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Early Phanerozoic Annelid Worms and Their Geological and Biological Significance

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Early Phanerozoic Annelid Worms and Their Geological and Biological Significance Early Phanerozoic annelid worms and their geological and biological significance M. F. GLAESSNER SUMMARY The earliest 'shdly' fossils deserve more de- those of serpulids. The problematic Angustio- tailed studies than they have received. Examin- chreida (Anabarites etc.), of similar age, also ation of the Cambrian genus Volborthella shows show such resemblances. The first appearance that it resembles in significant characters tubes in the geological record of mineralized skeletons of sabellariid worms, and on this basis the ('shelly fossils'), built according to various inter- morphology and ecology of the animal can be related modes in annelids and in a different reconstructed. The Cribricyathida, which in- manner in other Metazoa, is not a suitable clude Cloudina and range from late Precam- stratigraphic marker. The early differentiation brian to lower Cambrian, are not related to of annelid worms can now be documented Archaeoeyatha but are polychaete worm tubes palaeontologically. with some structural characters which resemble THE OPINION is still widely held that the first appearance of shelly fossils is the most important event marking the base of the Cambrian in stratigraphic sequences spanning the transition from Precambrian to younger sediments. In the classical view of stratigraphy which was dominant some twenty years ago, the oldest fossili- ferous Cambrian sediment was the famous Blue Clay which outcrops on the Baltic coast from Estonia to Leningrad. The first two biostrafigraphic zones in it were characterized by Platysolenites and Volborthella (Opik 1956). The systematic position of both these early "index fossils" is still controversial, as is that of most taxa from the earliest Cambrian. During the last decades the intensive study of early Cam- brian stratigraphy and fossils, particularly in Siberia but also elsewhere, has shown the occurrence of rich and varied faunas in strata which precede those with the oldest trilobites. Many authors recognize at the base of the Cambrian a Tom- motian Stage (Rozanov et al. i969, Rozanov i973, Cowie & Rozanov x974). The shelly fossils from these and slightly earlier strata were discussed recently by Matthews & Missarzhevsky (i 975). A careful study of the fine and ultra-structure of all these problematic early 'shelly' fossils, with due consideration of their mineralogy and diagenesis, may assist in solving the problem of the 'sudden' ap- pearance of shells in the fossil record. The morphology and systematic position of two of the early Cambrian fossils, Platysolenites and VolbortheUa, can now be clarified. The former genus represents not worm tubes, as had been generally believed, but foraminifera close to Bathy- siphon (Glaessner 1963 and in press). Volborthella is here assigned to the polychaete annelids and so are the enigmatic, tubular, calcareous Cribricyatha and Angustio- chreida. The first cribricyathids occur in southwest Africa in the Nama Group. The first Anabarites occur together with the 'conodontomorph' Protohertzina and the sabellidifid Paleolina in strata which are apparently older than the stratotypical Tommotian of the Aldan-Lena Region of the Siberian Platform and possibly (but Jl geol. Soc. Lond. vol. x3~, t976, pp. 259-275, 3 figs., 2 plates. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/3/259/4896980/gsjgs.132.3.0259.pdf by guest on 25 September 2021 26o M. F. Glaessner not necessarily, see Sokolov 1974) older than the basal beds of the Baltic Stage with the first Platysolenites. The study of these early shelly fossils throws some light on the relations between the formation of organic (mucous), agglutinated and cal- careous (secreted) shell material in at least one phylum of Metazoa, the earliest one in which the evolutionary step to shell formation occurred. Volborthella Schmidt, 1888 Lipps & Sylvester (I968, p. 334) concluded "that Volborthella was most likely a worm-like animal, possibly a small polychaete .... " It can be shown that there is much evidence in favour of including Volborthella, and the extinct family Volbor- thellidae Kiaer, i916 , in the Polychaeta. Material from Tallinn in Estonia which I have examined shows clearly the absence of septa or of any space that they might have occupied. It supports the views of many authors that the supposed septa are merely dark mineral grains in layers which are not always equally spaced or continuous. They are part of a tubular structure built of parallel layers of mineral grains arranged with their long axes (in section) sloping towards a narrow central cavity (Fig. Ia, P1. I Fig. I). This cavity was considered as a siphuncle by those who considered Volborthella to be a cephalopod. As Flower (i954) has pointed out, "Schindewolf's restoration of Volborthella (Schindewolf I928 , p. 7 o, Fig. i), showing thin distant septa terminat- ing in septal necks