Collazo et al (1994) describes a dual embryological origin for >neuromasts, the mechanoreceptors of the lateral line system, from >which organs including the otic organ are derived. Embryological >studies of Zebrafish, Siamese fighting fish and Xenopus show that >the neuromasts of the body and head are also neural crest derived. > > As for the ancestry of neural crest and ectodermal placodes, it >is believed that both these structures were derived from a common >precursor tissue (Smith et al., 1994) and they appear to be >derived either simultaneously or sequentially (Graveson, 1993). > > It therefore appears that if the interpretations of the hard >tissues in conodont 'teeth' is correct, that they are derived from >(cranial) neural crest, there is a fair chance that ectodermal >placodes existed. The phylogenetic position of conodonts is not >based on the eyes, but on the notochord, somites, caudal fin and >hard tissues (the last character placing conodonts crownward of >lampreys). If the eyes are not eyes, then the ability to develop >them would have had to have been secondarily lost. But of course, >they are eyes! > > Collazo, A., Fraser, S. E. and Mabee, P. M., 1994, 'A dual origin >for vertebrate mechanoreceptors', Science, 264, pp. 426-430. > > Graveson, A. C., 1993, 'Neural crest: contributions to the >development of the vertebrate head', American Zoologist, 33, pp. >424- 433. > > Smith, S. C., Graveson, A. C., and Hall, B. K., 1994, 'Evidence >for a developmental and evolutionary link between placodal >ectoderm and neural crest', The Journal of Experimental Zoology, >270, pp. 292-301. > > Best wishes, > > Phil Donoghue ******************************************************************* >Date: Fri, 26 May 1995 10:02:57 -0500 >From: gator@mail.utexas.edu (christopher brochu) >Subject: Definition vs. Diagnosis > I think two issues have been raised by this current argument >regarding conodonts - the actual phylogenetic position of >conodonts, and how we define our taxa. My response to John >Ruben's comment was an attempt to address the latter. > > Perhaps I shouldn't go any further with this, since I'm about to >unsubscribe for a few weeks, but this is an important issue. > > I may be misinterpreting Dr. Ruben's note, but it sounds like he >is defining Vertebrata on the basis of certain key characters - in >this case, encephalization. (If this is incorrect, Dr. Ruben, >accept my apology. Still, whether or not you personally use such >definitions, there are many who do.) According to this approach, >Vertebrata is defined by the presence, at least ancestrally, of an >enlarged brain and associated sensory capsules. > > I, on the other hand, advocate definitions on the basis of >ancestry. Conodonts should be considered vertebrates if their >ancestors were vertebrates, regardless of what characters they >have. We should diagnose our taxa with morphological characters; >such diagnoses will change as our data improve, but the >definitions will be more stable. This is particularly true for >nodes with lots of supporting characters; in the above example, >suppose Vertebrata were defined on encephalization and dermal >bone. Further suppose that conodonts had the latter, but not the >former. What would that do to our definition? > > The crux, as Dr. Ruben noted, is telling the difference between >primitive absence and secondary loss. Having only glanced at the >photos in the Nature articles, I'm not in a position to comment >either way. > > What are your thoughts on this matter? Should our definitions be >based on characters or ancestry? As an introduction to the >literature, I suggest a point/counterpoint in a recent issue of >Systematic Biology: > Lucas, S.G., 1992. Extinction and the definition of the class >Mammalia. S.B. 41:370-371. > Rowe, T., and J. Gauthier, 1992. Ancestry, paleontology, and >definition of the name Mammalia. S.B. 41:372-378. > There was also a more recent review by Kevin De Quieroz in Syst. >Bio., but I don't have that one in front of me. > chris ******************************************************************* >Date: Fri, 26 May 1995 11:36:22 +0100 (BST) >From: "Richard J. Aldridge" <ra12@leicester.ac.uk> >Subject: Conodont eyes > >Dear Jenny and everyone > > First, we're delighted to hear that the young zoologists at >Cambridge are being taught about conodonts as early vertebrates. >But we're bemused at your "disbelief" in the conodont eyes, and by >the way that the subsequent discussion seems to have wandered >rapidly into realms surreal. So perhaps for all readers we can >re-iterate the evidence for the hypothesis that the lobate >structures at the anterior of the conodonts represent eyes, and, >we hope, bring the arguments back to discussion of that evidence. > > Paired ring-like structures, expanding laterally outwards, are >preserved at the anterior end of two of the conodont animal >specimens from the Carboniferous of Granton, Edinburgh, and in >association with about 40 conodont apparatuses from the Ordovician >of South Africa. The fullest description of these is given by >Aldridge & Theron (1993, J. Micropal. 