THE FIRST NAMED EDIACARAN BODY FOSSIL, ASPIDELLA TERRANOVICA

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1 THE FIRST NAMED EDIACARAN BODY FOSSIL, ASPIDELLA TERRANOVICA by JAMES G. GEHLING, GUY M. NARBONNE and MICHAEL M. ANDERSON ABSTRACT. Aspidella terranovica Billings, 1872 was rst described from the late Neoproterozoic Fermeuse Formation (St. John's Group) on the Avalon Peninsula of eastern Newfoundland, approximately 1 km stratigraphically above the famous Ediacaran biota at Mistaken Point, and several kilometres below the base of the Cambrian. Aspidella has been reinterpreted perhaps more than any other Precambrian taxon, and has variously been regarded as a fossil mollusc or `medusoid', a gas escape structure, a concretion, or a mechanical suction mark. Our studies indicate that Aspidella includes a wide variety of preservational morphs varying from negative hyporeliefs with a raised rim and ridges radiating from a slit (Aspidella-type preservation), to at discs with a central boss and sharp outer ring (Spriggia preservation), to positive hyporeliefs with concentric ornamentation (Ediacaria preservation). Specimens occur in a continuum of sizes, with preservational styles dependent on the size of the specimen and the grain size of the host lithology; the elongation of specimens is tectonic. Aspidella is con rmed as a body fossil from observations of complex radial and concentric ornamentation, mutually deformed borders in clusters of specimens, and occurrence on the same bedding planes as certain distinctive Ediacaran taxa. Aspidella is indistinguishable from, and has priority over, several of the most common genera of late Neoproterozoic discoidal body fossils worldwide. Similar fossils from Australia are interpreted as holdfasts of frond-like organisms. The density of specimens in the Aspidella beds suggests levels of benthic biomass in the Neoproterozoic that could rival those of modern marine communities. The serial growth forms, Palaeopascichnus, Intrites, Neonereites renarius and Yelovichnus, associated with Aspidella, are interpreted as body fossils of unknown af nities rather than trace fossils. A new, trilobed, Ediacaran body fossil, Triforillonia costellae gen. et sp. nov., is described from the Aspidella beds of the Fermeuse Formation. T HE discovery and description of Aspidella terranovica Billings, 1872 in what is now known as the Fermeuse Formation of the St. John's Group, from the Avalon Zone of eastern Newfoundland (Text- gs 1±2), came at a time when geologists were grappling with the relatively new disciplines of stratigraphy and systematic palaeontology. The earliest references to these discoidal impressions realized their utility in establishing stratigraphical relationships on the Avalon Peninsula (Murray 1868, 1873; Billings 1872). It was another matter to explain enormous numbers of fossils of apparently soft-bodied organisms in very old strata. The fact that the Aspidella-bearing formation clearly underlay the local trilobite-bearing `Primordial' strata, with marked unconformity, added to the concern of later commentators. Despite regular reviews of the status of Aspidella (see Hofmann, 1971 and references therein, plus subsequent references by HsuÈ (1972), King (1980), Landing et al. (1988), Conway Morris (1989), and Jenkins (1989, 1992), there is little evidence that attempts were made to reinterpret these discoidal forms by more intensive collection and eld study in the 120 years following the rst publications. The status of Aspidella has remained uncertain for a number of reasons. Until 50 years ago, Precambrian rocks were generally considered to be free of macroscopic fossils, with the exception of stromatolites. Specimens of Aspidella show a variety of preservational morphs that, at face value, might be taken as different organisms. No fossils like Aspidella had been recovered from similar sedimentary rock types in the Phanerozoic. Consequently, it is not surprising that non-biological explanations were sought for the origin of large numbers of simple ovoid discs in Precambrian rocks. Renewed interest in global correlation, palaeobiology and taphonomy, in the last 40 years, has prompted reviews of unusual fossil deposits. In Newfoundland, the discovery of undisputed Ediacara-type fossils in the Mistaken Point Formation of the Conception Group (Anderson and Misra 1968; Misra 1969) meant [Palaeontology, Vol. 43, Part 3, 2000, pp. 427±456, 1 pl.] q The Palaeontological Association

2 428 PALAEONTOLOGY, VOLUME 43 TEXT-FIG. 1. Location map showing the Avalon and Burin peninsulas of south-eastern Newfoundland, outcrops of the Neoproterozoic Fermeuse, Mistaken Point and Gaskiers formations, and major fault lines. that Aspidella from the overlying formations could no longer be regarded as a pseudofossil simply because it was Precambrian. The remaining stumbling block to the renaissance of Aspidella, as the rst named member of the Ediacara biota, is to explain why such discs are apparently con ned to the late Neoproterozoic. The stratigraphical restriction of the Ediacaran-style of preservation, in relatively coarse-grained siliciclastic sedimentary rocks, has lead to speculation about the biological af nities of Ediacaran organisms. Seilacher (1984, 1989) proposed that these organisms were constructed from unusually tough biological materials to account for their enigmatic preservation. However, it is even more dif cult to explain why pseudofossils or certain sedimentary structures should be con ned stratigraphically. Hofmann (1971, 1992) in his reviews of all the published Precambrian fossils from Canada, demonstrated that shrinkage crack structures, rill marks and bedding surface wrinkle marks that originally had been described as fossils were best explained as sedimentary, mechanical, or diagenetic artifacts. Hofmann (1971, p. 5) suggested that so many pseudofossils and problematica are known from the Precambrian because they `receive special attention' in rocks otherwise devoid of `obvious organic remains'. It is dif cult to deny that the onset of bioturbation at the base of the Cambrian greatly reduced the survival potential of both sedimentary and non-resistant biological structures in shallow-water sedimentary environments (Jensen et al. 1998; McIlroy and Logan 1999; Seilacher 1999).

