An asymmetric segmented organism from the ...

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An asymmetric segmented organism from the Vendian of Russia and the status of the dipleurozoa a

Jerzy Dzik & Andrey Yu. Ivantsov

b

a

Instytut Paleobiologii PAN, Twarda 51/55, Warszawa, 00–818, Poland E-mail:

b

Paleontolo‐gical Institute of the RAS, Profsojuznaja 123, Moskwa, 117647, Russia

Available online: 10 Jan 2009

To cite this article: Jerzy Dzik & Andrey Yu. Ivantsov (1999): An asymmetric segmented organism from the Vendian of Russia and the status of the dipleurozoa, Historical Biology: An International Journal of Paleobiology, 13:4, 255-268 To link to this article: http://dx.doi.org/10.1080/08912969909386585

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Historical Biology. Vol. 13, pp. 255-268 Reprints available directly from the publisher Photocopying permitted by license only

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AN ASYMMETRIC SEGMENTED ORGANISM FROM THE VENDIAN OF RUSSIA AND THE STATUS OF THE DIPLEUROZOA JERZY DZIKa* and ANDREY YU. IVANTSOVb Downloaded by [Ohio State University Libraries] at 06:22 25 January 2012

a

Instytut Paleobiologii PAN, Twarda 51/55, 00-818 Warszawa, Poland and bPaleontological Institute of the RAS, Profsojuznaja 123, Moskwa 117647, Russia

A large animal, formally described as Yorgia waggoneri (Ivantsov, in press) has been discovered at a Vendian locality on the shore of the White Sea, in northern Russia. It is anatomically transitional between the Ediacaran "segmented worms" Dickinsonia and Spriggina. Like Dickinsonia, it had a metameric dorsal quilt, composed of a series of muscular chambers, and presumed intestinal caeca. Yorgia was unlike Dickinsonia, but similar to Spriggina, Marywadea, and Praecambridium, in that the anterior part of the dorsal quilt did not overhang the caeca. Rather, it was displaced posteriorly, with the medial chamber arched to the left. In effect, a lunate anterior region with ramified caeca was exposed dorsally. The left and right series of chambers were asymmetric, alternating along the midline. These organisms are not related to either annelids or arthropods, but possible homologies between characters of these animals and those of present day nemerteans and some deuterostomian phyla are proposed here. Keywords: Precambrian; Vendian; Russia; Mezen Formation; Metazoa; Evolution; Dipleurozoa; Dickinsonia; Spriggina; Yorgia

INTRODUCTION Extremely well preserved soft-bodied metazoan fossils have recently been discovered at a new locality, Zimnie Gory, which is near the village of Verkhnia Zolotica, on the shore of the White Sea in northern Russia (Grazhdankin and Ivantsov, 1996). Among those organisms is a species, about to be named Yorgia waggoneri (Ivantsov, in press), which resembles well known members of the Ediacara fauna with segmented bodies. Due to the unusually fine representation of morphological details in very fine-grained sediment, and more than one, distinctly different modes of preservation, these new specimens provide much more anatomical evidence than has been recognized in those from Australia (see * Corresponding Author. Email: [email protected] 255

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Glaessner, 1984). This new evidence may contribute to a more precise determination of the phylogenetic relationships of at least some of the Ediacarian organisms.

Downloaded by [Ohio State University Libraries] at 06:22 25 January 2012

THE LOCALITY The lower part of Unit 8 of the Yorga Beds of the Mezen Formation (Fedonkin, 1987) is exposed at Zimnie Gory. This is close to the base of the unit, which is defined by a sedimentary discontinuity that is locally characterized by conglomerates. The discontinuity is correlated with the boundary between the Redkino and Kotlino stages on the basis of lithological and putative palynological similarities (Stankovsky et al., 1987). This fossiliferous horizon thus lies well below the base of the Cambrian, and it is somewhat older than at least the youngest of the Ediacarian assemblages from Namibia (Narbonne et al., 1997; Jensen et al., 1998). The metazoan fossils occur at two horizons. The lower assemblage occurs at the base of a sand lens, filling an erosional channel. Here, rare and poorly preserved specimens of the new metazoan co-occur with several other fossils, including Kimberella (Fedonkin and Waggoner, 1997). The upper assemblage occurs about three metres higher in the section, at the base of a 10 cm thick sandstone bed. At this horizon, fossils of the segmented animals are abundant, and they are associated with several other, mostly undescribed metazoans.

