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On Thursday 22 May, Dick Jefferies writes: The title echoes two previous papers (Jefferies 1981, 1997). I was surprised to read, in the message from Jo Murukami of 14 May 1997, that the calcichordate theory of the origin of chordates had been successfully refuted by Claus Nielsen on pp. 377-378 of his 1995 book `Animal Evolution - interrelationships of the living phyla'. This was not what I remembered. However, Claus is a friend of mine and a very learned man, so I again looked up the pages referred to. I still do not think, of course, that his refutation was successful. Let me quote the passage in question with my comments enclosed in square brackets []. Nielsen: "A totally different phylogeny of the deuterostomes, the calcichordate theory, has been advocated in a series of recent publications by Jefferies (summarised in 1986)." [Comment: As Henry Gee has pointed out in his Paleonet message of 15 May, the matter has advanced considerably since I published my book. Most of what I said in it still stands but we now have much more extensive information. Thus, for example, Tony Cripps has worked on the cornute- mitrate transition (1989a, 1989b, 1990, 1991, Cripps & Daley 1994). Paul Daley (1992, 1995, 1996) and I (1990) have studied the solutes and, by implication, the events that occurred when echinoderms and chordates first separated from each other. Ian Woods has reconstructed locomotion in Cothurnocystis (Woods & Jefferies 1992). Fritz Friedrich has monographed the Cincta (1993). Individual mitrates have been studied by Mario Beisswenger (1994), Marcello Ruta (1997a, b) and Adam Craske (Craske & Jefferies 1989).] [As a consequence of all of this, we can now place all the `carpoid' groups (the solutes, cornutes, mitrates, ctenocystoids and helicoplacoids) in their broad phylogenetic position as stem-group dexiothetes, stem-group echinoderms, stem-group chordates, stem-group acraniates, stem-group tunicates or stem-group craniates (Jefferies, Brown & Daley 1996). In that same paper I made various guesses about gene expression in extant chordates, mainly on the basis of the theory of dexiothetism i.e. that the echinoderms and chordates had in their exclusive common ancestry an episode in which a Cephalodiscus-like ancestor lay down on the right side.] [In addition, Henry Gee has written an excellent general book (1996) which should make the whole problem more intelligible to interested outside readers. Also, Kevin Peterson has published criticism of the calcichordate theory (1994, 1995) and I have answered him (Jefferies 1997).] Nielsen: "It is based on a very detailed interpretation of the Palaeozoic group Carpoidea of Homalozoa, which by most palaeontologists, such as Ubaghs (1975) and Philip (1979), is regarded as non-pentameric echinoderms. The more traditional view is that the carpoids had an asymmetrical body with a series of openings, which could have housed retractile gills, and one articulated arm possibly with tube feet and possibly a feeding organ." [Comment: It is totally wrong to think that the views of the opponents of the calcichordate theory are consistent with each other. There is no generally accepted doctrine accepted by all sensible people, on the one hand, as opposed to the cranky opinions of Jefferies & Co. on the other. Thus Ubaghs, for example, believes that the jointed appendage of mitrates and cornutes was a feeding arm, but Philip does not. In my view, the Ubaghs interpretation is disproved by the solutes which possess an obvious feeding arm at one end of the animal and a homologue of the cornute and mitrate appendage (for me the tail) at the other end.] Nielsen: "Jefferies' interpretation of the series of openings is that they were gill slits like those of amphioxus and that the long jointed appendage was a tail with chorda and neural tube; the body should have housed a spacious branchial chamber like that of tunicates and cephalochordates. This group of organisms, called calcichordates, should then have given rise to the echinoderms and chordates. The theory completely disregards the similarities between the gill slits of enteropneusts and chordates." [Comment: This last sentence seems to refer to the classical comparison