12, 113-117), but a ready >reference for a couple of pictures is Geology Today July/Aug 1994, >p. 143. There are several possible interpretations of these >paired features, including a hood, semi-circular canals and >various sensory capsules; but their preferential preservation >suggests that they were sclerotized. This, together with their >size and their position above and anterior of the feeding >apparatus, is at least suggestive of the sclerotic capsules >surrounding the eyes. The similarity to structures interpreted as >sclerotic eye capsules in Jamoytius (Ritchie, 1968, Palaeontology >11, 21-39) is striking. Other interpretations seem, to us, to be >less tenable; but we would be most happy to hear other reasoned >arguments. > > And so to the features we suggest to be extrinsic eye muscles, >which are being dismissed as "any old bits of tissue." These >structures have been found to date only in the conodont animal >specimen from South Africa, described in our recent paper in >Nature (374, 800-803). They take the form of white patches with a >clearly fibrous texture, directly comparable with the muscle >fibres in the myomeres of the same specimen; the fibres are also >remarkably similar in pattern and orientation to those in muscles >surrounding the eye in extant lampreys. Preservation of this >material at the anterior of the animal has occurred only when >trunk musculature is also preserved. Significantly, the position >of these anterior fibrous patches is precisely the same, relative >to the feeding apparatus, as the position of the lobes discussed >above. Our interpretation cannot be refuted by mere dismissal or >expressions of disbelief. We would, of course, be very interested >to know of other possible interpretations consistent with the >morphological features of the specimen. > > Finally, we cannot understand why a brain and nervous system in >conodonts should be considered "unlikely". As has been pointed >out by others, we argue for a level of encephalization no more >complex than that of the lampreys, the most primitive of extant >unequivocal vertebrates. > > We hope these comments will help keep the discussion going. For >full details, of course, we refer everyone to the relevant papers. >But if anyone out there would like to see the evidence first-hand, >then all the most important conodont animal specimens are >currently here in Leicester. Anyone who wants to come and visit >us and them is most welcome, just contact us and ask. > > Dick Aldridge Sarah Gabbott ******************************************************************* >Date: Fri, 26 May 1995 13:10:49 +0000 >From: jac18@hermes.cam.ac.uk (Jenny Clack) >Subject: Conodonts' eyes again > >Well I'm glad to have stirred up such an ants' nest! My quarrel >is not with the concept of conodonts having eyes in principle; I >know what a craniate is, by the way, and I also am aware of the >link between neural crest and ectodermal placodes - that goes into >my lectures, too. My skepticism relates to the eyes as >reconstructed and figured - they look like those of a teleost, and >even more strange, are set right at the anterior end of the >animal, not a very craniate-like position. Yes, it's also partly >the size which worries me, and its implications of great >elaboration of the brain. To operate a teleost-like eye requires a >lot of complex software, not just the existence of cephalisation. >(It's a bit like trying to imagine a state-of-the-art colour >monitor attached to an old 64K BBC-B.) Maybe it was just the >drawing which looked incredible. If it had looked more like a >lamprey or even a hagfish, it might have been more convincing. >However I still get the impression that the lobate structures look >more like, say, an eversible pharynx, judging from the figures. >As a zoologist, I like to think about how (of if) the animals >we're talking about actually _work_, and that means taking on >board the likelihood of our reconstructions based on work from >other disciplines. Next time I come to Leicester, Dick, may I take >up your kind offer? Meanwhile, what do people think of >Philipe Janvier's suggestion that the teeth apparatuses were >pharyngeal? ******************************************************************* >Date: Fri, 26 May 1995 14:12:53 -0600 >From: cjanis@midway.uchicago.edu (Christine Janis) >Subject: Re:Them eyes is eyes > Re the issue of whether conodonts should be placed >"crownward of lampreys" (if, indeed, we accept that they ARE >vertebrates) ---- I'd like to raise an issue that I've not seen >discussed. Conodonts are all marine, right? (That's what the >geology types here told me: if I'm wrong, just ignore this entire >argument!) That strongly implies that, like hagfish, but unlike >all other vertebrates (which have freshwater representatives) they >still have body fluids of the same concentration of sea