Early Phanerozoic Annelid Worms and Their Geological and Biological Significance

Home , Anabarites, Hyolithellus, Salterella

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' appearance 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.

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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 calcareous (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 admixtures 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

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generally much larger in size than VolbortkeUa and have a wide central cavity compared 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 invaginated cones, pointing downward." They consist of a mixture of sand and clay in proportions "markedly different from that of the surrounding sediment." There

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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 ).

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is a "tendency for the long axes of the grains to lie parallel to the layers. The external 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 downward 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. Tubes built from sloping layers of agglutinated mineral grains are a distinctive feature of many SabeUariidae. Lipps & Sylvester (I 968) noted that Phragmatopoma Mtrch, I863, a colonial form, "selects tabular sand grains or shell fragments and orients them in conical layers that slope towards the closed end, such as the grains of Volborthella are oriented" (Fig. if). Kirtley & Tanner (x968) figured a section of two tubes of the closely related Phragmatopoma lapidosa Kinberg (Fig. I e). Scholl (I958) and Multer & Milliman (I967) observed the pronounced sorting of grains used by the worms in tube building according to composition, size and shape so that in these characters the tube material differs significantly from the surrounding beach satid. Many but not all living Sabellariidae are colonial. They cement tubes more or less tightly together with proteinaceous cement, which can become dia- genetically calcified. All sabellariids close their tubes with elaborate, tight-fitting opercula but there is no evidence of this adaptation in VolbortheUa Or in the Permian fossil described by Pickett. One difficulty in comparing VolbortheUa with such sabeliariid worms as Phala- crostemma and Phragmatopoma is the ratio of the width of the inner tube to the opening, which makes it impossible to fit bodies like those of the living forms into the fossil structures. This was noted by Lipps & Sylvester (x 969, p. 333). It is, however, significant that the Sabellariidae have a thin 'tail' without chaetae or parapodia, which is cylindrical and without external segmentation. In a specimen of Idanthyr- suspennatus Peters from near Adelaide, South Australia, I measured its diameter as -~ of that of the body which fits tightly into its tube. In Sabellaria spinulosa Leuckart the 'tail' is about ~ of the diameter of the body. Wilson (i929) observed that this 'caudal appendage' consists of 5o--6o internally separated segments and contains the gut. It is therefore of the same nature as the abdomen. According to Schinde-

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wolf (I934, p. 182) the central tube of Volborthella occupies { to ~- of the diameter of the fossil. This means that the posterior part of a sabellariid body would fit the central tube of Volborthella if it was as thin as the 'tail' of the living Sabellariidae, and extended backward instead of bending forward and lying alongside the abdomen in the tube. According to Dales (1952 , pp. 443-5) the larva of Phragmato- poma californica (Fig. IC) settles on the sea floor after two months or more of plank- tonic life and the tail grows out of its posterior portion. It is "provided with a pair of dark patches of pigment, probably acting as eyespots, the young worm being able to walk backwards with the aid of the mobile legs representing the parapodia of the first three abdominal segments, moving about quite actively for a short time after settling. After a brief phase of such activity, tube building commences and the caudal tail becomes normally reflexed" (Fig. id). According to Vovelle (1958) an inner organic (mucous) tube is formed by the thoracic glands. Sand grains are collected together with food particles from the turbid water by filiform tentacles of the two opercular peduncles, deposited on the edge of the tube by the ventral lips, and cemented by secreted proteinaceous granules. In Sabellaria the wall consists of a layer of single grains, as in the Pectinariidae which are known for the perfection of their tubes but in Phragmatopoma the wall of each tube in the colony is built from inward-sloping layers of grains and the tubes are cemented together. Phragmatopoma also builds an outer funnel around and beyond its tube aperture (Figs. I e, f). Kirtley (1968) considers this cup- or hood-like aperture as defining its feeding area, i.e. the area in which its tentacles are expanded and from which those of its neighbours in the colony are excluded, hence its absence in a non-colonial organism is not surprising. The abdomen of Volborthella and related forms was as slender as the 'tail' of the SabeUariidae but its thorax must have been as short as it is in some Serpulidae where the minimum number of somites is 3. These assumptions lead to a 'working model' of Volborthella as a functioning, tube-building, suspension-feeding polychaete worm (Fig. I a). Although morphologically distinct from the living representatives of the Sabellariidae, the Volborthellidae were probably closely related to this living family. The functional explanation of the structure of Vol- borthella which built a heavy tube around a thin 'tail' is that it provided an anchoring device in shifting silts and sands where no shells or larger particles for direct attachment of tubes existed. The suspension-catching and filtering tentacles must have projected freely from the aperture, which apparently had no tight fitting operculum. This could point to the absence or rarity of animals 'grazing' on extended worm tentacles where Volborthella lived. The layering of the grains probably reflects periodic feeding and (or) periodic growth. It may be significant from an evolutionary viewpoint that attached worm tubes are unknown from the early Cambrian fauna. The accumulation of tubes washed together on bedding planes like those figured by Karpinsky (19o3) shows that the anchoring device failed in circumstances such as storms during the life or after the death of the animals. The fossil Campitius titanius Firby & Durham (i974) appears to consist of Vol- borthella specimens which have been washed together in bands or patches or scattered through the sediment, similar to the occurrence of Volborthella tenuis in the Baltic Blue Clay.