between the gill slits of amphioxus, on the one hand, and those of the enteropneusts on the other. I dealt with this on pp. 25-27 of my 1986 book. In both groups the gill slits are fundamentally U-shaped, ciliated, supported by cartilage, crossed by trabeculae and associated with coeloms in the gill bars. However, the U-shape is a fairly obvious adaptation to increase the length (and therefore the number of cilia) of each slit. It could easily have evolved twice. Moreover the supporting cartilages are not in detail comparable and the trabeculae are an obvious strengthening device, easily acquired twice. Also the coeloms of the two groups differ in their relation to the gill bars - in amphioxus the primordial bars (between the U's) have coeloms and the tongue bars do not, while in enteropneusts the converse is true. Additionally, the ontogenetic development of the gill slits is different. It is very complicated in amphioxus: there is a primary left set in the larva, a secondary right set that appears suddenly at metamorphosis and a tertiary set added throughout the later life of the animal, at right and left behind the primary and secondary sets already present. There is no such complexity in enteropneusts where new slits are simply added posteriorly, symmetrically on right and left. I conclude that there is no good reason for regarding the detailed structure of the gill slits as being homologous between amphioxus and enteropneusts. I might add that the structure seen in acraniates such as amphioxus (Branchiostoma and Epigonichthys (= Asymmetron)) is never seen in other chordates, so I do not see why it is regarded as so important as an indication of what primitive chordate gill slits were like.] Nielsen: "The interpretation of the gill slits is proposed axiomatically : `Since the openings suggest outlet valves, they can plausibly be seen as gill slits.' (Jefferies 1986, p.197). However, it is very difficult to see how the pharynx of the reconstructed cornutes (for example Cothurnocystis; Jefferies 1986, Fig. 7.6) can be compared to the gill chamber of a living tunicate or cephalochordate; both of these types have a gill chamber with large areas of gills, which both carry the ciliary bands creating the water currents and support the mucus net which is the filtering device. If the mucus filter extended only across the row of gill slits in the cornute, the filtering area would have been disproportionately small both in relation to the size of the pharynx and to the size of the whole animal when the living organisms are considered. A possibility not considered by Jefferies is of course that the `gill slits' were merely exit openings for the filtered water and that the filter was a more extensive structure somewhere else in the `pharynx', but this brings the speculations into the realm of fantasy." [Comment: Fossil evidence does not answer all questions but has been too easily dismissed. It is much better than no evidence. This is particularly true for the mitrates, with their very complicated informative skeletons. In them the pharynx can be reconstructed rather fully. In several, often asymmetrical, details it compares with the pharynx of tunicates. These statements are based on interpretation of fossil evidence, not `fantasy'.] [Thus in the mitrates Mitrocystites, Mitrocystella and Placocystites plausible positions can be found for the right and left peripharyngeal bands, the ciliated organ (situated near where the peripharyngeal bands meet dorsally as in tunicates), the dorsal lamina (sloping downwards and rightwards in transverse section as in tunicates), the opening of the oesophagus (right of the mid-line as in tunicates), the posterior end of the endostyle, the retropharyngeal band (passing from the right posterior corner of the endostyle towards the opening of the oesophagus as in tunicates), the pharyngo-epicardial openings (right and left of the retropharyngeal band as in the tunicate Ciona) and the epicardia (situated posterior to the pharynx as in tunicates). Moreover, in the mitrate Placocystella the whole extent of the endostyle is indicated (Ruta 1997a). Also there is clear evidence, in all mitrates, that the left pharynx preceded the right pharynx in ontogeny, as in amphioxus. I