3 GEHLING ET AL.: EDIACARAN BODY FOSSIL 429 AGE STRATIGRAPHICAL UNITS Aspidella 'pizza discs' spindles fronds Zircon U -Pb (Benus 1988) TERMINAL PROTEROZOIC SIGNAL HILL GP ST. JOHN'S GROUP CONCEPTION GP CAPE BALLARD FM CUCKOLD FM FERRYLAND HEAD AND QUIDI VIDI FM GIBBETT HILL FM CAPPAHAYDEN FM RENEWS HEAD FM FERMEUSE FM TREPASSEY FM MISTAKEN POINT FM BRISCAL FM DROOK FM GASKIERS FM MALL BAY FM 565 ±3 Ma TEXT-FIG. 2. Stratigraphical units of the late Neoproterozoic succession above the Harbour Main Group on the Avalon Peninsula, Newfoundland, depicting known stratigraphical ranges and the relative frequencies of Aspidella and representative megafossils from the Mistaken Point assemblage; U/Pb date from zircons in volcanic ash layer on `Bed E', Mistaken Point Formation (Benus 1988). The fact that some problematic sedimentary structures are either entirely absent, or restricted to anoxic or hypersaline environments, in the Phanerozoic points to a variety of evolutionary changes in the Earth system across the Proterozoic±Palaeozoic transition (Hagadorn and Bottjer 1999; P uèger 1999). For example, the stratigraphical range of such phenomena as molar-tooth structures in carbonates (James et al. 1998) and microbial mat-bound lamination in siliciclastic sediment from shallow marine environments (Hagadorn and Bottjer 1997, 1999; Gehling 1999) may have been limited by the evolution of grazing and burrowing organisms. Only those sedimentary structures that were direct products of microbial binding disappeared from the record. While the preservation of purely inorganic structures may be limited by bioturbation, they should be present in appropriate lithofacies of any age. Like these non-actualistic sedimentary structures, the preservation of soft bodied organisms in late Neoproterozoic coarse siliciclastic strata was in part a function of taphonomic processes that were largely erased by the advent of effective bioturbation at the base of the Cambrian. This preservation depended on the formation of microbial mats and subsequent non-disturbance of sedimentary layering. Both sessile soft bodies and buried organic-rich horizons succumbed to the ecological changes across the Precambrian- Cambrian boundary. It is likely that the evolution of predation extinguished Ediacaran benthic and sessile communities, while penetrative burrowing destroyed the preservation potential for soft bodies in shallow marine environments (Narbonne 1998; Gehling 1999; Seilacher 1999).

4 430 PALAEONTOLOGY, VOLUME 43 Thus Aspidella was either the impression of a soft bodied organism, or it was a non-actualistic sedimentary structure that ceased to be preserved after the advent of Cambrian ecosystems. To distinguish between these two scenarios we weighed the evidence for inorganic and organic origins by studying stratal and bedding surface distributions, variation in shape and cross section, co-occurrence of other body fossils, and by making comparisons with Ediacaran fossils from the more diverse assemblages as well as with known discoidal forms of mechanical origin (cf. Cloud 1960; Conybeare and Crook 1968). FIELD STUDY In 1990 we examined newly exposed, convex discoidal casts on the soles of sandstone beds of the Fermeuse Formation cropping out in a recently widened road cutting on the main highway through the town of Ferryland (Text- g. 1). Large numbers of specimens resembling the holotype of Aspidella Billings, 1872 occur on thinner beds within the same facies as the large convex specimens. Our exploration of the Fermeuse Formation, from Cape Race on the south-east corner of the Avalon Peninsula to the type locality in St. John's, has revealed many sites where Aspidella is common (Text- g. 1). Specimens can still be studied in outcrop from the type locality at St. John's, where Aspidella-bearing strata are exposed in building excavations, road cuttings, and parking areas. The nest known exposure of the disc-bearing facies is 100 km south of St. John's, 1 km from Ferryland on the coast, just north of Aquaforte Harbour (Text- g. 1). In the zone above high tide and below the tree line, many tens of thousands of specimens can be studied on the surfaces of east-dipping beds. Additional specimens were collected from coastal and road-side outcrops around Ferryland. Weathered slabs of the heterolithic facies of the Fermeuse Formation were split, and part and counterparts were labelled for facing. Oriented specimens were collected for thin-section preparation and analysis of cleavage bedding intersection. Latex casts were made of selected specimens preserved in concave epirelief. Two sections were measured in coastal exposures through the Aspidella-bearing beds south and north of Ferryland (Text- g. 3). The preservation of discs is best in outcrops where bedding and cleavage are almost coincident, or where the sand content of beds is highest. Cleavage is more prominent in shale-rich beds and insigni cant in packages of sandy beds. Where the angle between the main cleavage direction and bedding is high, bed partings and any discs on them are poorly preserved. Discs are only abundant in the facies where thin to very thin sandstone beds alternate with silt and shale, in the upper part of the Fermeuse Formation. All types and gured specimens have been placed in the repository of the National Type Collection of Invertebrates and Plants, Geological Survey of Canada (GSC) in Ottawa, Canada. STRATIGRAPHICAL AND ENVIRONMENTAL CONTEXT Stratigraphy of Avalon Zone On the Avalon Peninsula, the late Neoproterozoic strata comprise an apparently conformable succession, some 6±7 km thick, overlying the massive glacial deposits of the Gaskiers Formation (BruÈckner and Anderson 1971; King 1980; Anderson and King 1981; Conway Morris 1989) (Text- g. 2). The Conception Group, divided into the Mall Bay, Gaskiers, Drook, Briscal and Mistaken Point Formations, is overlain by the Trepassey, Fermeuse and Renews Head Formations of the St. John's Group. Above the Gaskiers Formation, the Conception Group consists of medium to thickly bedded siliciclastics with relatively common ash tuffs and volcaniclastic-rich beds. In contrast, the St. John's Group is more thinly bedded with predominantly ner-grained siliciclastics. The Signal Hill Group mainly consists of thick packages of coarse-grained uvio-deltaic sediments from the uppermost part of the late Neoproterozoic succession. A maximum age for the glaciogene Gaskiers Formation is constrained by a Ma U-Pb zircon date from basement rocks of the Harbour Main (Krogh et al. 1988). High in the Conception Group, the most richly populated fossil bed in the Mistaken Point Formation was smothered by a crystal-bearing ash layer dated at Ma (Benus 1988). King (1980) correlated the top of the Signal Hill Group with the Rencontre Formation on the Burin Peninsula. The Rencontre Formation is overlain conformably by the

