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Journal of Paleontology, 85(2), 2011, p. 256–265 Copyright ’ 2011, The Paleontological Society 0022-3360/11/0085-0256$03.00

PROBLEMATICA, TRACE FOSSILS, AND TUBES WITHIN THE EDIACARA MEMBER (SOUTH AUSTRALIA): REDEFINING THE EDIACARAN TRACE FOSSIL RECORD ONE TUBE AT A TIME AARON SAPPENFIELD,1 MARY L. DROSER,1

AND

JAMES G. GEHLING2

1

University of California, Riverside, Department of Earth Sciences, 900 University Ave., Riverside, 92521, USA; and 2South Australia Museum, Adelaide, South Australia, Australia 5000

ABSTRACT—Ediacaran trace fossils are becoming an increasingly less common component of the total Precambrian fossil record as structures previously interpreted as trace fossils are reinterpreted as body fossils by utilizing qualitative criteria. Two morphotypes, Form E and Form F of Glaessner (1969), interpreted as trace fossils from the Ediacara Member of the Rawnsley Quartzite in South Australia are shown here to be body fossils of a single, previously unidentified tubular constructional morphology formally described herein as Somatohelix sinuosus n. gen. n. sp. S. sinuosus is 2–7 mm wide and 3–14 cm long and is preserved as sinusoidal casts and molds on the base of beds. Well-preserved examples of this fossil preserve distinct body fossil traits such as folding, current alignment, and potential attachment to holdfasts. Nearly 200 specimens of this fossil have been documented from reconstructed bedding surfaces within the Ediacara Member. When viewed in isolated hand sample, many of these specimens resemble ichnofossils. However, the ability to view large quantities of reassembled and successive bedding surfaces within specific outcrops of the Ediacara Member provides a new perspective, revealing that isolated specimens of rectilinear grooves on bed bases are not trace fossils but are poorly preserved specimens of S. sinuosus. Variation in the quality and style of preservation of S. sinuosus on a single surface and the few distinct characteristics preserved within this relatively indistinct fossil also provides the necessary data required to define a taphonomic gradient for this fossil. Armed with this information, structures which have been problematic in the past can now be confidently identified as S. sinuosus based on morphological criteria. This suggests that the original organism that produced this fossil was a widespread and abundant component of the Ediacaran ecosystem.

INTRODUCTION

are critical to the interpretation of T Neoproterozoic life as they currently provide the oldest definitive record of bilaterians (Valentine, 1994; Budd and RACE

FOSSILS

Jensen, 2000). However, reevaluation of purported Ediacaran trace fossils suggests that this record has been substantially exaggerated and, in accordance, the body fossil record underestimated by assigning problematic structures a trace fossil origin with little consideration for functional and taphonomic constraints on the interpretation of trace fossils (Droser et al., 2005). Jensen et al. (2006) have provided a critical overview of the Ediacaran trace fossil record and suggest that only a very small portion of that record has been appropriately interpreted and that the only definitive Ediacaran traces are simple, irregular, meandering trails parallel to bedding planes. Much of the difficulty arises from the preservation of soft-bodied tubular organisms, characterized by elongate and hollow structural organizations, in a manner superficially resembling trace fossils (Gehling and Droser, 2009; Tacker et al., 2010). Complicating the issue further, tubular organisms were common elements of the Ediacara biota (Droser and Gehling, 2008; Cohen et al., 2009; Tacker et al., 2010) yet they are rarely preserved as complete specimens. Viewed in hand samples, molds and casts of tubular organisms can be difficult to interpret and thus, are commonly mistaken for trace fossils; however, recent excavations of beds of the Ediacara Member of the Rawnsley Quartzite, South Australia, allow for large quantities of successive bedding surfaces to be examined providing the unique opportunity to view the fossil assemblage in a much broader context. The juxtaposition of poorly preserved specimens and those in higher taphonomic grade allows many structures to be resolved as taphonomic variants of more definitive body