TAPHONOMIC INTERPRETATION The fossils are represented either by easily recognizable, relatively deep depressions on the lower surface of a sandstone bed, similar to those typical of the Ediacara fossils (Wade, 1968; Runnegar, 1982; Glaessner, 1984; Seilacher, 1989, 1992, 1994), or by indistinct features emerging in low positive relief from the same surface. Fossils of the first type occur at both fossiliferous horizons; those preserved in positive relief are restricted to the upper horizon. The latter forms usually occur in close association with' specimens represented by deeper imprints, together forming groups that are oval in shape. A striking feature of these groups is that, in each case, there is only one deep impression on the base of the sandstone bed, but the number of associated fine, delicate forms preserved in positive relief varies, and they may be numerous. For instance, in specimen PIN 3993/5024 (Figure 1A) the deep imprint is associated


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FIGURE 1 Yorgia waggoned Ivantsov, Yorga Beds of the Mezen Formation at Zimnie Gory, near Vierchnia Zolotica, Arkhangelsk District, northern Russia. Scale bars equal 1 cm. (A) Specimen PIN 3993/2024, showing intersegmental organs (gonads?), preserved as depressions resulting from inferred post-collapse decay, and the anterior zone of exposed ramifying tubuli, which may represent intestinal caeca. Note the positive relief impression presumed to have been produced by the same organism below (see text). (B) Specimen PIN 3993/5018 with a sliver of mobilized sediment that penetrated the space between the external wall and the chambered unit, prior to its collapse. (C) System of dense, perpendicular striations, obliquely crossing the surface of impression PIN 3993/5011; probable collagen fibers. These specimens are housed in the Institute of Paleontology of the Russian Academy of Sciences (abbreviated PIN), collection number 3993, in Moscow

with six or seven finer impressions preserved in positive relief. In specimen 5018 (Figure IB) there is only one of these finer impressions, the smallest in size; in specimen 5028, there are two or three, and in specimen 5007 there are four. The planar dimensions of all imprints in any given group are the same, irrespective of their mode of preservation, but their sizes may be quite different from one group to another. Thus, it is probable that the delicate positive relief features, which produce very shallow depressions on the underlying claystone bedding plane, are

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impressions of a body that moved in and out of suspension several times under the influence of recurrent slow vortices, such as those produced by weak tidal currents behind local elevations on the sea bottom (Zimmermann, 1978; Park and Wang, 1991). In some cases (specimens 5028 and 5007), muscle contraction or post-mortem loss of internal body fluid resulted in a loss of turgor and thus a reduction in the diameter of the body, leaving a marginal imprint exposed beyond the area of preservation of the body fossil (Figure IB). To resuspend such a body without destroying its impression, the sediment must be cohesive (Schopf and Baumiller, 1998). Such cohesion was probably provided by gelatinous algal filaments, analogous to those of present day algal mats developed on quartz sand (Cameron et al, 1985). In fact, in specimen 3993, a dense fabric of pyritized algal filaments is preserved at the interface between the sandstone and the underlying mica-rich clay. These filaments also penetrate the underlying sandy clay. The algal mat was a source of organic matter. Due to its early diagenetic cementation by pyrite, the lower surface of the sandstone bed is smooth and reproduces the morphology of the impressions in great detail. The first 2 mm lamina of the sandstone bed, seen in a thin section taken from the margin of specimen 5035, is composed of angular, silt-sized quartz grains. This lamina is overlain by coarser sandstone with similar but larger grains. The interface between the layers is not distinct, but the fractured surfaces show some lamination. In the inferred succession of events, the touchdown impressions and cadavers (Figure 2) were initially covered by a lamina of muddy sediment, settling from suspension. Subsequently, numerous laminae of coarser sand were deposited, and under their load the soft bodies collapsed. This brought the sand from above and the plastic clay below into apposition, forming a depression in the base of the sandstone bed. Some specimens preserve parts of the morphology of such collapsed bodies, with apparently firmer elements preserved as elevations. This is especially true of the thickened septa, which are arranged in a pair of fields, on either side of the body. Some specimens apparently lost their internal body fluids before being deposited on the sea bottom (for instance specimen 5003) as they appear to be strongly deformed and folded. Elevated septa characterize even the smallest collapsed specimen, 5007. More commonly, however, the organisms experienced another stage of decay, which allowed clay migrating from below to fill in the space left by decomposing septal thickenings. This produced narrow depressions along the boundaries of the segments (Figure 1A). Thus, the intersegmental structures were evidently much more resistant to decay than other organs. There are no cuticular structures preserved in these fossils, either as carbonized remnants or as impressions, although a pyritic lining extends over parts of the surface of some of the specimens and pyritized algal filaments are abundant.