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This reconstruction of Volborthella is complemented by a consideration of the similar tubes of Salterella (Yochelson et al. 197 % Yochelson 197o ). Salterella sp. from Nevada has a narrowly conical calcitic outer shell wall, a body cavity (named by Yochelson et al. 'apertural cavity'), a central tube and a series of alternating layers of 'dark material and mineral grains.' In Salterella conulata Clark the 'inner deposits' between the tube and outer wall are calcareous. "Small shells contain vir- tually no inner deposits, but in large specimens as much as half the length of the shell is filled. This inner deposit is clearly secondary in the sense that it was deposited during life but after the outer shell had grown to some extent.., the inner deposit is laid down layer upon layer." (Yochelson I97 o, p. B4). The thin rim around the aperture (Yochelson 197o pl. 2 fig. 2, pl. 4 figs. 6, 7, pl. 5 fig. 2, pl. 6 figs. 3, 5, 7) is often directed slightly outward and surrounds a truncated, platform-like distal surface of the shell which would make a perfect seat for an operculum (not necessarily a calcified one). The other significant character is the "dark lines subparaUel to the shell but inclined apically and outwardly. Probably these are tension joints..." (Yochelson i97o , p. B4, pl. 2 fig. I, pl. 3 fig. 6, pl. 4 fig. 3, pl. 5 fig- 4, pl. 6 fig. 5.) While tension joints and cleavage traces are present, some dark lines are also clearly parabolic, with their convex curvature directed towards the aperture (Fig. 2e). They are comparable with the parabolic growth lameUae (Fig. 2a-d) seen in sections of serpulid worm tubes (G6tz 193I , Avnimelech 194I , Wrigley 195o , Schmidt 1951 , 1955). Hedley (i956a , b, 1958 ) described the tube formation of the serpulid worm Pomatoceros triqueter as follows: "During deposition the worm's branchial crown and operculum project out of the anterior tube opening, and the ventral and lateral collars are folded back over the surface of the tube. In the fold of the collar two glands, one on each side of the peristomium, produce calcium carbonate granules and these together with the secretion of mucopolysaccharide from the unicellular glands in the ventrolateral epithelium constitute the material of which the tube is constructed." GOtz (I93 I) makes the origin of the parabolic growth fines clear by referring to the deposition of each growth lamella under the reflexed collar so that its outer surface becomes a cast of the collar (Fig. 2d). In section the apex is formed where the inner surface of the collar is folded over and the outer limit of the parabola marks the outer limit of the folded part. The growth layers can be described as inverted hollow cones but their bases do not necessarily extend equally far down around the outer surface of the tube. The varying lengths of the inner and outer limbs of the parabolic fines in sections of various worm tubes, commented on by many authors, depend on the shape and position of the soft collar in various taxa and individuals. No great regularity of these growth lines either in section or on the outside of the tube can be expected when their somewhat haphazard construction mechanism is considered. The Serpulidae deposit also an inner calcareous layer parallel to the tube axis. This layer probably results from biogenic calcification of an equivalent of the simple mucous tube of many polychaetes. Salterella has an outer tube comparable with that of the serpulids, together with inner deposits comparable with the agglutinated tubes of Volborthella. In some species of Salterella they consist of quartz and heavy mineral grains in specimens from sandstones, and of carbonate grains in specimens found in limestones. The two genera appear to be