fossils of tubular organisms rather than trace fossils (Droser et al., 2005; Droser et al., 2006; Droser and Gehling, 2008). Glaessner (1969) described six trace fossil morphotypes, Forms A–F, from the Ediacara Member. Of these, only Form B, now Helminthoidichnites has been shown to be a definitive trace fossil. Of the remaining morphotypes, Form A, which was originally described as rows of fecal pellets, has been formally described as a body fossil (Funisia dorothea Droser and Gehling [2008]). Form C, which was originally interpreted as tight and regular meanders, is now regarded as the body fossil, Palaeopascichnus (Jensen, 2003), with possible affinities to Xenophyophores (Seilacher et al., 2003). Form D, originally described as a sinuous trail has been shown to be a large tubular organism commonly referred to as Aulozoon (Seilacher et al., 2003). In this paper we reinterpret specimens that would be included in Forms E and F of Glaessner (1969, p. 381–382) as body fossils of a tubular organism described here as Somatohelix sinuosus n. gen. n. sp. Observations of exceptionally preserved examples of S. sinuosus provide new insights into the taphonomy of soft-bodied Ediacaran tubes and form the basis for three taphonomic models presented herein. GEOLOGIC SETTING

The Adelaide Geosyncline, extending from the Mt. Lofty Ranges near Adelaide in the south to some 600 km north to the Flinders Ranges in South Australia, contains thick sections of Neoproterozoic to early Paleozoic strata (Sprigg, 1952; Preiss, 1990, 2000). In the Flinders Ranges, the Rawnsley Quartzite comprises the youngest Neoproterozoic formation and contains the majority of fossils of the Ediacara biota found in the Adelaide Geosyncline region with the only exceptions being a few enigmatic individual specimens of possible biologic origin contained within the underlying

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SAPPENFIELD ET AL.—TUBES AND TRACE FOSSILS WITHIN THE EDIACARA MEMBER Wonoka Formation (Gehling, 2000, 2007). The Rawnsley Quartzite consists mostly of light-colored medium-grained sandstone (Preiss, 2000) with abundant crossbedding and microbially induced sedimentary structures dispersed throughout the formation (Gehling, 1999). The Ediacara Member is defined as the heterolithic portion of the Rawnsley Quartzite consisting of siltstones, medium- to thick-bedded sandstones, and intercalated siltstone and sandstone units (Jenkins et al., 1983; Gehling, 2000). Within the Flinders Ranges, the Ediacara Member is 400–600 m below a basal Cambrian unconformity, but thins to approximately 10–30 m below this same unconformity in the westernmost portions of the outcrop area, indicating a more proximal depositional setting for these strata (Gehling, 2000; Droser et al., 2006). The thickness of the Ediacara Member follows this same trend with more distal portions of the member approximately 300 m thick within large incised valley fills and in more proximal settings reaching a maximum thickness of approximately 5–30 m (Gehling, 2000). The majority of beds within the Ediacara Member represent deposition in a shallow water setting between fair weather wavebase and storm wavebase (Fig. 1; Gehling, 2000). Beds range from storm beds up to 10 cm thick deposited within incised valleys at or near fair weather wave base to millimeter thick laminae deposited as waning storm surges (Droser et al., 2006). Fossil bearing outcrops of the Ediacara Member are distributed throughout the region of the northern and central Flinders Ranges in South Australia (Fig. 2), preserving the most diverse fossil assemblage of the Ediacara biota currently known (Gehling, 2000, 2007). Fossiliferous horizons at these outcrops show remarkable heterogeneity with immediately successive beds commonly preserving entirely different benthic communities (Droser et al., 2006). MATERIALS AND METHODS