5 GEHLING ET AL.: EDIACARAN BODY FOSSIL 431 TEXT-FIG. 3. Measured sections through the fossiliferous beds in the upper Fermeuse Formation; Aspidella is common in heterolithic facies; section F1, 300 m from the northern head of Aquaforte Harbour, 1 km south of Ferryland; section F2, on the south side of Freshwater Cove on Coldeast Point, in Ferryland on the Avalon Peninsula. Chapel Island Formation. The Precambrian-Cambrian boundary Global Stratotype Section and Point was designated within the lower part of Member 2 of the Chapel Island Formation at Fortune Head on the Burin Peninsula (Narbonne et al. 1987; Landing 1994). Range of Aspidella The stratigraphical range of discoidal specimens included in our revised diagnosis of Aspidella extends from the top of the Drook Formation, in the Conception Group, up into the Renews Head Formation at the top of the St. John's Group. Well-preserved specimens are uncommon low in the Fermeuse Formation,

6 432 PALAEONTOLOGY, VOLUME 43 where laminated siltstone is the most common facies. The upper, more heterolithic part of the formation bears the richest concentrations of Aspidella (Text- g. 2). The discs all but disappear in the transition to the Renews Head Formation where sandstone becomes more common than shale. Although the range of Aspidella extends down to overlap the Mistaken Point assemblage, specimens are rare in Mistaken Point fossil beds preserved by volcanic ash. Most discoidal holdfasts of frond-like fossils in the Mistaken Point assemblage are preserved in positive epirelief. However, isolated specimens of Aspidella, and some discoidal holdfasts attached to fronds, are preserved in negative hyporelief on horizons where a limonite crust is apparently associated with moulding and casting of soft-bodied organisms. The apparent absence of Aspidella above the St. John's Group, from any level in the Signal Hill Group, re ects the shallower water, commonly non-marine, environments represented by this uppermost part of the late Neoproterozoic succession on the Avalon Peninsula. Sedimentology of the Fermeuse Formation In the south-eastern part of the Avalon Peninsula the Fermeuse Formation is estimated to be some 1400 m thick (Williams and King 1979). Grading up from the Trepassey Formation, the lower part is composed of dark grey, pyritic shale with wispy lamination of silt and sand. This ne-grained facies exhibits large-scale soft sediment deformation. Preserved as 5±50-m-thick packages, this slump folding is distinguished from tectonic deformation by observed truncation of the deformed beds by at-lying strata at various levels. Soft sediment deformation decreases towards the top of the formation, but intraformational cut-outs are common throughout. The sense of stratal-dislocation and orientation of clastic dykes suggests westward mass slides that may represent the up-slope expression of slump-roll packages. Aspidella is rare in these facies. The upper part of the formation features increasing silt and sand in 1±2-m packages of coarsening and thickening upward beds (Text- g. 3). The coarser grained facies includes buff-coloured sandstone as wavy and lenticular beds, gutter casts, scours, and small-scale channel lls, hummocky cross-strati cation and starved current-ripple lamination. Hummocky cross-strati cation is weakly developed as 10±40-mm-thick sets of undulose drapes of nely laminated sand. Individual sandstone beds are cross-laminated, but continuous sets of current ripples are not common. Micro-cross lamination, rills, and scours indicate westerly-directed palaeocurrents. The Aspidella-bearing facies of the upper Fermeuse Formation vary from alternating millimetre-thick beds of light-coloured sand and dark-coloured shale, to thinly-bedded, sharp-based, lenticular sandstone with shale partings. Fossils appear to be absent from facies consisting of laminated siltstone and shale. Depositional environment The overall coarsening-upward sequence, represented by the Fermeuse and Renews Head Formations, is interpreted as a delta front and slope deposit. The large scale slumping and intraformational cut-outs were a product of rapid progradation over a tectonically active basin margin. Waning storm events produced wispy lamination of silt and sand in the shale facies of the lower part of the Fermeuse Formation. The increase in frequency and thickness of sandy packages, in the upper part of the Fermeuse Formation and the transition to the Renews Head Formation, represent general shoaling of the deltaic sequence. The Aspidella facies were deposited near storm wavebase in delta front and slope environments. Sand was deposited from suspension after the most intense storms, smothering all but the largest objects on the substrate. MORPHOLOGY OF ASPIDELLA Descriptions of morphs Billings (1872) originally named Aspidella from a small slab bearing two raised oval-shaped discs, without specifying one specimen as the holotype. On the metal plastotype, which is all that remains of the original slab, a small cross adjacent to the largest specimen may be interpreted as indicating the intended holotype

7 GEHLING ET AL.: EDIACARAN BODY FOSSIL 433 TEXT-FIG. 4. Aspidella terranovica Billings, 1872 type collection (GSC 221), epirelief preservation, from downtown St. John's and nearby localities (Billings 1872; Walcott, 1898); 1. A, metal plastotype of the holotype (GSC 221c); cross above presumed holotype. B, hypotype of a convex form (GSC 221b); positive hyporelief. C, hypotype, showing strong radial ornament (GSC 221a); negative epirelief. (GSC type 221c). It is an epirelief, 10±12 mm long by 8±9 mm wide, with a raised rim and sharp radial ridges extending from the border to a central ridge (Text- g. 4A). Walcott (1899, pl. 27, gs 7±8, 14±15) illustrated two other specimens in the type collection under the same name and type number, from the same locality (Text- g. 4B±C; GSC 221a±221b). These specimens extended the range of Aspidella to include discs with radial ridges and a central hollow, and others with concentric ridges and no radial ornamentation. Discs similar to those described by Billings and Walcott are extremely common in the Fermeuse Formation of the Avalon Peninsula, and in the course of our study we examined thousands of specimens in the eld and studied many hundreds of specimens in detail in the laboratory. These studies have demonstrated an intergrading plexus of forms that, in our opinion, represent different preservational aspects or formative stages of the same structure. Three end-member morphs can be recognized (Text- g. 5). Type morph. These small (5±25 mm in diameter) specimens closely resemble the holotype (Text- g. 4A). In hyporelief, a sharp border surrounds a broader convex zone that is cut by radial grooves converging on a central slit or invagination (Text- gs 4A, 5, 6A±B). The more attened specimens have a raised outer rim and radial grooves converging on the central pit (Text- g. 6J±K). At the lower end of the size range, each specimen is just a smooth convex ring with a central depression. Flat morph. In hyporelief these specimens are characterized by a slightly raised, at to low-relief disc, 30± 100 mm in diameter, with a central boss that is less than one quarter of the diameter of the whole disc (Text- gs 5, 6H±I,8E). The periphery is a raised rim with a sharp outer and inner edge. Between the rim and the boss, the disc is either smooth or ornamented by faint concentric rings and, more rarely, with ne radial grooves. Convex morph. These hyporelief casts vary in size from 5±110 mm in diameter and exhibit up to 4 mm of relief. The periphery is a low, at to concave ange rising sharply to a broadly convex disc with a prominent or slightly sunken central, convex boss (Text- gs 4B, 5, 6D±F). Fine, concentric grooves are common across the disc and boss, but tend to be concentrated at the outer edge of the boss. Relationship between morphs The three morphological groups of Aspidella can be represented as points of a triangle with integrating forms along the sides (Text- g. 6). The vast array of specimens available for study show an insensible intergradation of morphologies between the end members. While the Aspidella-type morph includes mostly smaller specimens, almost all larger specimens are at and convex morphs. Small specimens may conform to either the type or the convex morph, but rarely to the at morph, mainly because tiny, at

