Conodonts, Calcichordates and the Origin of Vertebrates
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Mitt. Mus. Nat.kd. Berl., Geowiss. Reihe l(1998) 81-92 19.11.1998 Conodonts, Calcichordates and the Origin of Vertebrates Jan Bergstrom ’, Wilfried W. Naumann 2, Jens Viehweg & M6nica Marti-Mus With 6 Figures Abstract Interpretation of early deuterostome evolution and relationships has been hampered by the lack of soft-part preservation in most groups. In addition, a recently revealed upside-down life orientation of vertebrates (the only real notoneuralians) compa- red to other bilateral animals has been misinterpreted as evidence for a unique body design in all deuterostomes, misleading any search for relatives. Regarding echinoderms, the variety of body plans is confusing. The interpretation of some fossils with echinoderm-type calcite skeletons as “calcichordate” ancestors of chordates, however, involves a hypothetical reconstruction of an unusual body plan and a long series of hypothetical transitions. The number of necessary steps is much lower if cephalo- chordates (amphioxus or lancelet) are derived directly from hemichordate enteropneusts. “Sensation interpretations” of fossils (Yunnunozoon, Cuthuymyrus) from Burgess Shale type deposits have added further confusion. Soft-part preservation of cono- dont animals, with V-shaped myomeres and a notochord, shows that they were segmented chordates, while probable eyes and teeth suggest that they were already on the vertebrate side. Key words: Deuterostomes, protostomes, amphioxus, conodont animals, hemichordates, calcichordates, notoneuralians. Zusammenfassung Die Interpretation fruher Deuterostomia hinsichtlich ihrer Evolution und verwandtschaftlichen Beziehungen ist in den meisten Gruppen durch den Mangel an Weichkorpererhaltung sehr erschwert. Die kurzlich entdeckte Tatsache, daB Verte- braten, d. h. die einzigen echten Notoneuralia, im Gegensatz zu anderen bilateral symmetrischen Organismen eine mit ihrer ursprunglichen Oberseite nach unten gerichtete Lebensstellung einnehmen, hat zu der irrtumlichen Ansicht gefuhrt, daB alle Deuterostomia uber einen im Tierreich einzigartigen Bauplan verfugen. Diese Interpretation brachte naturgemaB jede Suche nach Verwandtschaftsverhaltnissen auf Abwege. Hinsichtlich der Echinodermata ist die bauplanmanige Variation in der Tat verwirrend. Die Interpretation einiger Fossilien mit Echinodermen-ahnlichen Kalzitskeletten als ,,calcichordate“ Vorfahren der Chordata setzt jedoch die hypothetische Rekonstruktion eines ungewohnlichen Bauplans sowie eine lange Serie hypothetischer Ubergange voraus. Die Anzahl der notwendigen Schritte ist sehr vie1 geringer, wenn Cephalochordaten (Amphioxus oder das Lanzettfischchen) von hemichorda- ten Enteropneusta abgeleitet werden. Zusatzliche Verwirrung hat es durch sensationelle Interpretationen von Fossilien, wie Yunnanozoon und Cathaymyrus aus Burgess-Schiefer-artigen Ablagerungen gegeben. Weichkorpererhaltung von Conodonten- tieren, die V-formige Myomere sowie einen Notochord besitzen, zeigen, daB es sich um segmentierte Chordata handelte, wahrend sie die Prasenz moglicher Augenstrukturen und Zahne bereits auf die Seite der Vertebraten stellt. Schlusselworter: Deuterostomia, Protostomia, Amphioxus, Conodonten-Tiere, Hemichordata, Calcichordata, Notoneuralia. Introduction latter are classically considered as echinoderms, but in a series of publications by Jefferies and Attempts to trace the invertebrate origin of ver- others they are placed on the vertebrate lineage. tebrates usually recognise the other chordate The history of research will not be considered groups, the cephalochordates and urochordates here, since it has recently been thoroughly re- (tunicates), as the closest relatives. Some Palaeo- viewed by Gee (1996). zoic skeletonised fossils, the calcichordates or The different phyla and subphyla of deuteros- carpoids, also play a role in the discussion. The tomes are notably different from one another in Department of Palaeozoology, Swedish Museum of Natural History, PO. Box 50007, 104 05 Stockholm, Sweden. E-mail [email protected]> ’ Institut fur Zoologie, TalstraBe 33, D-04103 Leipzig, Germany. E-mail <[email protected]> Department of Palaeontology, Uppsala University, Norbyvagen 22, S-752 36 Uppsala, Sweden. E-mail <[email protected]> Received August 1997, accepted Juny 1998 82 BergstrBm, J., W. W. Naumann, J. Viehweg & M. Marti-Mus, Origin of Vertebrates basic designs. It is therefore no wonder that lated that the distribution of the two feeding there is great disagreement between interpreta- methods marks two major branches of animals. tions, with attempts to derive deuterostomes As a result, some tentaculate phyla were added from many places between the base and the top to the deuterostomes. However, few features, if