West of the northern Flinders Ranges at Nilpena (Fig. 2), 20 fossil bearing beds have been excavated to expose successive bedding surfaces. Fossils are preserved on the base of beds and thus, beds must be inverted following excavation. Excavation of beds begins with the removal of overburden. Uncovered beds are initially traced in place onto a clear plastic film and labeled for reassembly in a nearby location. The clear plastic film is overturned and laid out in a flattened area. The bed is then removed, inverted, and reassembled on the plastic film. Excavated beds are divided into 50 cm grids and all structures preserved on that surface, of both known and unknown affinity, are then recorded as to their location on the bed, size, orientation, and any other definitive characteristics. This process has yielded approximately 200 m2 of observable bedding surface, with excavations still in progress. A total of 191 specimens of Somatohelix sinuosus n. gen. n. sp. have been recovered from three excavated surfaces as well as from hillside talus. Specimens were labeled with the prefixes ‘‘NV’’, ‘‘NPB’’ and ‘‘NSTC’’ to identify the specific bed on which the fossil was found. Additional specimens were recovered at South Ediacara (Fig. 2) and labeled ‘‘SE’’. Sedimentary facies at SE and NSTC suggest deposition near fair-weather wave base, potentially prior to beds at the NSB and NPB localities which were deposited as mass-flow sands below wave base. The letter ‘‘F’’ at the end of specimen prefixes identifies specimens recovered from hillside talus while the absence of this letter identifies a specimen located on an excavated bed. These prefixes are followed by numbers

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specific to each specimen, corresponding to the spreadsheet provided as supplementary material. Measurements were taken in the field as well as by using ‘‘ImageJ’’ image processing and analysis software (Rasband, 1997–2008) on digital photographs. While field notes and photographs are key components in data collection, latex molds are especially useful for making exact replicas of samples left in the field to maintain the integrity of this fossil Lagersta¨tte, and for producing casts of natural external molds of specimens. All type materials and figured specimens recovered from talus have been deposited in the South Australia Museum paleontological collections (prefix SAM). Figured specimens found on excavated bedding planes have been left in place to maintain the integrity and utility of the field localities and the excavated surfaces. SYSTEMATIC PALAEONTOLOGY

Genus SOMATOHELIX new genus Diagnosis.—As per species. Type species.—Somatohelix sinuosus new species. Etymology.—From Greek soma meaning body and helix meaning helical or coiled, assigned based on the interpretation that the organism which produced this fossil had a helical shape prior to burial. SOMATOHELIX SINUOSUS new species Figure 3 Diagnosis.—Sinuous tubes with as many as two complete undulations of equal dimensions. Sinusoidal curvature is consistent and size independent with an average one quarter wavelength to amplitude ratio (Jl:A) of approximately 3. Circular in cross-section with a constant diameter for the entire length of the fossil. Lateral margins parallel to the fossil axis are uniform and curved throughout and contain no internal or external ornamentation. Description.—The holotype and paratypes of Somatohelix sinuosus range from 5–11 cm in length and 3–6 mm in width. Length measurements do not represent a maximum or minimum size range for this fossil as the completeness of any of these examples is questionable. The largest potential example found thus far is approximately 15 cm in length and 7 mm wide. In specimens where body termination can possibly be inferred, these ‘‘terminal’’ ends are typically rounded. All specimens show a consistent diameter throughout their entire length. Lateral margins are best defined in the smallest specimens and degrade in larger diameter specimens. Folds are common and are distinctly located at the areas of maximum distance from the zero point of undulation (the peaks and troughs in the sine wave pattern). This is consistent with the original organism having had a helical shape that had been compressed upon burial such that mostly the uppermost curves of the helix are molded. Sinuosity, measured in terms of the one quarter wavelength to amplitude ratio (Jl:A), varies only slightly among the type specimens from 2.5 to 3.8. The average value is 3.06. Sinuosity within a specimen does not change, exhibiting a consistent Jl:A value that does not vary any more than 60.1, which is within error margins of slight inaccuracies made while measuring. While the Jl:A value of any given specimen varies only slightly, the dimensions of any single specimen’s wavelength and amplitude is not consistent with all specimens. Small specimens of Somatohelix sinuosus have a smaller wavelength and amplitude than larger specimens, however, their Jl:A