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taxonomically distinct as there is no evidence of a biogenicaUy calcified outer shell in Volborthella. The structures of both kinds of tubes are compatible with known tube-forming processes in riving polychaete worms though outer calcareous wails are no longer combined with inner agglutinated deposits. In riving forms either the production of non-calcified organic tubes is combined with agglutination processes, or organic matter is deposited together with calcium carbonate. The heavy deposits in the aboral part of the tube had probably the same anchoring function in Salterella as in Volborthella. This function seems to have disappeared with the perfection of cementing mechanisms for various kinds of tubes, including those of the reef-building Sabellariidae and those of the worms which attached themselves to the newly evolved brachiopods and mollusks.

Late Precambrian and early Cambrian calcareous worm tubes: Order Cribricyathida The distinctive tube formation and layering in the Serpulidae suggests the assignment to the polychaetes of a formerly problematic group of tubular fossils, the Cribricyathida. Describing the new genus Cloudina from the Nama Group of southwest Africa, Germs (x972 , p. 753) noted its resemblance to SaltereUa and placed "Gloudina in the Class Cribricyathea, with a possible relation to the serpulids."

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i " I I ii i E L 1 mm, C O FiG. 2. Tube structure in serpulid worms and SaltereUa conulata. A. Hamulus, Cretaceous (after Wrigley, x95o ). B. Ditrupa plana, Eocene (after Wrigley, I95o). C. Hamulus octocostatus, Cretaceous (after Avnimelech, I94I ) D. Serpula convoluta, Jurassic (after G6tz, x93x). E. Salterella conulata, lower Cambrian (diagrammatic reconstruction of section from illustrations in Yochelson I97o ). In Figs. 2A-D the arrow is in the tube lumen, pointing towards the aperture. The direction of growth lamellae is amphidine in A, exodine in B, D, dominantly endodine in C, E. o----outer layer, i--inner layer of tube.

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Dimensions of cribcricyathids are distinctive but compatible with this assignment: length often 5-xo mm, rarely as much as 20-25 mm, width o.3-I.o mm, rarely 1.5-3.o mm (Yankauskas I969, I972 ). Cloudina hartmanae Germs (nom. corr. herein) is 2.5-6.5 mm wide and 8-I5o mm long, thus overlapping with but greatly exceeding the size range observed by Yankauskas. C. rierakeae Germs is o.3-I.3 mm wide and 1.5-I2 mm long. Germs described his fossils under "Phylum: Archaeocyatha (?)" but at the same rime Hill (r972) placed the class Cribri- cyathea under "Phylum uncertain (probably not Archaeocyatha)" and "prob- lematical microfossils" while Yankauskas (I972) proposed a new Phylum Cri- bricyathi. The distinction between biogenically deposited calcite, calcified organic matter, and infillings of original voids is difficult and complicates reconstruction of their basic moi'phology. Some parts of the walls were dolomitized during fossilization and appear as raised areas in etched or weathered specimens (Germs i972 , Fig. 3dfl, P1. I Figs. 8-i I). The tube walls seem to have contained organic material. Their distortions suggest some flexibility and do not appear to be entirely due to irregular growth. The tubes are often incompletely preserved in a manner which suggests decay or incomplete fossilization rather than fracturing, reminiscent of some agglutinating foraminifera with walls of quartz or carbonate grains and much organic cement which are often irregularly deformed by compaction of the enclosing sediment. As in the arenaceous foraminifera, the distortions of the shells of cribricyathids together with misinterpretation of sections and the states of pres- ervation have led to excessive taxonomic splitting. This was corrected partly by Hill (I972) and partly by Yankauskas (I972). Hill also reduced the excessive morphological terminology applied by Yankauskas. These revisions and my studies of limited material from southwest Africa make the link between cribricyathids and polychaete worms tubes more plausible than it appeared to Germs when he first suggested it. The "peripterata" (Vologdin, Yankauskas) are dark, probably organic layers in the wall which are often oblique and slope inward and toward the apex of the tube though they may be irregularly transverse (P1. ~, Fig. i, 5, Germs i972 fig. 3). The oblique layers resemble the layers in the outer wall of Salterella, and their boundaries may be parabolas with the apex directed towards the aperture. They may be incomplete so that internally the wall layers merge