8 434 PALAEONTOLOGY, VOLUME 43 TEXT-FIG. 5. Morphological end members of Aspidella represented on a single block. In hyporelief, the type morph is at to convex with radial grooves and central invagination; at morph has raised marginal rim and central boss; convex morph has a prominent central boss, radial and concentric grooves; diagrammatic cross-section with pro les in hyporelief; upper block diagram shows part and counterpart in opposite relief. specimens are dif cult to recognize. Between the type morph and the convex morph, a small boss occurs within the central invagination (Text- g. 6C±D). As the largest invaginated specimens are less than 30 mm in diameter, size appears to be a critical factor in the preservation of the different morphs of Aspidella (Text- g. 7, y-axis). The relative proportion of sand and clay making up a bed is a signi cant factor in controlling the preservation of the different Aspidella morphs (Text- g. 7, x-axis). However, on almost any surface where large numbers of discs of one morphological group occur, usually there are a few specimens of another morphology (e.g. Text- gs 8E±F, H, 9). This observation supports the view that such assemblages represent preservational variants of a single species, where individual biological variation and inhomogeneities in a sedimentary surface account for the morphological differences. If the particular morph was exclusively determined by the sedimentary context, it could be argued that the resultant discs were either mechanical responses to particular sedimentary processes or different species without overlap of habitat. The intergradation between morphs is too continuous to easily invoke several species. For the larger specimens, the thickness of the casting sand was the single most important factor that determined

9 GEHLING ET AL.: EDIACARAN BODY FOSSIL 435 TEXT-FIG. 6. Triangle of morphological variation, with named form-genera illustrating the end members. Diagrammatic cross-sections and examples of the intergrading series of morphs, in hyporelief preservation, described clockwise from the apex. A±B, Aspidella-type morphs with radial grooves converging on an invaginate centre. B±E, loss of invagination, increasing convexity and prominence of central boss, toward the Ediacaria convex morph. E±I, decreasing convexity and prominence of central boss, toward the Spriggia at, annulate morph. I±A, increasing invagination of the centre, and reduced size, toward the type morph. A, GSC 221; 1. B, D, GSC ; 1. C, GSC ; 0 8. E, GSC ; 0 3. F, GSC ; 0 8. G, GSC ; 0 5. H, GSC ; 0 5. I, eld specimen; 0 5. J, GSC ; 0 8. K, GSC ; 0.8. which morph was preserved. Thus the principal variables responsible for these morphs are disc diameter and sand to clay ratio (Text- g. 7). The density of discs on any surface ranges from high to patchy, with local crowding in the form of chains or clusters of similar-sized specimens (e.g. Text- g. 9A). Discs of all sizes occur as isolated clusters with mutual deformation of common borders (Text- g. 8G). Overlapping borders can be observed, but only where discs are impressed from different levels in the sediment. In such cases, the disc with sharper outlines and ornamentation is separated from the more diffuse, under-printed disc by a single lamina of sediment.

10 436 PALAEONTOLOGY, VOLUME 43 TEXT-FIG. 7. Graphical relationship between disc diameter (mean of ellipsoidal dimensions, y-axis) and bed classes (xaxis), based on a qualitative estimate of the sand to shale ratio of the host beds for measured specimens of Aspidella; Aspidella-type morphs (squares) and Spriggia morphs (crosses) are preserved in shale with thin sandstone laminae; Ediacaria morphs (circles) are convex casts one soles of 10±60-mm sandstone beds, above counterpart moulds in silty shale. Cross-sections through specimens preserved in convex hyporelief show in ll from above, or sagging of laminae into the central part of the disc (Text- g. 10). Simple convex morphs were cast continuously with the sand of the overlying bed (Text- g. 10B). Small convex discs, on the soles of interlaminated sand and shale, were preserved by small sand plugs isolated from or in continuity with an overlying lamination of sand (Text- g. 10A, C). Flat morphs were cast by shale or thin laminae of sand, with a plug of sand forming the central boss. Rarer button-like variants of the type morph (5±12 mm in diameter) consist of a prominent convex rim and a sunken boss with an invaginated centre (Text- g. 8H). In section, the rim and central boss are the lower edges of an inverted cone-shaped sand body in continuity with, or just detached from, an overlying sand lamina (Text- g. 10D). The rim is the terminus of down-turned sand laminae that appear to have abutted a sack-shaped body before its collapse. No examples of upward displacement of laminae are known; rather, it appears that in all cases sediment collapsed into a vacated space over the site of the disc. The complete intergradation between type, at, and convex morphs negates the argument for a biological origin for one morph and a mechanical or diagenetic origin for another. Aspidella, as de ned by the type morph, represents an end member of a plexus of morphs for which a single biological model is suf cient. The morphological, size and spatial associations of discs on each surface are consistent with the view that they represent communities of benthic organisms that were buried and cast in place. Model These disc-shaped morphs are the products of casting of the basal impression of a collapsible or hollow