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FIGURE 1—South Australia Precambrian stratigraphy and generalized stratigraphic section of the Rawnsley Quartzite.

value is still approximately equal to 3. The best examples of Somatohelix sinuosus preserve as many as 1.5 wavelengths. When present on substantially exposed bedding surface (.1 m2), Somatohelix sinuosus can consist of up to 10 specimens per square meter. Individuals are loosely packed with little consistency in spatial and size distributions among different surfaces. A sinusoidal pattern is preserved in all specimens of this fossil despite preservational style or taphonomic grade. This includes specimens that were contorted upon burial (Fig. 3.1). Both positive and negative examples of this fossil show the preservation of distinct folds at the peak of undulation (Figs. 3.1, 3.3, and 3.5–3.7). Positive portions of Somatohelix sinuosus commonly cease at the peaks of these sine waves. Four specimens are directly connected to circular features on one end (Figs. 3.1, 3.2 and 4.1, 4.2). In one of these specimens where the circular feature appears to be incontrovertibly attached to the fossils, the interior of the circle contains

indiscernible features resembling wrinkle marks (Fig. 3.1). These circular features are present as both positives and negatives on the base of beds. Etymology.—Latin for sinuous, assigned based on the shape of this fossil. Holotype.—SAM P45898 (South Australia Museum). Paratypes.—SAM P45894, SAM P45895, SAM P45901 (South Australia Museum). Occurrence.—Ediacara Member, Rawnsley Quartzite, South Ediacara and 9 km south, at Nilpena, South Australia. Discussion.—In the Ediacara Member, Somatohelix sinuosus is preserved only on the base of beds (hyporelief). Like Funisia dorothea (Droser and Gehling, 2008), S. sinuosus is preserved in both positive and negative relief, and both positive and negative relief occur within an individual specimen. However, Somatohelix sinuosus differs from Funisia in having no serial partitions. The highest quality specimens of S. sinuosus are present in positive relief as internal casts appearing as though

SAPPENFIELD ET AL.—TUBES AND TRACE FOSSILS WITHIN THE EDIACARA MEMBER

FIGURE 2—Distribution of fossiliferous horizons of the Pound Subgroup throughout the Flinders Ranges. Stars denote the two localities utilized in this study.

the tube could be ‘‘plucked’’ from the base of the bed. These infilled portions of the tube are typically no longer than a few centimeters due to the difficulty of injecting the tube with sediment. Sediment infilling commonly ceases in the areas of the fossil with the highest amplitude. A TUBE OR A TRACE FOSSIL? MORPHOLOGICAL AND TAPHONOMIC EVIDENCE

Taphonomic evidence plays a central role in differentiating Ediacaran tubular taxa from trace fossils as both occur in a variety of preservational styles and both preserve parts and counterparts. Among morphologies that at least superficially resemble trace fossils are Glaessner’s (1969) Forms E and F. These morphologies are briefly discussed by Glaessner (1969) and only one of them (Form E) is figured in the paper (Glaessner, 1969, fig. 5f, p. 380). Examination of hundreds of structures on excavated beds suggests that morphologies that would be included in Forms E and F are not distinct trace fossils but are actually taphonomic variants of Somatohelix sinuosus. Somatohelix is preserved as symmetrical and uniformly sinusoidal grooves and ridges on the base of beds within the size ranges of both Forms E and F. The picture and description of Form E are consistent with specimens of Somatohelix as undulations can be seen throughout the figured specimen (Glaessner, 1969, fig. 5f, p. 380). The preservational mode mentioned in the text for Form E is observed within several examples of Somatohelix (Fig. 3). The ‘‘clay film’’ mentioned in the description of Form F is a distinct feature of several examples of S. sinuosus both wellpreserved as well as those with poor preservation (Fig. 4.1). Armed with a range of taphonomic variants, and wellpreserved examples of this fossil based on data obtained from the large quantities of exposed bedding surface at Nilpena, the likelihood of Somatohelix sinuosus, and thus Forms E and F, having a trace fossil origin can be critically evaluated.