PLATE I. ~facingt~age) FIo. i. Volborth~lla tenuis Schrnidt. Scanning electron microscope picture of broken upper surface of tube showing size and shape of grains and roughly layered arrangement (upper left to lower right). The specimen was sectioned longitudinally and the left-hand side of the broken upper surface is seen sloping away slightly from the source (upward in the picture). ETEC Autoscan picture by Dr K. Bartusek. Scale bar Io pm ( × I6OO). FIG. ~. Cloudina hartmanae Germs. Late Precambrian, Schwarzkalk Limestone Member, Kuibis Formation, Nama Group; Q uaggaspoort Farm, District Bethanie, southwest Africa. Weathered surface of rock specimen showing irregularly shaped and layered longitudinal and transverse sections of tubes. × 3.3, scale in mm.

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PLATE 1 266

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PLATE 2

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with the carbonate matrix filling the tubes. Such walls have been described as consisting of 'chambers,' but in fact they correspond to walls consisting of completely outlined calcareous rings (Germs 1972, Fig. 3), except that the organic deposits lining the wall layers may be incompletely calcified. Additional irregular hollows in or on the outer walls of the tubes (bullate texture, Yankauskas 1972 , p. I64) are often seen in serpulid tubes. In transverse sections of Cloudina the rings do not completely encircle the tubes (P1. 1 Fig. 2), and may not be visible everywhere under the dark, peripheral layer (P1.2 Fig. I, 2). The 'inner wall' of some Pterocyathidae and Cribri- cyathidae is probably a partly calcified inner layer. This layer may have lined an organic inner tube which is not preserved or a calcareous wall which through diagenesis has become indistinguishable from the matrix. Cribri- cyathean tubes were probably laid down as carbonate a \ /~ a I granules with much organic cement. The layering of the outer wall which takes the form of inverted, invaginated, \ // incomplete cones (Fig. 3a) indicates periodic growth. Be- tween the conspicuous growth lines the carbonate substance d of the wall now is recrystallized. The fine transverse / / FIO. 3 Cloudina hartmanae. Idealized model of tube structure. A--Longitudinal section shows transversely ridged outer layer and sloping half- a~ at rings, with diagenetic (?) internal structure indicated in upper part. B--transverse section I n'lfft below a-a' in Fig. 3A. t---t

PLATE 2. (fa~ page) Flos. z--6. Cloudina hartmanae Germs. Late Precambrian, Schwarzkalk Limestone Member, Kuibis Formation, Nama Group. Figs. I-2 from Driedoornvlakte near Schlip, southwest Africa. Photographs presented by Dr G. J. B. Germs. Figs. 3-6 from same rock specimen as PI. z fig. 2. FIG.z--oblique lontigudinal section and transverse sections. × 3"3- F1o. 2--random sections on polished surface. × i. F xo. 3qweathered, transversely ribbed, granular surface of tube. × 5, scale in mm. Fro. 4---same specimen as P1.2 fig. x on upper left, largerandsmaller specimens in longitudinal and oblique sections below. × 2. Fxo. 5--two portions of longitudinal sections of tubes showing spindle-shaped sections of calcareous half-rings in left wall fragment only (cf. Fig. z). x 4"5- F1o. 6---weathered specimen showing cone-in-cone arrangement of tube wall layers (cf. Fig. z). × 5, scale in ram.