supplemented by connecting rings, finds absolutely no support in any of the published material," nor is it compatible with the material which I have examined. It is not surprising that Spath (I936 , p. i59 ) referred scornfully to the view that "the supposed cephalopod Volborthella" could have a functional phragmocone with 2o septa to the ram. Schindewolf (i934) defended his view of the existence of septa against Gtirich's (i934) findings partly by referring to analogies with internal molds of "Orthoceras" from Devonian sandstones, which led him to interpret pyrite grain stringers in the dark layers as the result of organic admixtures in the original septa. It is more likely that they indicate organic ad- mixtures in more argillaceous layers; there are no calcareous layers parallel to the conical stratification of the mineral grains and hence no septa. Yochelson et al. (i97o) concluded that the absence of an outer shell in Volborthella is the result of diagenetic alteration, after comparing it with Salterella which has a laminated cone of mineral grains with a central tube and also a conical, calcitic outer shell. The Baltic specimens are certainly abraded, broken and often current-sorted. Gtirich (i934) observed calcite grains on the margins of his sectioned specimens but neither calcareous outer shells nor specimens in undisturbed life position in the sediment have been described. Karpinsky (I9o3) speculated that the shell may have consisted of conchiolin, later removed by diagenesis. In their discussion of the systematic position of Volborthella, Lipps & Sylvester (i 968) considered various agglutinated foraminifera. Apart from agglutination of sand grains there is no morphological resemblance between this group of Protozoa and Volborthella. The authors also compared this genus with worms which build tubes of sand grains. They remarked that "the tubes built of sand grains are Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/3/259/4896980/gsjgs.132.3.0259.pdf by guest on 25 September 2021 E rty eha o oi anndia worm generally much larger in size than VolbortkeUa and have a wide central cavity com- pared with the tube thickness" (p. 333). Tubes of an unnamed worm from the late Permian of New South Wales (Pickett I972 ) resemble Volbortkella and may indicate the unknown life position of its representatives. These structures (Fig. Ib) "are made up of a series of in- vaginated cones, pointing downward." They consist of a mixture of sand and clay in proportions "markedly different from that of the surrounding sediment." There E E o E Fxo. x. Comparison of Volborthella and Sabellariidae. A. Reconstruction of Vol- borthella tenuis. Tube shown in section (after Schindewolf x93x, pl. I8, fig. 2 x). Hypothetical reconstruction of body in ventral view. B. Diagram of longitudinal section of worm tube from the upper Permian of New South Wales (based on Pickett t972, pl. 2o, fig. I). C. Sabellariid larva at settling stage. D. Young sabellariid in tube. (C-D after Dales I952 , diagrammatic, greatly magnified). E. Section through two adjoining specimens of Phragmatopoma lapidosa. Diagram showing structure of the tube and extended and contracted position of worms (from Kirfley & Tanner 1968 ). F. Portion of the tube of Phragmatopoma californica, diagrammatic (SchoU, x958 ). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/3/259/4896980/gsjgs.132.3.0259.pdf by guest on 25 September 2021 262 M. F. Glaessner is a "tendency for the long axes of the grains to lie parallel to the layers. The ex- ternal diameter of the tubes is not constant." It averages on different bedding planes from I3 to 36 ram, with a maximum of 62 ram. It is not known whether these measurements are of the width of tube openings or whether they were taken on bedding planes cutting different levels of eroded, elongate, conical tubes. The complete tubes end in funnels 4 ° mm wide, 2o-3 ° mm deep, and continue down- ward as tubes 3-4 mm wide "for 2o mm or more." These worms occur in large numbers, up to 5oo per m S, but not as colonies of contiguous tubes. They are found in situ, not washed out of the sediment and lying on bedding planes. They are much larger than Volborthella but the ratio of the diameter of the central tube to that of the entire structure is similar and so is the slope of the layers (as seen in P1. 2, fig. I of Pickett x972 ). The terminal funnel seems to flare to about Io times the tube diameter. Pickett compared these Permian fossils with various ecological groups of living polychaetes including "suspension feeders, e.g. Sabella" but concluded, mainly on the basis of his observations on population density and derivation of sand grains, that they were terebeUids, detritus feeders with ex- tensile tentacles. He did not mention SabeUariidae. Another tubular fossil closely resembling VolbortheUa is the lower Cambrian Salterella (Yochelson et al. I97O, Yochelson x97o ). This fossil has in addition to the inner tube and the sloping layers of mineral grains an outer calcitic shell and for this reason it will be discussed later.
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