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Observations.—The presence of distinct folds within Somatohelix sinuosus provides robust evidence against a trace fossil origin (Fig. 3) (Droser et al., 2005; Cohen et al., 2009). Folding is common even in low taphonomic grade specimens of S. sinuosus and thus resolving folds within this fossil as folded trace fossils is difficult. Orientation data from two separate excavated beds at Nilpena suggests strong alignment among specimens of Somatohelix (Fig. 5). This alignment is consistent with other current related features on these beds such as tool marks as well as other body fossils. A key aspect of the trace fossil record that adds substantially to its utility is that trace fossils cannot be transported except as reworked sedimentary clasts. Evidence of alignment among problematic structures is thus a strong indicator that the aligned structures are not trace fossils (Droser et al., 2005; Tacker et al., 2010). In the tightest portions of turns within Somatohelix sinuosus, where a trace fossil would be expected to either remain consistent or widen as is observed in other definitive Ediacaran traces such as Helminthoidichnites (Droser et al., 2005), S. sinuosus narrows significantly (Figs. 3.1 and 3.3). None of the specimens of S. sinuosus suggest that portions of the fossil extend into or out of the bedding plane. The narrowed portions of this fossil are commonly associated with folding of the original organism. On the other hand, as an organism moves directly forward, whether on top of the substrate or interstratally, the trail or burrow it leaves will maintain a consistent diameter, only widening as the animal turns. This is true even with organisms that move by peristalsis where the dimensions of the body of the organism change. The terminations of S. sinuosus are commonly either pointed or splayed outwards but are rarely round (Figs. 3.1, 3.7, and 5. 2) as this fossil rarely, if ever, preserves complete specimens. The tearing of soft-bodied organisms and modular growth such as is observed in some anthozoans can produce frayed or angular terminations of structures preserved in the fossil record, however, the corners of trace fossil galleries are very rarely sharp. Paleodictyon is the noted exception to this rule (Droser et al., 2005). Positive relief examples of Somatohelix sinuosus appear as though they can be ‘‘plucked’’ from the bed, being differentiated from the surrounding materials by distinct lateral margins (Figs. 3.2 and 3.5–3.7). This suggests that these are not backfilled burrows as the sediment that fills these cavities is separate from the surrounding matrix yet is composed of the same material. Backfilled or infilled open burrows in a homogeneous siliciclastic setting would not be separated as such from the surrounding materials (Gehling and Droser, 2009). Our observations thus preclude a trace fossil interpretation for Somatohelix sinuosus including the original Form E and Form F of Glaessner (1969). Features observed within S. sinuosus are consistent with a soft-bodied tubular organism (Droser and Gehling, 2008; Gehling and Droser, 2009). ‘‘Tube’’ is commonly used to refer to what is likely a phylogenetically disparate set of organisms, grouped only by all of these taxa having a hollow and elongate constructional morphology (Jensen, 2003; Droser and Gehling, 2008; Cohen et al., 2009; Gehling and Droser, 2009). Taphonomic evidence suggesting a hollow body cavity (discussed in further detail below) and the elongate sinusoidal curvature of S. sinuosus is thus consistent with placement among this group. Interpretations and reconstruction.—Somatohelix sinuosus is most commonly found as negative external molds on the base of beds implying some rigidity to the original organism’s

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FIGURE 3—Somatohelix sinuosus (all examples are on the base of beds (hyporelief)): 1, (SAM P45897), sinuous specimen with circular structure indicated by arrow; 2, (SAM P45898), holotype specimen, circular feature indicated by arrow; 3, (SAM P45899), preservation of even undulations, an