of the protostome (gastroneuralian) tree. any, unite tentaculates and deuterostomes except We should note that St. Hilaire (1822) ex- for the upstream collecting system. It should be plained the differences by suggesting that verte- noted that an upstream collecting system is typi- brates rolled over to an upside-down posture cal for groups in which the second larval ciliary compared to protostomes (gastroneuralians). band (metatroch) develops into the adult ciliated The classical argument for this suggestion was tentacles. Therefore it seems more likely that that the vertebrate blood circulates in a direction this unique similarity is caused by convergent opposite to that in protostomes. A rolling-over functional adaptation. would mean that there is a fundamental identity Nielsen (1987) thus interprets the downstream in direction, the difference being only apparent. and upstream collecting systems as key charac- Recently, the upside-down explanation has re- ters for two major lineages of bilateral animals ceived unexpected support from the field of de- without seeing a functional link between them. velopmental science by the discovery that corre- Furthermore, his model offers no explanation to sponding dorso-ventralling genes in insects and the molecular phylogenetic trees which do not vertebrates act on opposite sides of the body in coincide with his tree. It also causes striking con- the two groups (Arendt & Niibler-Jung 1994; tradictions in the interpretation of classical mor- Niibler-Jung & Arendt 1994; Holley et al. 1995; phological characters (for instance by interpret- De Robertis & Sasai 1996). This opens comple- ing ctenophorans as deuterostomes), and fails to tely new perspectives for understanding the ori- explain the existence of orthologous genes in bi- gination and evolution of deuterostomes as well laterians. as for comparative morphology. It should be noted that protostome and deu- It should be noted that the situation in deuter- terostome ciliary patterns are not as clearcut as ostomes other than vertebrates has not yet been the names may imply. Nielsen (1987: 217) refers elucidated. Therefore the molecular arguments to a polychaete worm (Owenia) which is similar do not tell us where in the evolutionary tree roll- to deuterostomes in having rnonociliate cells in ing-over occurred. This is discussed below. the prototroch and metatroch. Dcuterostomes We recognize that the ultimate solution of the may have a protostome-type telotroch with com- question of vertebrate origins should be consis- pound cilia (Nielsen 1987: Fig. 8). tent with the basic designs and phylogenetic po- We note (1) that ciliary bands always develop sitions of “calcichordates”, urochordates, and ce- from the ectoderm adjoining the hypothetically phalochordates. slit-like gastropore, (2) that ciliary bands serve filter-feeding and swimming, (3) that primary lar- vae grow to a maximum size (delimited by cili- Ciliary bands and deuterostome relationships ary band arrangement, relative length of the lar- va, and inflation of it, see Emlet 1994), and (4) Any data matrix that yields correct information that selection controls the maximum size. reveals a disordered distribution of morphologi- In the trochophora larva, the ciliary band cal characters among phyla; notably there is no germ gives rise to the epitroch (prototroch) and clear separation between protostomes and deu- hypotroch (mesotroch). Functional fusion results terostomes. If a matrix shows a more ordered in a double band structure, the downstream col- separation, there is reason to question its quality. lecting system, which transports food particles to In the data matrix of Nielsen (1987: Fig. 33), the mouth when the two constituent ciliary some data appear to reflect the author’s expecta- bands beat in opposite directions. Efficiency was tions rather than observation (see Bergstrom further increased by fusion of cilia and by the 1997). For instance, hemichordates are said to development of a telotroch. have their central nervous system on the dorsal There are alternatives to this trend, for in- side, which is hardly the case - at least not with stance the development of a simpler upstream regard to the main longitudinal trunk where it is collecting system. Adult bivalves with ciliary in fact ventral. Nielsen correctly noted the differ- feeding have shifted to deuterostome-type up- ence between upstream and downstream food stream-collecting (Nielsen 1987: 225). In this collecting in pelagic larvae. From this he specu- case, however, the shift has not been taken over Milt. Mus. N;rf.kd. Berl.. Geowiss. Keihe l(1008) 83 Fig. 1. Downstrcarn and upstream collecting systems in marine larvae. In a typical trochophore larva, food particles collecl on the lee side of cilia. Cells are typically multiciliale, and cilia (long