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FIGURE 2 Taphonomic interpretation of the under surface of a slab, viewed at an oblique angle, with multiple impressions of a specimen of Yorgia preserved in positive relief (pi) and a single deep imprint (ci). These are interpreted as "touch-down" marks (pi), made by an animal periodically dislodged by bottom vortices, on a clayey bottom that was consolidated by a microbial mat; and as the mold of the soft-bodied animal, where it was finally buried by an influx of suspended sediment

RESTORATION OF THE ANATOMY Given the taphonomic interpretation just proposed, it follows that the elevated impressions of the organisms preserved on the base of the upper sandstone bed represent their surfaces in positive relief. These show clear evidence of segmentation, with segments diminishing in size and becoming progressively more

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arched towards the hypothesized posterior end of the animal. Segmentation is barely visible at the opposite, presumably anterior end, which is usually poorly preserved. This may mean that this part of the organism was more inflated and convex, so its margin was not closely pressed against the sediment surface. The boundaries between the segments were expressed externally by faint, narrow, transverse furrows. A similar furrow runs along the mid-line of the body, separating the two series of lateral segments. The lateral segments on opposite sides of the medial furrow do not match precisely across the mid-line, however. Those on the right are shifted somewhat posteriorly relative to those on the left. In some specimens of Yorgia wagonneri, preserved as collapse imprints (5018, 5015, and 5024), muddy sediment expanding from the anterior end partially covers the relief produced by the collapse (Figure IB). The surface of this sediment masks details of the positive relief of the body's exterior. This surface is flat and nearly featureless, but on the left side of specimen 5018 there are some faint indentations, corresponding to the body segments. These may represent injections of sediment into the body. The lack of relief implies that the septa, represented now by furrows which are locally preserved within the rock, were very weak and did not prevent the movement of sediment within the cavity, as the organism decayed. This also means that structures preserved in relief, visible elsewhere on the specimen, were internal and separated from the body surface by a narrow space (Figures 3C, D). The external wall of the body appears to have been structurally complex, as shown by the wrinkled surface of imprint 5011. It consisted of densely spaced (less than 0.5 mm) riblets, corresponding to original fibres which ran obliquely across the segments (Figure 1C). Two perpendicular systems of fibres were present, presumably in separate layers. The surface of the collapsed chambered units of the body is also segmented (Figure 3C). The boundaries between segments correspond to those on the external body surface (Figure IB). Boundaries between segments and the medial septum are generally well preserved and the pattern of collapse suggests that the segments were originally fluid-filled chambers. The furrows between chambers frequently show regularly distributed pits (Figure 1A). These may represent thickenings of the wall, perhaps vertical skeletal fibres of vessels extending between the chambers. The exterior surface of the segmented body is wrinkled (Figure IB). The wrinkles run more or less transverse to and across the segments. They are arranged fan-like, radiating outward from the posterior end of the body (Figure 3C). The spacing between wrinkles is usually rather wide (about three wrinkles are equivalent in width to one segment). The wrinkling may signify the existence of a fibrous organic skeleton, developed above the chambers. These fibres would have held the whole chambered structure firmly together, especially in the posterior part of the body, where they converged and


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FIGURE 3 Restoration of preserved organs of Dickinsonia costata Sprigg, shown from the ventral side (A) with intestinal caeca (intc) and dorsal serial chambers (me), for comparison with our restoration of Yorgia waggoneri Ivantsov. This is shown as a section cut horizontally across the body chambers (B) exposing possible gonads (g); with the chambered unit exposed dorsally (C) and covering the level where ramified tubuli (rt), may represent intestinal caeca; and (D) an external dorsal view of the body covers. Note that the medial chamber is straight in Dickinsonia, but laterally displaced in Yorgia, where the left and right series of chambers are offset from one another, in an alternating arrangement

were most closely spaced. At the anterior end of the body, and at the margins where the segments curve back posteriorly, the fibres diverged, producing a less firm structure. The most intriguing feature of the body is the asymmetry that is expressed in the single, anteriormost, unpaired and wider chamber (Figures 1 A, 3C). Its right ramus extends only at a short distance to the right of the medial septum, whereas the left ramus continues to the body margin in the same way as the other chambers. In front of this anterior chamber there is a zone of ramifying tubules, shown by specimens 2024 (Figure 1A) and 5007. These probably represent canals. They

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fan out anteriorly, radiating from a point hidden below the medial septum. These structures are complex but their detailed arrangement, most clearly visible in specimen 2024 (Figure 1A), is hard to trace. They terminate close to the anterior margin of the body, but are separated from it by a very narrow, finely crenulated

rim.