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FIGURE 4—Two additional specimens with circular depressions: 1, (NV03), sinuous specimen with circular depression within the white square; 2, (NP07), specimen of Somatohelix sinuosus with direct attachment to a potential holdfast within the white square. Scale bars are both 1 cm.

structure that was commonly able to withstand burial without collapsing (Fig. 6). Specimens expressed as negatives are typically not as deep as the maximum radius of a specimen’s cross-section. This is consistent with slight compression of the tube upon burial (Fig. 6.2). Unlike most Ediacara Member fossils, the highest quality specimens of S. sinuosus are present in positive relief as internal casts produced by sediment being injected into the hollow body of the organism (Fig. 7) resulting in a fossil that appears as though it could be ‘‘plucked’’ from the base of the bed (Fig. 7.4). These infilled portions of the tube are typically no longer than a few centimeters, probably due to the difficulty of injecting the tube with sediment. Sediment infilling commonly ceases in the areas of the fossil with the highest amplitude. Complete specimens of Somatohelix sinuosus are rare due to the likelihood of collapse of portions of the tube upon burial (Fig. 8). Collapsing the body of an organism following burial results in ambiguity in the majority of structures potentially produced by the organism (Gehling, 1999; Droser et al., 2005). The robustness of an organism’s structure is dependent upon not only the body material, but also the way in which the structure is organized. The presence of a hollow internal cavity causes Somatohelix to have a higher propensity for collapse than those organisms that lack this cavity. Specimens of Somatohelix exhibiting preservation following collapse, like infilled specimens, are observed as positives on the base of beds. Also similar to infilled specimens, these examples are typically less than a few centimeters in length and rarely make up the entirety of the preservational style observed in a single specimen. This style of preservation can be distinguished from infilling as collapsed portions of the tube show little to no relief extending from the base of the bed and are bound by faint rather than pronounced margins. Despite the rigidity observed in the specimens present as negatives on the base of beds, Somatohelix sinuosus is interpreted here to be soft-bodied due to the folding and apparent flexibility of the organism expressed in several examples (Figs. 3.3 and 3.5–3.7). Folding is also noteworthy

within S. sinuosus because this property is consistently at areas of highest variation from the zero point of undulation. This consistency in both the presence and location of these folds implies that the original organism likely had a three dimensional element to its overall shape. By ‘‘unfolding’’ this fossil, the overall shape is best described as a loosely coiled helix (Fig. 9.1). This interpretation is consistent with the sinuosity of this fossil reflecting helical pitch when the organism was in life position. This three dimensional shape would have been compressed onto a two dimensional surface upon burial yielding the sine wave morphology of this fossil as well as the folds observed in the areas of highest amplitude (Fig. 9.2). Folds do not reflect a preference in clockwise or counter-clockwise rotation of the helical body of the organism. A helical constructional morphology is conducive to an erect life position as it provides the organism with the advantage of increasing surface area while minimizing the height at which the organism extends into the water column. This makes it less exposed to high wave energy within the shallow portion of the water column. Because of the three dimensional element of the helical shape, this organism could not have been in complete contact with the substrate in a prone position. The helix would have to extend slightly into the water column while having portions of the organism in contact with or slightly buried within the substrate. Having only isolated portions of the tube in contact with the substrate provides the organism with little advantage for feeding or current resistance. A prone position would also be disadvantageous to filter or suspension feeding as well as photosynthesis which are the overwhelmingly dominate consumption strategies seen in modern sessile organisms. Variation in the linearity of the zero point of undulation also provides evidence for an erect position of the original organism. The zero point of undulation is again a hypothetical axis around which the sine wave pattern undulates. If Somatohelix sinuosus were a prone organism, the zero point of undulation would be mostly linear in the majority of instances that this organism is preserved in the fossil record.