have argued all this in several publications (e.g. Jefferies 1981).] [It is therefore likely that mitrates fed, as tunicates do, by a mucous filter secreted by the endostyle, held anteriorly by the peripharyngeal bands, rolled up in the dorsal lamina and passed rearwards to the opening of the oesophagus. There is some evidence that the internal surface of the pharynx was corrugated where the mucous filter was situated and such corrugations may have served to hold the filter away from the pharyngeal wall. Very likely, also the corrugations would be ciliated to provide a pump for the feeding current. In that case the functions of the gill slits, deduced to have existed, would be simply to let water into the atria.] [Cornutes would have fed in the same way except that there was no left pharynx. The lack of such a pharynx does not prevent feeding in larval amphioxus where the endostyle is fully functional (Olsson 1983).] [The fact that the cornutes, like larval amphioxus, had left gill slits only is a bizarre resemblance. All those who consider, like Claus Nielsen, that the calcichordate theory is fantasy, should contemplate this resemblance for a couple of minutes.] [In most mitrates the evidence for gill bars and slits is circumstantial. However, in two mitrates we now have direct evidence - the stem-group acraniate Lagynocystis (Jefferies 1973, 1986; Gee 1996, Fig. 4.14, p. 243) and the stem-group tunicate Jaekelocarpus (Jefferies, 1997, Fig. 5, p. 6).] Nielsen: "A functionally even more improbable explanation is the interpretation of the closely related Scotiaecystis, which had a long series of closely fitting, chevron-shaped ossicles in the same position as the `gill slits' of Cothurnocystis. These ossicles would appear to close the slits, but the following explanation was offered: `When water pressure was high inside the head the dorsal integument would inflate upwards. The chevron complex, bisecting the integument and therefore situated along the line of maximal stretching, would itself be stretched and gaps between the chevrons would open, allowing water to escape' (Jefferies 1986, p.207). It is unclear how the pressure inside the head/ pharynx would be created. In living tunicates the pharynx/ gill chamber is kept expanded by the elastic tunic and the ciliated gill bars pump the water through the mucus filter out of the pharynx; there is, accordingly, a slightly higher pressure at the exhalant siphon than inside the filter (Riisgard 1988). If the pressure were to have been higher inside the pharynx of Scotiaecystis, a mechanism unknown in living tunicates or cephalochordates would have been present, and the discussion again becomes mere fantasy." [Comment: In order to pass water through a mucous pharyngeal filter of primitive chordate type there has to be a pump. From the engineering point of view, this pump can be located anywhere in the stream. It can be upstream of the filter, as with the muscular contractions of salps or the velar pump of ammocoete larvae, though such a pump may require a valve upstream to prevent back-flow. Or it could be downstream of the filter, depending on the action of cilia in the gill slits, as in ascidians or amphioxus. Or, conceivably, it could be between the filter and the gill slits, depending on the action of cilia in a corrugated pharyngeal wall as may well have been true in mitrates like Mitrocystella. Pressure will be at a maximum immediately downstream of the pump, wherever the latter is located, but will decrease by friction wherever the current passes, or passes through, an obstacle, as with the oral or branchial siphons of a tunicate or with the mucous filter itself. Both in Cothurnocystis and Scotiaecystis, the integuments were flexible and probably muscular and the walls of the pharynx in Cothunocystis seem to have been corrugated and may have been ciliated. Upstream valves could have been present at the mouth (closed by a sphincter) and perhaps at the junction of pharynx and buccal cavity (the velum). Consequently muscular or ciliary action, or both, could easily have forced water through the mucous filter and opened the gill slits by passive response, as their outlet valve structure suggests. Such a mechanism is not fantasy but reasonable supposition. Contrary