11 GEHLING ET AL.: EDIACARAN BODY FOSSIL 437 TEXT-FIG. 8.Various morphs of Aspidella, from the Fermeuse Formation. A±B, latex cast of eld specimen from south of Cape Race. A, convex morph preserved in negative epirelief, in laminated sand and siltstone; GSC ; 0.8. B, retrodeformation of image A. C±H, specimens with hyporelief preservation from Ferryland. C, type morph with marginal rim; GSC ; 1 5. D, type morph, with partly overfolded invagination; image retrodeformed; GSC ; 1. E, attened preservation on sole of shale with thin sand laminae; three larger at morphs with varying relief; several attened type morphs; note parallel axes of elliptical deformation in all specimens; GSC ; 0 5. F, retrodeformed image of at to convex morphs; note radial grooves; cast by sand penetrating through thinly laminated bed; GSC ; 1. G, cluster of small, at to convex morphs with mutually deformed boundaries; GSC ; 1. H, type morphs showing central invagination with recessed bosses; GSC ; 0 8.

12 438 PALAEONTOLOGY, VOLUME 43 TEXT-FIG. 9. Aggregations of Aspidella on bedding surfaces from Ferryland. A, specimens preserved in positive relief on the sole of 10±15-mm-thick sandstone, having suffered minimal tectonic deformation; mainly clusters of convex morphs with a small numbers of type morphs (thin white arrow, upper right), and one at morph (thick white arrow, centre right); clusters of small round beads (black arrow, centre left); GSC ; 0 8. B, part (left) and counterpart (right) of tectonically deformed specimens in shale-rich beds; mainly type morphs with invaginate centres; some typeconvex morph intermediates with small central boss within invaginate centres (black arrows), and one convex morph (white arrow); GSC ; 0 75.

13 GEHLING ET AL.: EDIACARAN BODY FOSSIL 439 TEXT-FIG. 10. Thin sections cut perpendicular to bedding through specimens of Aspidella from Ferryland; 3. A, sand- lled plug of a convex specimen in mud and silt; note differential compaction of overlying and underlying laminae; GSC B, convex specimen at the base of a graded to parallel-laminated sandy event bed; note the truncation of sand laminae below the parting surface; GSC C, sand casts of two specimens, each truncating and deforming the underlying laminae; GSC D, specimens, at different levels, separated by a at shale lamination, each showing slumping and complex ll by sand during collapse of the Aspidella organism; GSC bulb-shaped organism that was buried beneath successive layers of sediment. As only the basal surface is clearly cast, the structure of the top of the organism is unknown. For simplicity, the model organism will be treated as if it were bulb-shaped (Text- g. 11). Variations in the geometry and ornamentation in these discoidal fossils, with increasing size, are regarded as the products of ontogenetic changes and taphonomic responses in varying micro-sedimentary environments. TAPHONOMY OF ASPIDELLA The restoration of Aspidella requires (in order) tectonic, sedimentary, and biomechanical retrodeformation before the signi cance of these discoidal bodies can be assessed. After removing the effects of tectonic deformation, the morphology of discs can be analyzed to distinguish characters due to sediment compaction from those re ecting the original preserved body structure. Almost all the discoidal forms are found in heterolithic facies of the Fermeuse Formation, varying from wavy and lenticular, thinly bedded sandstone interleaved with silt and shale, to wispy, sand lamination in shale. As mentioned above, disc morphology is strongly but not exclusively facies controlled. Discs exhibit clear outlines and sharp ornamentation rather than the diffuse outlines and irregular geometry that might be expected if they were the products of uid escape or soft-sediment loading. Tectonic deformation On the Avalon Peninsula, the rocks of the Conception and St. John's groups have been affected by two orogenic episodes, one in the latest Proterozoic, Avalonian Orogeny, and the other in the middle

14 440 PALAEONTOLOGY, VOLUME 43 TEXT-FIG. 11. Bulb-shaped model for Aspidella, based on casts of the ventral surface; illustrating relative attening of the body, and prominence of the basal boss, with increasing size of the bulb as growth kept pace with background sedimentation. The dorsal surface is unknown, but for limited evidence of a tubular extension or stem-like attachment (see Text- g. 14). Palaeozoic, Acadian Orogeny (see Landing et al. 1988, and references therein). The latter was responsible for the development of fracture cleavage in rocks of both groups. (The tectonic history of the Avalon Peninsula appears to have involved at least two phases of cleavage deformation in rocks of the Conception and St. John's Group.) Consequently, most bedding surfaces have suffered relative shortening perpendicular to the principal cleavage direction. Cleavage is least apparent in packages of sandstone beds, especially where the clean sand beds are greater than 10 mm thick. Surface structures were least disturbed where the angle between bedding and cleavage was smallest. Conversely, where the intersection approached right angles, most surface structures are dif cult to identify. The formation of slaty cleavage has resulted in differential compaction around wavy and lenticular sand beds in heterolithic facies, and the consequent development of irregular bedding surfaces in shale beds that originally would have been planar. Hofmann (1992), following HsuÈ (1972), con rmed that the oval shape of the type specimens of Aspidella was a function of tectonic deformation and had no biological signi cance. On any surface, the long axes of the ellipsoid specimens are parallel to the intersection between bedding and the principal plane of cleavage (Text- g. 9B). This effect is emphasized by the parallel alignment of the central groove or counterpart ridge in specimens such as the type material (Text- g. 4A). Retrodeformation of images of the disc-bearing surfaces demonstrates that all specimens were originally circular in plan view (Text- g. 8A±B), and that the slit-like centre of the type morphs is largely a product of coincidence of cleavage and radial grooves (Text- g. 8D). Sedimentary compaction The style of deformation of a bulb shaped object within soft sediment depends on the original shape of the bulb, the response of the structural materials, and the particular composition, thickness and order of lamination involved in burial. In the simplest case, a bulb seated with the basal protuberance in clay, suffers burial under a layer of sand at least as thick as the exposed height of the bulb. Collapse or decay of a hypothetical upper surface would allow sand to cast the basal impression in the underlying clay (Text- g. 12A2±D2). In clay rich sediment, loading and dewatering would, in time, produce considerable shortening of the mould relief, resulting in at-morphs. Where the bulb was originally embedded in alternating sandy and clayey laminae, the differential compaction of clay and sand would produce a stepped pro le in the wall of the bulb before collapse and sand casting. That part of the pro le adjacent to sand would be conserved, while the pro le adjacent to clay would suffer attening and shortening, like the walls of a concertina (Text- g. 12A1±D1). Epireliefs of larger, convex discs, preserved by multiple laminae of sand, silt and clay, indicate a history of progressive collapse of the bulb and staged ll of the resulting depression (Text- g. 13). The pro le of a disc, cast by clean uncompressible sand in a single event, would be conserved only when the substrate was suf ciently sandy to hold the mould. An isolated plug of casting sand, formed by collapse and decay of a bulb, surrounded by clayey sediment, would deform the overlying and underlying laminae during the