r apparent fold in the lower portion of the fossil indicated by arrow, and an infilled portion in the upper left; 4, (SAM P45894), multiple juxtaposed specimens preserved in both positive and negative relief; 5, (SAM P45895), evenly undulating tubular fossil with indication of a hollow structure implied both by the infilled portion on the right and the nature of the infill where indicated by arrow. Infilling disrupted at the peak of undulation and a subsequent fold running along the length of the fossil is preserved; 6, (SAM P45900), folded specimen with infilled portion indicated by arrow, infilling ceases at peak of undulation and an apparent fold is preserved; 7, (SAM P45901), evenly undulating tubular fossil with indication of a hollow structure implied both by the infilled portion on the left and the nature of the infill where indicated by arrow, infilling abruptly ceases at the peak of undulation. Scale bars are all 1 cm.

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FIGURE 5—Rose diagrams with orientation data for excavated beds NPB and NSTC. 1, NPB; 2, NSTC.

FIGURE 6—External mold of tubular specimen: 1, tube following collapse on a relatively firm substrate; 2, slightly compressed tube upon burial; 3, decay of organic material and injection of underlying sediment; 4, SAM P45899 (4* denotes the portion of the external mold of the tube that is reconstructed in the image above and to the right of the photograph). Scale bar is 1 cm.

This fossil is rarely found maintaining a consistent directional trend of the zero point of undulation throughout its length indicating various positioning of the organism due to collapse prior to or upon final burial (Figs. 3.3 and 3.6). Comparison between Somatohelix sinuosus and other Ediacaran tubes.—Recent work has shown that organisms with a tubular structural organization were considerably more common in the Ediacaran and Cambrian than previously appreciated (Droser et al., 2005; Droser and Gehling, 2008; Cohen et al., 2009). Many of these fossils are characterized by simple, indistinct forms with few, if any, distinguishable characteristics. Only one tube from the Ediacara Member, Funisia dorothea, has received a systematic description to date (Droser and Gehling, 2008). That fossil can be up to 30 cm in length and consists of linearly arranged, serial units that rarely branch. Funisia dorothea can be recognized in the fossil record from either its distinctive ‘‘buds’’ which make up the length of the body or as grooves with scalloped edges which mark the separations between the units (Droser and Gehling, 2008). The sinuous morphology and lack of serial units within Somatohelix sinuosus makes it distinct from this morphology. The smooth outer sheath of Somatohelix sinuosus is also not consistent with the scalloped margins observed in Funisia dorothea. Currently, no specimens of Somatohelix sinuosus have been discovered that display any type of branching. Cohen et al. (2009) re-evaluated Ediacaran tubes preserved as ribbon-like carbonaceous compression fossils found in the Nama Group in Namibia, originally described as Vendotaenia antiqua Gnilovskaya (1971). This morphotaxon is abundant within the Nama Group and Cohen et al. (2009) observed a number of preservational traits similar to Somatohelix sinuosus such as folding and curvature. Folds are rare in V. antiqua and are not preserved in the peaks of undulation as in S. sinuosus. V. antiqua is also commonly seen to branch and does not preserve the consistency in sinuosity observed within S. sinuosus. Furthermore, specimens of V. antiqua are just 0.6–2.1 mm wide, and apparently exclusively preserved as carbonized, flattened tubes with some evidence of internal serial divisions (Cohen et al., 2009). Despite the relative simplicity of tubes in comparison to more complex Ediacara morphologies, recognition of the importance of these simple architectures is critical to a more complete understanding of the early history of life. The fossil record of Ediacaran tubular organisms has provided the earliest directly observable evidence of fundamental steps in


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FIGURE 7—Internal casting of tubular specimens: 1, tube following collapse; 2, burial with sediment filling the hollow cavity; 3, decay of organic material and lithification; 4, SAM P45901 (4* denotes the infilled portion of the specimen and is the portion of the fossil reconstructed in the image above and to the right of the photograph). Infilled portion is separated from the surrounding sediment by well-defined margins. Infilling ceases at the peak of the sine wave. Scale bar is 1 cm.