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layering seen locallyin some thin sections (Germs 1972, P1. I Fig. I) may be the result ofrecrystallization. It is unrelated to the irregular but conspicuous external transverse ribs on the outer surface of the tubes ofCloudina hartmanae ('annular ridges and depressions', Germs I972 , p. 753), some of which appear to consist of agglutinated mineral grains. On weathered and on polished sections they are dark (P1.2, Figs. I-4) ; the outersurface of the tubes as well as the boundaries of the main growth layers seem to be rich in organic matter. The cribricyathids differ from Salterella in the absence of a narrow inner tube and of inner agglutinated cones; serpulid tubes are mostly rigid and attached. IfYankauskas's statement (I972 p. I66) that cribricyathid tubes are strictly bilaterally symmetrical is correct, it reinforces the view that they were inhabited by a bilaterally symmetrical animal such as an annelid worm. Present knowledge therefore supports Germs's view that the Cribricyathida are best considered as an extinct Order of the Class Polychaeta. Yochelson & Herrera (1974) have reached similar conclusions concerning the morphology and affinities of Cloudina. They described the probably related C. ? borelloi from the lower Cambrian of Argentina. The differences in wall construction and the consistently curved and infilled lower end of the tube indicate that this fossil is generically distinct from Cloudina. The Order Angustiochreida Under the heading "Fossils with calcitic tubular shells (Order Polychaeta ?)," Missarzhevsky (in Rozanov et al. I969, p. I52, x55 ft.) described from the Tommo- tian Stage of the lower Cambrian of Siberia species of Coleoloides Walcott, Coleolus Hall and Coleolella Missarzhevsky (Family Coleolidae Fisher, i962 ) and the new genus Anabarites ("Family incertae," type species A. trisulcatus Missarzhevsky). Val'kov & Syssoev (i97o) assert that the new Order Angustiochreida in which they place Anabarites and related forms cannot be assigned to any existing Phylum or Class. According to Matthews & Missarzhevsky (i 975) their zoological affinities cannot be clearly indicated, but the available morphological data seem to support Missarzhevsky's earlier hesitant assignment of these tubular fossils to the Annelida Polychaeta. The living Serpulidae differ from Anabarites and related genera in having tubes that are generally internally smooth, with circular cross section. Their remaining characters are found also in serpulid tubes: the longitudinal twisting and curvature, the rows of pinhole depressions in grooves, and even the unusual surface sculptures. A vaguely spiral arrangement of shell elements is in agreement with the constructional principle of the serpulid tube. It is deposited under the collar of the bilaterally symmetrical animal which has to twist in its tube to add material to all parts of its margin.

Form and function of polychaete worm tubes Worm tubes serve as supporting structures for suspension feeding and detritus feeding by means of tentacles; as protective organs, particularly for the tentacles when re- tracted and provided with an operculum; and as water supply tunneb for sediment dwellers. The geologically early worm tubes discussed here were probably sup-