water. >This leads me to suspect that it is highly unlikely they could be >placed crownwards of anything else except hagfish on the >vertebrate cladogram. (We have evidence of bone being lost in >various vertebrate groups, but physiologists tell us that body >fluid dilution is pretty much a one way street --- although I know >there was some dissention to this idea in a recent issue of >BioScience.) > > Christine Janis ******************************************************************* >Date: Sat, 27 May 95 12:17 BST >From: pmb19@esc.cam.ac.uk (Paul M Barrett) >Subject: Re:Them eyes is eyes > >I'm not sure that the arguement about body fluid concentration can >be used to place limits on the phylogenetic position of conodonts. >Maybe freshwater lampreys evolved their osmoregulatory strategies >independantly of other vertebrates; maybe conodonts did have >freshwater representatives which are unkown; maybe condont animals >had the right kidney structure etc. but 'chose' not to be >freshwater (why are there marine and freshwater lampreys - are >their osmoregulatory systems radically different?). Marine >lampreys are not placed below freshwater lampreys on the Craniate >cladogram because they don't live in freshwater. This doesn't >preclude conodonts from being more crownward than lampreys: maybe >they retained marine habits whilst the less crownward (?) lampreys >went their own adaptive way. With independant invasions of >freshwater by teleosts, other groups of actinopts and a return to >the sea by sarcopts can be really use body fluid concentration in >this way? Personally, I find the histological evidence quite >convincing. However, how much should we believe in any of the >Craniate cladograms so far published? Forey and Janvier have >changed their positions several times in the past two or three >years and we're dealing with cladograms based on only two or three >characters per node (at the moment anyway). In this sort of >situation the discovery of one new character or creature could >seriously rock the boat... > > Paul M. Barrett ******************************************************************* >Date: Fri, 26 May 1995 15:17:22 +0100 (BST) >From: Mark Purnell <map2@leicester.ac.uk> >Subject: an eye for an eye, etc. > >Before we dispense with conodont eyes and move on to conodont >teeth, just a couple more simple points: the eyes are not as large >as drawn in some reconstructions; they are about 3 per cent of >body length in the Scottish animals. Their absolute size is about >the same as that of the eyes of the smallest lampreys and, >incidently, seems to be at about the minimum size limit for a >functioning vertebrate eye (hence their large size relative to >body length). It is not the strongest of arguments, but Northcutt >(1985) has used similarity in eye size to suggest that the eyes of >osteostracans were similar in complexity to those of lampreys; I >think the evidence (size considerations and other stuff) is >consistent with conodonts having eyes that were similar in >complexity to those of lampreys (for more details you'll have to >wait for my ms on conodont eyes and vision in Lethaia 28:2). >Regarding parsimonious interpretations, as several people have >pointed out, a whole load a characters other than the eyes suggest >that conodonts are craniates; this being the case you would expect >them to have eyes. Surely, therefore, paired structures in the >head area that look like the eyes in other fossil agnathans are >most parsimoniously interpreted as eyes. > > >Meanwhile, what do people think of Philipe Janvier's suggestion > >that the teeth apparatuses were pharyngeal? > > > There are several problems with this interpretation. My primary >objection is that conodonts were macrophagous (microwear evidence >in recent Nature paper); if their anteriormost teeth were back in >the pharynx, how the hell did they get hold of their food! Most >extant macrophagous aquatic vertebrates are suction feeders, >relying on rapid expansion of the buccal cavity to generate the >negative pressure required. Without jaws, conodonts could not do >this. It is also interesting that those macrophagous aquatic >vertebrates that depend least on suction have the most toothy >grins. Conodonts probably could not grin, but they were certainly >well endowed in the toothiness department! > > MARK ******************************************************************* >Date: Fri, 26 May 1995 17:25:28 -0400 >From: rowe@lepomis.psych.upenn.edu (Mickey Rowe) >Subject: Re: Conodonts' eyes again > > jac18@hermes.cam.ac.uk (Jenny Clack) writes: > > > To operate a teleost-like eye requires a lot of complex >software, > > not just the existence of cephalisation. (It's a bit like >trying to > > imagine a state-of-the-art colour monitor attached to an old >64K > > BBC-B.) > As another one of the ants in the nest you've stirred up, I'm >wondering if you (and the "neuro folks" you've been talking to) >might be a little bit presumptuous here. While I expect