to Nielsen, there is no reason why the pump should have taken the form of cilia in the gill slits and in the case of Cothurnocystis and Scotiaecystis I have never supposed that it did.] Nielsen: "The reconstructions of notochord and spinal chord with ganglia in the articulated extremity and of nerves, ganglia with eyes, even, in the head/body of the Middle-Ordovician Mitrocystella and other mitrates, interpreted as early vertebrate ancestors, appear as extreme examples of wishful thinking." [Comment: Such blanket dismissal of fossil evidence is depressingly common but difficult to counter. I assert, however, that fossil evidence is better than no evidence and that there are complicated observed features on which the reconstructions are based. My students and I have published photographs of the fossil evidence for the nervous system in many places. Thus the spinal ganglia of the mitrate Lagynocystis, for example, are shown in Jefferies (1973, Pl. 43, figs 45, 46). The spinal ganglia, connected with the dorsal nerve cord overlying the notochord, of the mitrate Mitrocystites can be seen in Jefferies (1973, Pl. 39, Figs 32, 33). The spinal ganglia of the mitrate Chauvelia are to be seen in Cripps (1990, Fig. 3i). The bipartite brain of Mitrocystites and Mitrocystella, divided into prosencephalon and deuterencephalon with the optic foramen antero- ventral to the prosencephalon, is shown in Jefferies & Lewis (1978, Pl. 11, Figs 110-113, Pl.13, Figs 121, 123). The branching nerves of the palmar complex, mainly trigeminal but also optic, can be seen in Jefferies & Lewis (1978, Pl.9, Figs 98, 99). Cripps (1990, Figs 12-14) has demonstrated how, in the mitrate Chauvelia, the acoustic and lateralis ganglia are directed connected with the part of the brain where, by comparison with modern fishes, the acustico-lateralis nuclei ought to have been located.] [The calcichordate theory is not fantasy. It is tied down to fossil evidence in hundreds of observed details of which it provides a coherent explanation. I cannot understand, therefore, why some dismiss it so easily, whereas the work of Garstang (1928), for example, is widely accepted without any evidence whatever.] Nielsen: "Consequently, I reject the calcichordate theory on functional grounds, in accordance with a number of other authors who have rejected it on other grounds (Philip 1979, Ubaghs 1975, Jollie 1982)." [Comment: The functional grounds are very weak. Also, as already stated, it is not true that the cited authors agree with each other.] [Any phylogenetic argument based on fossils is difficult to present because neontologists have no feel or respect for fossil evidence. I emphasise, however, that the calcichordate theory has great explanatory power, presupposes the close relationship of chordates with echinoderms which is generally accepted, and is based on innumerable detailed observations.] [In particular, unlike its rivals, it can explain the numerous asymmetries of echinoderms and primitive chordates as Haeckelian recapitulations. Claus Nielsen, and those who agree with him, should confront the fact that the gill slits of Cothurnocystis, like those of larval amphioxus, are left gill slits only.] [I hope Paleonetters will have some sympathy for my position.] [I am grateful to my friend David Hardwick, of the Civil Engineering Department, Imperial College, London, for highly professional advice on pumps.] References. Beisswenger, M. 1994. A calcichordate interpretation of the new mitrate Eumitrocystella savilli from the Ordovician of Morocco. Paleontologische Zeitschrift 68, 443-462. Craske, A.J. & Jefferies, R. P. S. 1989. A new mitrate from the Upper Ordovician of Norway, and a new approach to subdividing a plesion. Palaeontology 32, 69-99. Cripps, A. P. 1989a. A new stem-group chordate (Cornuta) from the Lower and Middle Ordovician of Czechoslovakia and the cornute-mitrate transition. Zoological Journal of the Linnean Society 96, 49-85. Cripps, A. P. 1989b. A new genus of stem chordate (Cornuta) from the Lower and Middle Ordovician of Czechoslovakia and the origin of bilateral symmetry in the chordates. Geobios, 22, 215-245. Cripps, A. P. 1990. A new stem-group craniate from the Ordovician of Morocco and the search for the sister