15 GEHLING ET AL.: EDIACARAN BODY FOSSIL 441 TEXT-FIG. 12. Preservation of Aspidella morphs. A1±D1, sand casting of a bulb embedded within alternating thin sandy laminae and clay. After smothering by sand, the bulb collapses and begins to decay. A stepped pro le in the cast is produced by differential compaction of the substrate. Dewatering and compaction of the clay produces attening of that part of the pro le in contact with the clay; the uncompressible sand conserves portions of the pro le (D1, Ediacaria morph). A2±D2, D3, sand casting of a collapsed bulb, embedded in clay, causes the wall to concertina producing a shallow cast with rugosities (D2, Spriggia morph), or the basal protrusion may invaginate under hydrostatic pressure, during collapse and before complete decay of the organism (D3, Aspidella-type morph). subsequent history of compaction of the surrounding clay. This effect is apparent in thin sections of the button-like variants of the type morph, such as those in Text- gure 8H. The sand plug, formed either as the cast of the basal protuberance of a large specimen or a complete cast of a small specimen, acted as a rivet by puncturing and deforming the laminae above and below the bulb (Text- g. 10). Biomechanical deformation The position and concentration of rugosities on the surface of Aspidella show general concordance for specimens in particular size classes on the same surface. As such, they might either be interpreted as growth lines, for a single recruitment of organisms, or a mechanical response of the body wall to compaction of a convex pro le with changing slope. It is quite likely that growth rings would re ect minor sedimentary events, and thus be almost indistinguishable from collapse rings. They are unlikely to represent internal structures, such as muscle bands, because they are absent on some specimens, and on others they vary in relief and position depending on the sedimentary context. Compaction caused the body

16 442 PALAEONTOLOGY, VOLUME 43 wall to concertina in the zone of maximum slope, at the junction between the boss and the zone of widening, and also at the outer rim. The resultant closely spaced rugosities may even be off-centre due to slightly oblique compression. Thus, the relative positions where rugosities are concentrated give clues to the original pro le of the convex base of the bulb, provided that the in uence of variable compaction due to alternating sand and clay is rst taken into account (Text- g. 12). Some radial ornamentation is found on most specimens. In some convex discs, sharp radial grooves occur on both the outer zone and the boss. More commonly, radial grooves are faint and con ned to the outer zone of at morphs and low convex morphs. In the type morph, radial structures may be strong and regular or absent. Only where radial grooves are most intense, in the deeply recessed base of the type morphs, is there reason to suggest that they represent tension in the integument during invagination. The absence of invagination in specimens more than 25 mm in diameter suggests a change of material strength with size. As a characteristic of the integument, whether under tension or compression, radial structures cannot be explained as stretch marks alone. The sharp, regularly spaced radial grooves appear to be original. The combination of collapse of an organic-walled bulb, differential compaction of both the substrate and the casting medium, followed by cleavage deformation, produced a variety of disc-shaped casts with varying pro les. By demonstrating a gradational series of morphologies, and determining that these were not mutually exclusive, a taphonomic solution has been found for these morphs of Aspidella, and its biogenicity can be assessed from associated evidence. ORGANIC EVIDENCE There is no consistent or convincing inorganic explanation for the preserved suite of discoidal forms. The sharp outlines of discs on sole surfaces and complex casting observed in sectioned specimens are highly atypical of gas or uid escape (cf. Cloud 1960). Mechanical load casts and small-scale soft sediment deformation (cf. Conybeare and Crook 1968) are rare in the heterolithic facies of the Fermeuse Formation, and con ned to sandstone beds at levels where Aspidella is absent. Sectioned specimens show the collapse of laminae into a vacated space, resulting in the casting of the basal mould of an originally three-dimensional object. In addition to the morphology of the discs, several lines of evidence support the interpretation of Aspidella as a body fossil. These include comparison with other fossil assemblages, associations with other fossil taxa and microbial mat textures, and growth characteristics. Comparative morphology. Once the Aspidella morphs have been retrodeformed, they cannot be distinguished from other Ediacaran discoidal taxa, and would be identi ed variously as Ediacaria, Cyclomedusa, Spriggia, Tirasiana, Irridinitus,orPlanomedusites if found in other Ediacaran sites around the globe. The Aspidella plexus of Newfoundland shares key characteristics with discoidal fossils from other continents, which identify them as body fossils rather than pseudofossils. Associations. The serial sets of curved sausage-shaped and bead-like elements in close contact, known variously as Palaeopascichnus, Intrites, Yelovichnus and Neonereites renarius are common on many of the beds where Aspidella is concentrated (Pl. 1). These serial forms have been described from late Neoproterozoic fossil assemblages on the East-European Platform of Russia (Fedonkin 1985), Burin Peninsula of Newfoundland (Narbonne et al. 1987), South Australia (Glaessner 1969, gs 5C±D; Haines 1990; Gehling 1991, pl. 6, g. 2), and Wales (Cope 1983). Yelovichnus was observed (JGG) in the same beds as Ernietta in the Kliphoek Member of the Kuibis Formation in southern Namibia. The interpretation of these serial forms as trace fossils is not consistent with the branching arrangement observed in many examples (Pl. 1, gs 3±4), or the ring-like collapse of elements found in these taxa (Pl. 1, gs 1, 3). The biological af nities of Palaeopascichnus, Intrites and Yelovichnus are likely to be with body fossils of organisms that grew on the substrate. It is unclear whether they are bacterial colonies, algae, egg masses (Text- g. 9A) or even serially arranged budding specimens of Aspidella. The coincidence in style of preservation and co-occurrence on the same bedding planes of these

