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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.-Acra Zoologica (Stockholm) 77: 101-122.Left-right asymmetry in Dexiothetica (= echinoderms + chordates) results mainly from dexiothetism-an episode in their ancestry when an animal resembling the Recent pterobranch Cephalodiscus lay right-side-downwards on the sea floor. Casrericystis sprinklei belongs to the dexiothete stem group. The history of the echinoderm stem group is reconstructed. Chordate bilateral symmetry evolved by six successive steps. Tail-head overlap occurred independently in craniates and acraniates. The neural crest would have existed in the latest common ancestor of extant chordates, or even earlier. Gross asymmetries occur in extant chordates in organs derived from the calcichordate head, but not in those derived from the calcichordate tail. The anterior boundary of hox gene expression in vertebrates corresponds to the anterior end of the calcichordate tail. Left-right organ pairing (an important step in the origin of chordate bilateral symmetry) may have involved the interaction of a symmetrizing morphogen, produced from the anterior end of the tail, with a lateral morphogen (Wilhelmi's morphogen), produced in ontogeny at first from the left and later from the right. This mechanism may still act in the metamorphosis of amphioxus and in mirror-imaging in vertebrate twins. Wilhelmi's morphogen may be related to one or more members of the dorsal cascade of Drosophila.

The paper describes a new member of a group of Lower Palaeozoic marine fossils which partly bridge the gap between echinoderms and chordates. Evidence suggests that this group included the ancestors of the vertebrates. Its members are traditionally regarded as primitive echinoderms, but are better seen as primitive chordates with echinoderm affinities. They form a basal subphylum of chordates-the Calcichordata Jefferies 1967. The Calcichordata, in accordance with an early suggestion by GislCn, are probably ancestral to all living chordates.The new calcichydate is named Reticu/ocarposvhanui gen et sp. nov. I t comes from the Lower Ordovician Sirka Formation (Llanvirn) of Sdrka near Prague, Czechoslovakia and is placed in the family Amygdalothecidae Ubaghs 1970. I t is important because of its position in the Calcichordata. This group is divided into two very different orders-the Cornuta and the Mitrata. The Cornuta are the more primitive order and gave rise to the Mitrata, which had the structure of giant, calcite-plated tunicate tadpoles. Many features show that the new species is a very advanced cornute, closely related to the stock that gave rise to the mitrates. For this reason it is important in the general history of the chordates, since some primitive mitrate was probably the latest common ancestor of the living chordate subphyla i.e. of tunicates, of amphioxus and its allies and of the vertebrates.Being a rnitrate-like cornute, the new species allows the cornutes and mitrates to be compared more confidently than before. Four results are especially important. Firstly it is likely that the stem (=tail) of mitrates is equivalent only to the anterior part of the stem of cornutes. This is significant, because traditional views as to which was the upper surface in rnitrates have been based on stem homologies now seen as false. Secondly Reticulocarpos hanusi is adapted to stay up on very soft mud, using only the strength of the mud for support. The mitrates, on the other hand, supported themselves on soft mud by a much more reliable method resembling buoyancy. Thirdly, the new form had paired transpharyngeal eyes which are otherwise known only in mitrates, and which are the earliest type of paired eyes in chordates. Fourthly, it becomes possible to homologize the thecal plates of cornutes with those of mitrates.Reticulocarpos hanusi represents an important phase in chordate evolution dominated by the necessity of staying up on mud by a very precarious method. During this phase many pre-adaptations for swimming were acquired. Primitive mitrates, descended from a very similar form, were probably the first chordates that could swim.V prici je popdn Reticulocarpus hanusi n. gen. n. sp., novL druh primitivnich chordhi z podkmene Calcichordata Jefferies, 1967. Calcichordata jevi blizkt vztahy k ostnokoicum a takt se jim do znaEnC miry podoba ji. Zahrnujeme do nich dva Tidy, Mitrata a Cornuta, ktert byly diive, spolu s dalgimi dv'ema Mdy, Soluta a Cincta fazeny do th'dy Carpoidea Jaekel, 1900. 6 9 7 0 R. P. S. JEFFERIES...

The chordates, hemichordates (such as acorn worms) and echinoderms (such as starfish) comprise the group Deuterostomia, well established as monophyletic. Among extant deuterostomes, a skeleton in which each plate has the crystallographic structure of a single crystal of calcite is characteristic of echinoderms and is always associated with radial symmetry and never with gill slits. Among fossils, however, such a skeleton sometimes occurs without radial symmetry. This is true of Jaekelocarpus oklahomensis, from the Upper Carboniferous of Oklahoma, USA, which, being externally almost bilaterally symmetrical, is traditionally placed in the group Mitrata (Ordovician to Carboniferous periods, 530-280 million years ago), by contrast with the bizarrely asymmetrical Cornuta (Cambrian to Ordovician periods, 540 to 440 million years ago). Using computer X-ray microtomography, we describe the anatomy of Jaekelocarpus in greater detail than formerly possible, reveal evidence of paired gill slits internally and interpret its functional anatomy. On this basis we suggest its phylogenetic position within the deuterostomes.

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