hagfish >and lamprey to be much better analogues of conodonts than any >other living animals, there are a lot of other animals with camera >eyes and not much brain power. Check out Fig. 22a in: > > Land, M. (1981). "Optics and Vision in Invertebrates", in Autrum, >H., Ed. _Handbook of Sensory Physiology VII/6B Vision in >Invertebrates_, Springer-Verlag, New York. > > If you didn't have the caption, I could probably convince you >that the eye in that picture came from a fish. I doubt I could >similarly show you this sea snail's nervous system and fool you >the same way. If you want to go to even further extremes >(admittedly with smaller eyes), check out the description of the >cubomedusoid's eyes and lack of brain in: > > Brusca, R. C. and Brusca, G. J. (1990). _Invertebrates_, Sinauer >Associates, Sunderland, MA. > > While grafting a modern teleost eye onto a conodont might be like >your analogy, a superficial resemblance between a modern teleost >eye and the proposed conodont eye doesn't imply that the conodont >needed a teleost brain. > > -- Mickey Rowe ******************************************************************* >Date: Tue, 30 May 1995 08:49:34 +1000 >From: S.Turner@mailbox.uq.oz.au (Dr Susan Turner) >Subject: Re: Conodonts' eyes > >Gee, Jenny, I wonder what I have begun but I hope the Aldridgites >will forgive me. I was partivularly interested in Henry Gee of >Nature's response to my casting aspersions on Phil Janvier's >opinion - he says that if Phil says conodonts are vertebrates then >it must be so. Well, I am a bit fed up with men's club science and >there are a lot of people out here in the colonial wiolderness who >don't yet "believe" - I am just acting at the new Doubting >Thomas/Devil's advocate I suppose - we shall see. I think it is >fair enough to discuss conodont animals in the early vertebrate >course but please put both sides of the argument because there are >a lot of problems still with conodonts being teeth - papers are >fothcoming but are taking time to wend through the mill. >Hope life is swinging along - will see you in Newcastle, Sue ******************************************************************* >Date: Tue, 30 May 1995 07:58:43 GMT >From: henry@chiswick.demon.co.uk (Henry Gee) >Subject: Re: Conodonts' eyes > In message <199505292250.AA16502@dingo.cc.uq.oz.au> >vrtpaleo@usc.edu writes: > I was partivularly interested in Henry >Gee of Nature's response > > to my casting aspersions on Phil Janvier's opinion - he says >that if Phil > > says conodonts are vertebrates then it must be so. Well, I am a >bit fed up > > with men's club science ... > > Pardon?? My respect for Philippe Janvier's opinion on the >matter rests on three things. First, he has thought about the >issues a great deal. Second, he has been markedly sceptical >about the alleged chordate or vertebrate affinities in the past, >and has said so, prominently, in print (see for example Forey and >Janvier, Nature vol 361, pp129-134). Third, he had the courage >to admit, frankly, in print (same journal, in an article >accompanying the latest eyes/teeth papers), that the latest work >finally convinced even him that conodonts were something to do >with vertebrates. Now then, is there anything in the above that >depends on the gender of either Philippe Janvier or me? Would >things have been different were we called Philippa and Henrietta? >I think not. So what's all this about 'men's club science' then? >Sorry, Sue, but your posting is redolent less of scientific >discourse than loony feminist paranoia. > > -- Henry Gee ******************************************************************* >Date: Tue, 30 May 1995 16:46:15 +1000 >From: S.Turner@mailbox.uq.oz.au (Dr Susan Turner) >Subject: Re: Definition vs. Diagnosis > >Dr Alain Blieck (Univ. Lille) says: > > Concerning this problem, I think that , for the time >being, no serious cladistic analysis of the conodont-chordate >relationships has been processed. So it is clear that all the >arguments used for demonstrating the vertebrate nature of >conodonts is just a matter of authority's argument (do you say >that in English ?). An histologist "demonstrates" the nature of >conodonts on one or two histological features. A morphologist does >the same from a few morphological features, and so on. The >"cladogram" shown by Phil Janvier in Nature 374 (6525) is nothing >else than an image to provoke the readers. It is not a cladogram >based upon a published matrix of characters which may be tested by >anybody, once published. So not serious as I said. All the >recently published papers in Nature and other spectacular journals >are of the same kind. Not based upon a full analysis of the higher >taxa involved. ******************************************************************* So there you have it. Dr Mark A. Purnell Department of Geology, University of Leicester University Road, Leicester LE1 7RH, U.K tel: 0116 2523629 fax: 0116 2523918
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