17 GEHLING ET AL.: EDIACARAN BODY FOSSIL 443 TEXT-FIG. 13. Casting of Aspidella by multiple laminae of sediment, based on epirelief exposure of large discs with partial erosion of laminated ll (see Text- g. 14). Continued collapse of the bulb allows casting by a series of laminae from successive events. characteristic Ediacaran taxa with large numbers of Aspidella requires that the latter cannot be inorganic in the presence of the former. At least two other distinctive body fossil taxa occur with Aspidella. They include two specimens of the tentaculate disc Hiemalora Fedonkin, 1981, which is known from Ediacaran fossil sites worldwide. A new trilobed fossil, Triforillonia costellae gen. et sp. nov., showing typical Ediacara-style preservation, occurs in the same beds as Aspidella in a coastal section at Ferryland (see Systematic Palaeontology). The occurrence of limited numbers of Aspidella (as isolated discs) within the Mistaken Point assemblage, from the underlying formations in the Avalon succession, is another indicator of its biological af nities by association. In the Fermeuse Formation, the low diversity of body fossils may re ect deeper environments than represented by richer Ediacaran assemblages. It may also re ect taphonomic conditions that restricted preservation to casts of embedded, sessile organisms. The volcanic ash beds responsible for the preservation of the more diverse Mistaken Point assemblage, found together with Aspidella in the underlying formations, are all but absent in the Fermeuse Formation. Mat textures. Reticulate bedding surface textures, considered to be the moulds of microbial mats (Gehling 1999), are present on some Aspidella beds (Pl. 1, g. 5). These `elephant skin' surface textures are common on beds with Ediacaran body fossils in north-western Canada (Narbonne and Hofmann 1987; Narbonne and Aitken 1990; Narbonne 1998), Russia (Fedonkin 1992), Australia (Gehling 1999) and most other occurrences of the Ediacara biota world-wide. The texture is associated with an iron oxide or carbonaceous coating. Bacterially-induced mineralization of a sole veneer is suspected as the mechanism responsible for enhancing the preservation of external moulds of soft bodied organisms in the Ediacara biota (Gehling 1999). Mutual deformation. Clusters of larger specimens have mutually deformed boundaries (Text- g. 15), a characteristic of the Ediacaran form-taxon Cyclomedusa in Australia (Wade 1972, pl. 4, g. 1; Gehling

18 444 PALAEONTOLOGY, VOLUME , pl. 5, g. 1) and north-west Canada (Narbonne and Aitken 1990, pl. 1, g. 6). In examples where the apices of each specimen are well separated, it appears that the clusters represented competition for space by several growing individuals that had been well separated as juveniles. Both the geometric regularity of individual specimens of Aspidella, and the mutual contacts between discs, differ signi cantly from those observed for interfering bacterial or microbial mat colonies with concentric growth (see Glaessner 1969, gs 2±3; Gerdes et al. 1994, g. 3d). The clear evidence of physical distortion of individual walls in the clusters of Aspidella (Text- g. 15B) con rms the mechanical integrity of each bulb. Aspidella as a holdfast. The preservation of Aspidella is typical of most other Ediacaran discs in that it provides exquisite documentation of the lower surface of the organisms but little information about its upper surface. However, certain bedding planes in the Fermeuse Formation at Ferryland have several large discs that show an epirelief impression of an associated stem-like structure (Text- g. 14). Two other specimens of Aspidella, preserved in positive hyporelief, were observed on the base of a rare ash layer in the Fermeuse Formation, on the west side of Conception Bay. Each disc had a stem and poorly preserved frond-like extension. Stems are preserved emerging from the upper sides of discs in Ediacaran fossil deposits worldwide, and complete fronds are attached to discoidal holdfasts in the Ediacara biota of Australia (Jenkins and Gehling 1978), the Mistaken Point assemblage (Misra 1969; Seilacher 1992), and the Charnian assemblage of Leicester, England (Ford 1958; Jenkins and Gehling 1978). Even in these assemblages, the casts of discoidal holdfasts are preserved with far greater frequency than the impressions of the frond-like extensions, and especially the complete organism with holdfast, stem and frond. Therefore, by comparison, Aspidella may represent the holdfast of a frond-like organism that is not preserved except for rare traces of the connecting stem or stalk. However, the fact that some larger specimens had stem-like extensions to their upper surfaces does not justify extrapolation to all specimens of Aspidella, and especially the very small ones. TAXONOMIC IMPLICATIONS Synonymy Aspidella terranovica Billings, 1872 was the rst formally described and named Ediacaran body fossil. The judgement of Billings (1872) and Murray (1868) in realizing that the strata bearing Aspidella were pre-cambrian in age, and thus predating the oldest known shelly fossils, has been vindicated despite the doubts expressed in the intervening 130 years. The potential implication of our interpretation of Aspidella in all its morphs as a single taxon is clear. Most of the Ediacaran discoidal taxa described in the last 50 years are almost certainly junior synonyms of Aspidella. However, as Aspidella itself is as yet an incompletely known organism, it is not practical to propose a formal synonymy that may require revision EXPLANATION OF PLATE 1 Other fossils occurring on Aspidella-bearing slabs. Fig. 1. Yelovichnus gracilis Fedonkin, 1985, GSC ; convex hyporelief preservation of three serial sets of arcuate rod-like elements, fragmented in part, and associated with small type and convex morphs of Aspidella. Collapsed elements on one set (black arrow), and a chain of collapsed bead-like bodies (white arrow, GSC ) resemble Intrites punctatus Fedonkin, 1980a; Fig. 2. Neonereites renarius Fedonkin, 1980a, GSC ; convex hyporelief, chains of beads within clusters of small, convex morphs of Aspidella, GSC ; 0 5. Fig. 3. Palaeopascichnus delicatus Palij, 1976, GSC ; convex hyporelief, serial set of arcuate, to straight, short, rod-like elements, some collapsed; 1. Fig. 4. Undescribed chains of deformed bead-like bodies (arrows, GSC ) with Aspidella, GSC ; 0 5. Fig. 5. Bed sole reticulate network known as `elephant skin' texture; interpreted as cast of microbial mat (Gehling 1999); fossiliferous facies of the Fermeuse Formation, Ferryland; GSC ; 1.