Body thickness can be inferred from the marginal part of specimen 5002, which is preserved within the sandstone and only slightly compacted. The margin is represented by a vertical wall. Its upper edge seems to represent the surface of the specimen as it was buried, before collapse under the sand load. Its height roughly corresponds to the width of a segment. The segments were thus slightly higher than they are wide and the body, although dorsoventrally compressed, was not completely flat.

RELATIONSHIP TO OTHER EDIACARIAN FOSSILS Despite so distinctive an anatomy, some features of the new organism's body plan suggest a close evolutionary relationship to other members of the Ediacarian biota. Most of these possible relatives of Yorgia are small in size, so they may be comparable to its early developmental stages. Insight into the course of ontogeny is provided by comparison of the two most completely preserved specimens, which differ considerably in size. Specimen 5007 is 45 mm long, while 5024 (Figure 1A) is about 200 mm long. These specimens do not differ in the number of segments, which is 25 in both cases, but they show very different proportions. The juvenile has a much larger anterior part, with ramified tubules that are much more pronounced. The narrowing of the posterior segments is also more striking. Extrapolating from this trend, one may infer that in even smaller specimens the anterior part would be proportionally larger still. If the organism did not have the final number of segments immediately upon hatching, or during the larval stage, segments are likely to have been added sequentially at the posterior end of the body. Vendomia menneri, from the Valdai series of the Onega Peninsula (Paliy et al, 1979), which is 4 mm long and has six segments, seems to fit in with such a pattern of development. A similar number of segments characterizes the 14 mm long Vendia sokolovi, which is rather differently preserved but from the same strata, in the Yarensk borehole (Keller, 1969). At comparable sizes, the White Sea organism described here would have had more segments, as suggested by the indistinct imprint of specimen 5014, which is 21 mm long but has at least fifteen segments. Segments somewhat similar to those described here are also present in poorly preserved



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specimens of Valdainia plumosa from the Lomozov Beds of the Mohylev Formation, in the Ukraine (Fedonkin, 1987). Ramified tubular structures in the anterior region of the body and posteriorly arched segments which alternate along the mid-line occur in the Ediacaran organisms Marywadea ovata (Glaessner and Wade, 1966; Glaessner, 1976, 1984) and Praecambridium sigillum (Glaessner and Wade, 1966, 1971; Glaessner, 1984). In these respects, they are very similar to Yorgia. Spriggina floundersi, with its very elongated body (Glaessner and Wade, 1966), is also similar. Specimens of Marywadea and Spriggina show impressions of more resistant transverse serial structures on both sides of the body axis in their "thoracic" parts. These are virtually identical in shape, extent, and location with the proposed gonads (Figures 1 A, 2B) of Yorgia, especially as these are expressed in the smallest, juvenile specimen. All these organisms were apparently closely related. The Zimnie Gory organisms show no evidence of setae or any other cuticular structures, although the surface of some specimens is covered by a pyritic film which would have reproduced mineral or organic skeletal structures if they had been present. This makes it unlikely that these organisms were related to annelids or arthropods, as Glaessner and Wade (1966, 1971) have alleged in regard to the forms from Ediacara. However, it hardly solves the riddle of their zoological affinities. Evidence of affinity does come from comparisons with another Ediacaran animal, Dickinsonia costata, which shows basically the same body plan (Glaessner and Wade, 1966; Wade, 1972; Runnegar, 1982; Glaessner, 1984; Gehling, 1991, Jenkins, 1992). Individual body segments of Dickinsonia represent fluid-filled chambers separated from each other by walls (Seilacher, 1989). Some specimens show changes in the diameter of the body (Runnegar, 1982) similar to those observed in the White Sea organism, apparently resulting from muscle contraction or loss of turgor pressure inside the chambers. Wrinkles parallel to the margin (Runnegar, 1982) also occur in both Dickinsonia and the White Sea organism. The main difference is that in Dickinsonia the wrinkles, here interpreted as resulting from the fibrous nature of the internal skeleton, run parallel to the body margin along the entire length of the body, holding the chambers firmly together throughout. As a result, the anteriormost unpaired unit is not expanded laterally and drawn to the left. Rather, it is directed straight forward (Figure 3A), preserving the generally symmetrical organization of the body that is also expressed in the lateral parts of the segments, which are not offset from one another. Dickinsonia grew by the addition of new metameric units at its posterior end; in its early ontogenetic stages the anteriormost unit was roundedly triangular in outline (Runnegar, 1982). The chambered part of the body in Dickinsonia extends much further anteriorly, relative to the ramified tubular region, as shown