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porting structures for suspension feeders. The formation of tubes by annelid worms differs from that in molluscs or brachiopods in that particulate material may be actively transported to the building site. Agglutination of detrital grains by cemen- tation with secreted mucoproteins or mucopolysaccharides occurs in many in- stances; this occurs also in foraminifera where the entire operation is conducted by a single cell. In annelids agglutinated material consists mainly of sand grains collected together with food particles either from the sediment or from the sediment- laden water. The grains were observed in Sabdlaria (VoveUe 1958) to be actively transported and placed in position by lip-like organs of the head. The calcareous tubes of the Serpulidae are not secreted by epithelial cells but constructed by the collar, from calcium carbonate granules secreted by two glands and transported to the building site together with mucopolysaccharides secreted by epithelial cells (Hedley I956a, b, 1958 ). The construction of inner and outer parchment-like tubes "between which fine sand and mud particles are placed" (Hartman i944, P. 43) has been observed in Diopatra tridentata. Mucous, agglutinated and calcareous tubes in annelid worms seem to be the results of closely related and co- ordinated physiological processes. The morphological differences between the collars of the worms and the mantles of brachiopods and mollusks are reflected in differences of the layered structures of calcareous shells resulting from their activi- ties. In cribricyathid tubes the layers are added mainly on the inside, like stacked, inverted, open cones, often with flaring outer edges (peripterata). They are seen in transverse sections to be incomplete, not reaching around the periphery of the tube. In modern serpulids the tube building process has been elaborated, resulting in stacked, open cones arranged so that the layers slope either inward (endocrine) as in Vermiliopsis and Mercierella or outward (exocline) as in Ditrupa, or else the layers are regularly parabolical (amphicline) as in some Serpula and Hamulus. It is likely that the distinctive processes of tube production in annelid worms will be found reflected in their fine structure when tubes are examined by scanning electron microscopy. The formation of organic-walled tubes seems to be ubiquitous and largely independent of the environment and of feeding mechanisms in polychaete worms (excluding active predators). The agglutination of sand grains on or in the organic tube walls seems to occur in turbid near-shore environments. The selective advantage arising from the utilization of particles rejected from the food stream seems obvious. The production of calcareous tubes, whether formed from agglutinated or secreted particles, is linked with conditions leading to carbonate sedimentation. The available palaeontological data are at present insufficient for an elucidation of the shifting ofannelids into new adaptive zones which must have been the under- lying reason for their rapid and early diversification. The absence or rarity of competitors which had not evolved or gready diversified at that early stage, such as coelenterate reef builders, brachiopods, molluscs and arthropods meant that numerous ecological niches in the neritic and littoral environment were open when the coelomate grade was reached. The recognition of abundant annelid fossil remains with distinctive characters, at least partially documenting their early differentiation, opens a significant field for future research.

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Stratigraphic distribution Platysolenites, long considered as a worm tube but now assigned to foraminifera, characterizes the lower part of the lower Cambrian (correlated with the Tommo- tian Stage of Siberia by Rozanov x973) from Poland to Leningrad, the Moscow syneclise and Scandinavia. Its range overlaps in places that of Volborthella which occurs in the middle and upper lower Cambrian on the Russian Platform, in the Balticarea and in Scandinavia. These total ranges are at least of Stage magnitude and thereforenot suitable for regional finezoning. The youngest reported occurrence of VolbortheUa is in the middle Cambrian of Czechoslovakia (Prantl i948 ). "V. cortcavi" Walthier, I949 from the Kippens Formation of the lower Cambrian of Newfoundland is probably SaltereUa conulata Clark. The distribution of this species in the lower Cambrian of Eastern North America was discussed by Yochelson (i97o). The lower Cambrian rocks with Salterella from central Nevada described by Yochelson et al. (197o) "may be correlative with part of the Harkless Formation in southwestern Nevada and adjacent California," from the base of which Vol- bortheUa tenuis was described by Lipps & Sylvester (I968). If Campitius is a Vol- bortheUa, its range in California extends to the lower part of the Campito Formation (base of Fallotaspis range zone). The Cribricyathida were found in Siberia and considered by Radugin (I966) as late Precambrian and by Vologdin (I966) as lower and middle Cambrian. Stratigraphic data now available (Hill I972 p. EI34, Yankauskas I972 ) place them definitely in the lower Cambrian. Semikhatov (I974, p. 6I) concluded that the "presence" in the Nama Group of southwest Africa "of cribricyathids which were previously unknown from the Precambrian contradicts" its Precambrian age. This does not follow. The two Siberian cribricyathids with which Cloudina was compared by Germs and which they closely resemble, belong to the Family Vologdinophyllidae Radugin which is almost entirely confined to the lower Cambrian Ust-Kundat 'Horizon,' correlated with the Tommotian Stage (Yank- auskas I972 , Fig. I). The differences between Cloudina and the lower Cambrian cribricyathids are of at least generic and probably greater taxonomic significance. The fallacy of the argument that all cribricyathids must be of lower Cambrian age because those found in Siberia are lower Cambrian becomes obvious when they are to be considered as related to polychaete annelids rather than to Archaeo- cyatha. The latter are world-wide in their distribution and unknown anywhere below the Zone of Aldanocyathus sunnaginieus and its equivalents. A tubular fossil which is probably also a polychaete, Anabarites trisulcatus MJs- sarzhevsky, occurs in possible equivalents of the late Precambrian Yudoma Complex in northern and northwestern Siberia (Missarzhevsky in Keller et al. i974). It was found also in the succeeding Kessyussa Formation on the flanks of the Anabar Massif, with the gastropod Anabarella plana Vostokova and in the Tommotian Stage in the eastern part of the Siberian Platform (Matthews & Missarzhevsky 1975). It appears that tube-building worms occur below the base of the Cambrian, wherever that may be drawn by eventual consensus (Cowie & Rozanov x974, Cowie & Glaessner I975). The proposition that the first appearance of shelly fossils