evolution such as skeletonization (Germs, 1972), predation (Bengtson and Zhao, 1992; Hua et al., 2003), and sexual reproduction (Droser and Gehling, 2008). In addition, consistent with our interpretation of Form E and Form F as body fossils of a single tubular organism rather than trace fossils, it has been proposed by several workers that the bulk of what have been described as Precambrian trace fossils are actually poorly preserved specimens of tubular body fossils (Jensen, 2003; Droser et al., 2005; Jensen et al., 2006; Droser et al., 2006; Cohen et al., 2009; Tacker et al., 2010). CONCLUSIONS

The interpretation of the origin of a structure based on isolated specimens viewed in hand sample has substantially overinflated the Precambrian trace fossil record within the Ediacara Member and in Precambrian strata as a whole. The critical evaluations of Ediacaran ichnotaxa provided by Jensen (2003), Droser et al. (2005) and Jensen et al. (2006) have not only reduced the number of Precambrian trace fossils, but have also recognized that there is a considerable quantity of distinct morphotypes previously misinterpreted as trace fossils that still remain undescribed. As a prime example,

FIGURE 8—Casting of specimen following collapse of the tube: 1, tube following collapse on relatively firm substrate; 2, burial and collapse of the tube; 3, decay of the organic material and lithification; 4, SAM P45896 (4* denotes the portion of the external cast of the tube that is reconstructed in the image above and to the right of the photograph). Margins are faint, trending from left to right. Scale bar is 1 cm.

Somatohelix sinuosus escaped recognition as a component of the body fossil assemblage for more than forty years as a result of its original interpretation as a trace fossil (Glaessner, 1969). Similarities between structures produced by tubular organisms and trace fossils makes differentiating between the two problematic. Despite the low levels of complexity observed in S. sinuosus, this fossil provides further basis for the argument that tubular constructional morphologies are responsible for many structures previously interpreted as trace fossils and that a single systematic description can potentially lead to the reinterpretation of multiple problematic forms. The reinterpretation of trace fossils as body fossils of tubular organisms also suggests that these organisms were prolific elements of the Ediacara biota which has fundamental implications on interpretations regarding the synecology of Precambrian macroscopic communities.

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FIGURE 9—Reconstructions of Somatohelix sinuosus: 1, life position; 2, collapse of the organism producing a sinuous, mostly two dimensional corpse out of the original three dimensional structure.

The phylogenetic affinity remains problematic for the original organism which produced Somatohelix sinuosus. Modern analogues of organisms with a helical constructional morphology are found not only in disparate Kingdoms, but also in multiple domains, utilizing this shape for very different functions. For example, a helical shape is utilized for maximizing surface area in the black coral Cirripathes spiralis, locomotion in some bacteria, wind resistance in climbing plants, and the list goes on. Until material is discovered which provides evidence of a particular ecologic strategy, the question of what this organism is will likely remain unanswered. ACKNOWLEDGMENTS

This research was supported by a National Science Foundation grant (EAR-0074021) and a NASA grant (NNG04GJ42G NASA Exobiology Program) to MLD and an Australian Research Council Grant (DP0453393) to JGG. ADS is grateful for support from the American Federation of Mineralogical Societies. We are indebted to R. Fargher and J. Fargher for access to their property and permission to excavate fossiliferous beds. Fieldwork was facilitated by D. Rice, M. Dzaugis, M. E. Dzaugis, R. Droser, D.A. Droser and members of the Waterhouse Club. N. Hughes, D. Garson, C. Sappenfield and E. Clites provided helpful comments. We also thank B. Pratt, S. Jensen and an anonymous reviewer for comments that greatly added to this contribution. REFERENCES

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ACCEPTED 7 NOVEMBER 2010