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by those rare specimens where the tubular region is preserved in the form of an internal filling of sediment (Runnegar, 1982; Jenkins, 1992; Dzik, 1997). The system of tubules appears to be much simpler in Dickinsonia than it is in the White Sea organism. Some specimens of Dickinsonia show a sediment-filled cylindrical gut beneath the segmented part of the body (Runnegar, 1982; Jenkins, 1985, 1992), which supports the idea that these were truly intestinal caeca (Figure 3A). Thus, there seems to be a common anatomical organization for these Vendian organisms, ranging from the oval Dickinsonia to the elongated Spriggina, with Yorgia representing an intermediate form in this morphocline. The basic body plan consists of dorsal, fluid-filled segments, a straight gut (not preserved in Yorgia) with metameric caeca, and an external, segmented body cover that was strengthened by transversely arranged, oblique fibres. The fleshy, cylindrical organs distributed along the septa which separate the chambers in the posterior part of the body might represent gonads.

PHYLOGENETIC POSITION WITHIN THE METAZOA The hydraulic structure of the dorsal chambers implies the presence of muscular cells in their walls and an osmoregulatory system capable of maintaining a constant fluid pressure during the life of the organism. Similar hydraulic structures occur in a wide array of living animals, most notably the coelomic chambers of chaetognaths and the rhynchocoel of nemerteans, but this does not necessarily imply any direct relationship. In nemerteans, the rhynchocoel is represented by a dorsal, muscular coelomic chamber (Turbeville and Ruppert, 1985; Turbeville, 1991). The coelomic cavities of chaetognaths develop as muscular units with their walls composed of myoepithelial cells. The basal membrane of those coelomic chambers forms a sort of exoskeleton, with a layer of collagenous fibres that are oriented obliquely in alternating directions (Kapp, 1991; Duvert, 1991; Pedersen, 1991; Shinn, 1997). In fact, the walls of the chambered units of several Ediacaran animals, especially Ernietta from Namibia, show both flexibility and elasticity that suggest they were composed of a collagenous fabric (Dzik, 1998; Dzik, in press, b), as fibrous polysaccharides (chitin, cellulose) are not so elastic. This is inconsistent with non-metazoan interpretations (Retallack, 1994; Zhuravlev, 1993) of these organisms. The presence of serial chambers in Ediacarian organisms that differ substantially from Dickinsonia in their body plans (Narbonne et al, 1997) suggests that this was a primitive (plesiomorphic) trait that they have retained in common, in their evolutionary radiation.


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Too great a time separates the Ediacaran faunas from present day metazoans to make direct comparison of details of their anatomies useful. Regrettably, there is little information on the anatomies of Early Palaeozoic unskeletonized organisms that can be used in interpreting Yorgia. Among Cambrian animals, the only one showing even remote similarities to the White Sea metazoan is Odontogriphus omalus from the Burgess Shale (Conway Morris, 1976). Its oval, elongate body was subdivided into an anterior lunate part with an oral apparatus and a trunk with numerous narrow segments. The segments extend across the mid-line of the body, unseparated by a median septum, which makes them unlike those of Yorgia, but similar to those of Dickinsonia. Along each side of the body of Odontogriphus runs a row of darker oval structures (possible gonads), which may correspond to the organs located at chamber boundaries in Yorgia. The straight, wide intestine of Odontogriphus is not unlike that of Dickinsonia. Little more can be inferred from the only known specimen of Odontogriphus. Dorsal metameric units, probably representing muscular chambers also occur in the Early Cambrian chordate Yunnanozoon (Dzik, 1995). The presence of a dorsal (presumably coelomic) space and metameric gonads are traits shared not only with enteropneusts but also with nemerteans. Thus, these are likely to have been primitive features that occurred widely among Precambrian metazoans. These similarities may support the old idea of a relationship between the Nemertini and the Chordata (Jensen, 1960, 1988; E. N. Willmer, 1974, 1975; Dzik, 1995), which share the dorsal hydraulic skeleton and an inability to secrete chitin (P. Willmer, 1990; Wagner, 1994). They also carry similar homeotic genes (Kmita-Cunisse et ah, 1998), although one must bear in mind that the presence of the same series of homeotic genes is, in itself, a plesiomorphic character, and molecular phylogenetic evidence remains controversial (Turbeville et ah, 1992; Winnepenninckx et ah, 1995). Another line of evidence which has been exploited to suggest a chordate-nemertean relationship is the similarity between the stylets of the nemertean proboscis and the oral denticles of the most primitive Cambrian conodonts (Dzik, in press, a). The only member of this group for which any anatomical information is available, Odontogriphus, shows at least superficial similarities to Dickinsonia, as noted above.