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should be taken to mark the base of the Cambrian is therefore unattractive. The expressions 'shelly' or 'skeletal' fossils are ambiguous; Rozanov (1973, p. 149 ) refers to Anabarites as "strictly not a skeletal fossil," and little precision is gained by using 'mineralized tissues.' It is scarcely possible to distinguish in a mm-sized, primarily mucous tube, whether mineralization was biogenic or diagenetic. Both processes are known to occur in polychaete tubes (Hedley 1958 , Kirtley & Tanner i968 ) and there are transitions from agglutinated to secreted 'mineralization' in annelid tube building. These complexities make it unlikely that the appearance of mineralized skeletons, not only in annelids but also in brachiopods, molluscs, archaeocyathids, etc., could have had a common cause and could have occurred at a sufficiently precise moment in time to satisfy the requirements of modem stratigraphic boundary correlation.

Outline of early annelid evolution In his review of the annelids, Dales (I963 p. 64) stated: "In considering the evolution of the Polychaeta we are faced with two difficulties. First, the existing families are very distinct, an aspect perhaps of their antiquity, and secondly, there are no fossils worth mentioning" (my italics). The antiquity of the annelids and their early differentiation are no longer in doubt. The late Precambrian Ediacaran fauna contains two, at present monotypic, families of polychaetes, the Dickinsoni- idae Harrington & Moore and Sprigginidae Glaessner, in addition to several trace fossils probably produced by detritus-feeding or grazing polychaetes. The Dickinsoniidae are considered to be ancestors of the living Spintheridae which Clark (1969) placed in the Order Amphinomorpha. The Sprigginidae differ in morphology and adaptation and may be placed in the Phyllodocemorpha, or at least in the vicinity of that Order. The Volborthellidae are clearly related to the living SabeUariidae and thus could belong to the Order Spiomorpha. The late Precambrian to early Cambrian Cribricyathida and Angustiochreida appear to be extinct Orders. The tubes of Hyolithelminthes and Coleolidae which are common in the lowest Cambrian are often ascribed to annelids but they require further study. Hyolithellus has been placed in the Pogonophora (Poulsen 1963) and a number of organic-walled tubes (Sabellidites, Paleolina, Saarina and others) from the late Precambrian and early Cambrian of the U.S.S.R. have been placed in this Phylum by Sokolov (x967). In this context it is interesting that the Pogonophora may be Protostomia, related to annelids or at least an independent Phylum not as closely related to the Deuterostomia as had been thought previously (Norrevang I97O , Barrington 1974). The early differentiation of the annelid worms may be related to several factors. One may well be the adaptation of some polychaetes to exploitation of the supply of detritus not only after its incorporation in the sediment but while it is still in suspension. This capability, which involved various kinds of tube construction for support, must have given them a considerable selective advantage over their presumed ancestors, the Platyhelminthes. It also must have attracted them to turbulent waters where there was more organic matter in suspension, and in consequence produced further adaptations to this particular environment in tube