group of the Craniata. Zoological Journal of the Linnean Society 100, 27-71. Cripps, A. P. 1991. A cladistic analysis of the cornutes (stemchordates). Zoological Journal of the Linnean Society 102, 333-366. Cripps, A. P. & Daley, P. E. J. 1994. Two cornutes from the Middle Ordovician (Lllandeilo) of Normandy, France, and a reinterpretation of Milonicystis kerfornei. Paleontographical (A) 232, 99-132. Daley, P. E. J. 1992. The anatomy of the solute Girvanicystis batheri (? Chordata) from the Upper Ordovician of Scotland and a new species of Girvanicystis from the Upper Ordovician of South Wales. Zoological Journal of the Linnean Society, 105, 353-375. Daley, P. E. J. 1995. Anatomy, locomotion and ontogeny of the solute Castericystis vali from the Middlke Cambrain of Utah. Geobios, 28, 585-615. Daley, P. E. J. 1996. The first solute which is attached as an adult; a Mid-Cambrian fossil from Utah with echinoderm and chordate affinities. Zoological Journal of the Linnean Society 117, 403-440. Friedrich, W.-P. 1993. Systematik und Funktionsmorphologie mittelkambrischer Cincta. Beringeria, 7, 3-190. Garstang, W. 1928. The morphology of the Tunicata and its bearing on the phylogeny of the Chordata. Quarterly Journal of Microscopical Sciences, 72, 51-187. Gee, H. 1996. Before the backbone - views on the origin of the vertebrates. Chapman & Hall, London, 360pp. Jefferies, R. P. S. 1973. The Ordovician fossil Lagynocystis pyramidalis (Barrande) and the ancestry of amphioxu. Philosophical Transactions of the Royal Society, (B) 265, 409-469. Jefferies, R. P. S. 1981. In defence of the calcichordates. Zoological Jounal of the Linnean Society 73, 351-396. Jefferies, R. P. S. 1986. The ancestry of the vertebrates. British Museum (Natural History), London, 376pp. Jefferies, R. P. S. 1990. The solute Dendrocystoides scoticus from the Upper Ordovician of Scotland and the ancestry of chordates and echinoderms. Palaeontology 33, 631-679. Jefferies, R. P. S. 1997. A defence of the calcichordates. Lethaia, 30, 1- 10. Jefferies, R. P. S., Brown, N. A. & Daley, P. E. J. 1996. The early phylogeny of chordates and echinoderms and the origin of chordate left- right asymmetry and bilateral symmetry. Acta Zoologica (Stockholm), 77, 101- 122. Jefferies, R. P. S. & Lewis, D. N. 1978. The English Silurian fossil Placocystites forbesianus(Barrande) and the ancestry of the vertebrates. Philosophical Transactions of the Royal Society (B) 282, 205-323. Jollie, M. 1982. 1982. What are the `calcichordata'? And the larger question of the origin of chordates. Zoological Journal of the Linnean Society, 75, 167-188. Nielsen, C. 1995. Animal Evolution - Interrelationships of the living phyla. Oxford University Press, Oxford, 467pp. Olsson, R. 1983. Club-shaped gland and endostyle in larval Branchiostoms lanceolatum (Cephalochordata). Zoomorphology, 103, 1-13. Peterson, K. 1994. The origin and early evolution of the Craniata. 14-37 in Prothero, S. R. & Schoch, R. M. (eds): Major features of vertebrate evolution. Short courses in Paleontology 7. The Paleontological Society and the University of Tennessee, Knoxville, Tenn. Peterson, K. 1995. A phylogenetic test of the calcichordate scenario. Lethaia, 28, 25-38. Philip, G. M. 1979. Carpoids - echinoderms or chordates? Biological Reviews 54, 439-471. Riisgard, 1988. The ascidian pump: properties and energy cost. Marine Ecology Progress Series, 47, 129-134. Ruta, M. & Theron, J. N. 1997a. Two Devonian mitrates from South Africa. Palaeontology, 40, 201-243. Ruta, M. 1997b. Redescription of the Australian mitrate Victoriacystis with comments on its functional morphology. Alcheringa 21, 81-101. Ubaghs, G. 1975. Early Palaeozoic echinoderms. Annual Review of Earth and Planetary Sciences, 3, 79-98. Woods, I. S. & Jefferies, R. P. S. 1992. A new stem-group chordate from the Lower Ordovician of South Wales, and the problem of locomotion in boot- shaped cornutes. Palaeontology, 35, 1-25. ==================================================== R P S Jefferies Department of Palaeontology The Natural History Museum, London SW7 5BD, England Telephone: +44 (0)71 938 8713 Fax: +44 (0)71 938 9277 JANET: r.jefferies@uk.ac.ic.nhm INTERNET: r.jefferies%nhm.ic.ac.uk
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