CONCLUSIONS Among unusually well preserved fossils from Zimnie Gory, the segmented organism described as Yorgia waggoneri by Ivantsov (in press) appears to be anatomically transitional between Dickinsonia and Spriggina. These organisms

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seem to represent a monophyletic group, for which the name Dipleurozoa (Harrington and Moore, 1955) is available. This taxon is characterized by the unique anatomy of its members, including: muscular, dorsally located metameric body chambers which are probably coelomic in character, functioning as a hydraulic skeleton; dense, serially arranged organs that probably represent gonads; and metameric intestinal caeca. It has already been proposed that the presence of a dorsal coelomic cavity is a plesiomorphic character within the clade of metazoans which lack the ability to secrete cuticle, and that the rhynchocoel, notochord, and myomeres could have been derived from it. Acknowledgements We are grateful to Piotr Roniewicz (Warsaw University) for comments on sedimentological aspects of our taphonomic interpretations and to Tomasz Baumiller (University of Michigan) for reading the manuscript and improving its style. The review by James Valentine is also gratefully acknowledged. The study was made possible by grant number 6 PO4D 010 13 from the Polish Committee on Scientific research. References Cameron, B., Cameron, D. and Jones, J. R. (1985) Modern algal mats in intertidal and supratidal quartz sands, northwestern Massachusetts, U.S.A. In Biogenic Structures: Their Use in Interpreting Depositional Environments, edited by H. A. Curran, pp. 211-223. Tulsa, Oklahoma: Society of Economic Paleontologists and Mineralogists, Special Publication 35. Conway Morris, S. (1976) A new Cambrian lophophorate from the Burgess Shale of British Columbia. Palaeontology, 19, 199-222. Duvert, M. (1991) A very singular muscle: the secondary muscle of chaetognaths. Philosophical Transactions of the Royal Society of London, B 332, 245-260. Dzik, J. (1995) Yunnanozoon and the ancestry of chordates. Acta Palaeontologica Polonica, 40, 341— 360. Dzik, J. (1997) Wczesna filogeneza zwierzat tkankowych w zapisie kopalnym. Kosmos, Warszawa, 45, 657-686. Dzik, J. (1998) Organic membranous skeleton in the Vendian Petalonamae from Namibia. Geological Society of America, North-Central Section, 32nd Annual Meeting, Abstracts with Programs, 30 (2): 15. Dzik, J. (in press, a) The origin of the mineral skeleton in chordates. Evolutionary Biology, 31. Dzik, J. (in press, b) Organic membranous skeleton of the Precambrian metazoans from Namibia. Geology, 27. Fedonkin, M. A. and Waggoner, B. M. (1997) The Late Precambrian fossil Kimberella is a mollusc-like bilaterian organism. Nature, 388, 868-871. Fedonkin, M. A. (1987) Non-skeletal fauna of the Vendian and its place in the evolution of Metazoa. Trudy Paleontologiceskovo Instituta, AN SSSR, 226, 1-178. [In Russian] Gehling, J. G. (1991) The case for Ediacaran fossil roots to the metazoan tree. Memoirs of the Geological Society of India, 20, 181-124. Glaessner, M. F. (1976) A new genus of Late Precambrian polychaete worms from South Australia. Transactions of the Royal Society of South Australia, 100, 169-170. Glaessner, M. F. (1984) The Dawn of Animal Life. A Biohistorical Study. Cambridge: Cambridge University Press. Glaessner, M. F. and Wade, M. (1966) The late Precambrian fossils from Ediacara, South Australia. Palaeontology, 9, 599-628. Glaessner, M. F. and Wade, M. (1971) Praecambridium - a primitive arthropod. Lethaia, 4, 71-77.



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