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construction and eventually colony formation. Elsewhere, the well known physiological adaptation of many annelids to low-oxygen environments may have been a pre-adaptation dating back to a time of generally lower oxygen levels (Glaessner I972 ). There are clear relations between burrowing efficiency, the formation of the coelom, and annelid segmentation (Clark x964). Further progress of annelid evolution during early Cambrian time can be considered here only briefly as work on key faunas is now in progress. The uppermost lower Cambrian of Kangaroo Island, South Australia (Emu Bay Shale, Daily 1956, Pocock 1964, 197o) contains a rich fauna of trilobites, phyllocarid Crustacea (Isoxys and Tuzoia), and a large number of annelid worms including a new species of Palaeoscolex, a genus also known from the lower middle Cambrian of Utah and the Tremadocian of England, and new genera and species apparently related to the living Scoldepis (--Nerine), Family Spionidae, Order Spiomorpha) and the Opheliidae (Order Drilomorpha). Of the first five Orders ofpolychaete annehds listed by Clark (i 969), four appear to have been present in late Precambtian to early Cambrian times. The Eunice- morpha with their characteristic dental apparatus appear only in the Ordovician. The observation that most of the less advanced polychaete Orders are represented as fossils in early Phanerozoic sediments, albeit not necessarily by their most primitive genera and families, supports the view that annelid systematics as re vised by Dales (I962, I963) and Clark rests now on a firmer basis than earlier classifications. Clark has tightly said (p. 46) that of all annelids polychaetes present the most intractable problem of phylogeny. Regrettably, the Errantia-Sedentaria dichotomy which he proposed to discard as purely ethological, still appears as the major taxonomic subdivision of the polychaetes in modern textbooks. As in many other groups of Metazoa, the evolution and systematics of the Anne- lida will be understood only as a result of closer integration of modern zoological and palaeontological data. This in turn will remove sources of confusion from the biostratigraphy of the late Precambrian and early Cambrian. Polychaetes and possibly other coelomate worms are represented in the fossil record of that period of time, not only by numerous trace fossils which give few indications of the morphology and systematic position of the animals, but also by preserved 'soft' bodies and by some of the first 'shelly' fossils. There is indeed fossil evidence of "consolida- tion and possible radiation of coelomate worms" (Valentine I973, pp. 445-7) in the "fourth phase" of metazoan diversification and of "the fifth phase" which "includes the development of mineralized skeletons by a variety of lineages, mostly epifaunal, creating the Cambrian fossil record." Like other boundaries between phases in earth history, including the history of life, when studied in detail these two boundaries appear as times of more or less rapid transitions. They are not ready made time markers for Ediacaran and Cambrian sediments, suitable for the operational requirements of the stratigrapher.

ACKNOWLEDGEMENTS. I thank the following for specimens and information: Dr S.J. Edmonds, lately at the Zoology Department, University of Adelaide; Dr G. J. B. Germs, Geological Survey, Southwest Africa; Professor R. F. Hecker, PalaeontologicalInstitute, Moscow; Dr D. W. Kirtley, Harbor Branch Foundation Inc., Florida; Dr H. J. Oertli, Soci~td Nationale des P~troles d'Aqui-

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mine, Pau, France; Dr A. Yu. Rozanov, Geological Institute, Moscow; Dr E. L. Yochelson, U.S. Geological Survey. Uloudina obtained through the courtesy of the Anglo-American Company and Aquitaine South-West Africa.

References

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Received 29 April x975; revised typescript received 19 August I975.

MARTIN F. GLA~SS~R, Centre for Precambrian Research, Department of Geology and Mineralogy, University of Adelaide, Adelaide, South Australia 5000.

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