19 GEHLING et al., other fossils PLATE 1

20 446 PALAEONTOLOGY, VOLUME 43 TEXT-FIG. 14. Field photographs (1 km south of Ferryland) of negative epireliefs of two discs, lled with multiple laminae of sand and silt, showing traces of a stem-like structure aligned with the centre of the disc in each case. A±B, original surface with overlay sketch representing three laminae exposed as ll of cast; the stem was cast by two laminae; disc diameter 85 mm; stem 15 mm wide, and at least 160 mm long from the margin of the disc. C, stem preserved in negative epirelief and disc cast with multiple laminae; stem 30 mm wide and 220 mm long; disc diameter approximately 90 mm. after future discoveries are made. Instead we recommend that in future systematic descriptions of discoidal fossils of the Aspidella plexus, authors refrain from using new names, especially where specimens are of unknown orientation, few in number, or poorly preserved. The commonly used names of junior synonyms should be retained as form-genera to represent certain styles of preservation. Of the many Ediacaran discoidal taxa, those that conform to the plexus of forms encompassed by Aspidella are tabulated as probable synonyms (Table 1). The three end members of the Aspidella plexus (Text- gs 5±6): `Aspidella' for invaginate morphs, `Spriggia' for at, annulate morphs, and `Ediacaria' for convex morphs, represent the most parsimonious form-genera under which Ediacaran discoidal fossils can be accommodated. These names carry no implied interpretation of the biological origins or af liations of discs, unlike such genera as Cyclomedusa and Charniodiscus. The original descriptions of Ediacaria, Beltanella, Cyclomedusa, Tateana, and Madigania by Sprigg (1947, 1949) were based on one to three specimens of each taxon, and their diagnoses expressed in terms that assumed they were either scyphozoans or hydrozoans. After more material was collected, Glaessner and Wade (1966) demonstrated that some of Sprigg's genera were junior synonyms of Ediacaria and Cyclomedusa. At the same time, they introduced new genera, such as Medusinites, and continued to interpret discoidal fossils as the aboral impressions of medusoids with unknown oral sides. As more discoidal fossils have been described from Ediacaran localities around the globe, new taxa have been erected to account for the gradations between previously named genera, with consequent claims for increased diversity. Jenkins (1989), in attempting to revise interpretations of some discoidal forms, suggested that most discoidal taxa were benthic polyps. Since then, Ediacaria, Cyclomedusa, and TEXT-FIG. 15. Clusters of convex morphs with mutually deformed boundaries. A, solitary convex morph and cluster of 5 more attened specimens; GSC ; 0 7. B, cluster of at-convex discs with imaginary section X-Y; GSC ; 1 2. C, reconstruction of X-Y sectional view of two crowded bulbs, embedded in a clayey substrate. D, sand cast formed after collapse and burial with counterpart cast in shale.

21 GEHLING ET AL.: EDIACARAN BODY FOSSIL 447

22 448 PALAEONTOLOGY, VOLUME 43 TABLE 1. Probable junior synonyms of Aspidella terranovica Billings, 1872.?1933 Paramedusium africanum GuÈrich Namibia at morph 1947 Ediacaria indersi Sprigg South Australia convex morph 1947 Beltanella gilesi Sprigg South Australia convex morph 1947 Cyclomedusa davidi Sprigg South Australia convex morph 1949 Protodipleurosoma wardi Sprigg South Australia at-type morph 1949 Tateana in ata Sprigg South Australia convex morph 1949 Cyclomedusa radiata Sprigg South Australia at-convex morph 1949 Cyclomedusa gigantea Sprigg South Australia convex morph 1949 Madigania annulata Sprigg South Australia at morph 1966 Cyclomedusa plana Glaessner and Wade South Australia at morph 1972 Planomedusites patellaris Sokolov Ukraine at morph 1972 Medusinites patellaris Sokolov Ukraine at morph 1976 Tirasiana disciformis Palij Ukraine convex morph 1976 Tirasiana coniformis Palij Ukraine convex morph 1977 Tirasiana concentralis Bekker Russia, Ural Mts convex morph 1980b Paliella patelliformis Fedonkin Russia, White Sea at morph 1980b Protodipleurosoma rugulosum Fedonkin Russia, White Sea type morph 1981 Cyclomedusa minima Fedonkin Russia, White Sea type-convex morph 1981 Cyclomedusa delicata Fedonkin Russia, White Sea at-convex morph 1983 Irridinitus multiradiatus Fedonkin Ukraine type morph 1986 Spriggia wadea Sun South Australia at morph 1987 Vendella larini Gureev Ukraine type morph 1987 Glaessneria imperfecta Gureev Ukraine at morph 1988 Jampolium wyrzhykoowskii Gureev Ukraine at morph (in Ryabenko et al. 1988). Medusinites and Spriggia have been the generic names most commonly assigned to Ediacaran discoidal fossils in north-western Canada, Australia and Finnmark. Discoidal fossils from the East-European Platform include a number of distinctive taxa that are probably junior synonyms of Aspidella (in the strict sense) as well as being encompassed within the Aspidella-type form genus. Irridinitus multiradiatus Fedonkin, 1983 and Vendella larini Gureev, 1987, from the Ukraine, share radial grooves converging on an invagination within a disc that has a rim standing out in positive hyporelief. Organic connections In view of the apparent association of stem-like impressions with some epirelief casts of discs in the Fermeuse Formation, the possibility that Aspidella may be the holdfast of a larger frond-like organism presents certain potential problems of priority and identi cation. While Aspidella has priority, for taphonomic reasons, it does not appear to preserve characters that would enable the larger organism to be identi ed. The preserved discoidal holdfasts of Charniodiscus and Charnia show considerable variability in form and do not appear to be suf ciently distinctive to enable identi cation of the frond taxon without the presence of the frond. However, even where preservation is poor, frondose genera such as Charniodiscus and Charnia are still clearly identi able without the discoidal holdfast. Consequently, where discs and fronds are preserved in structural continuity, the whole organism has been referred to the frond genus name rather than the disc genus name (Jenkins and Gehling 1978). Where there is no clear evidence of a structural connection with another named organism, discoidal fossils should be referred to existing form-genera. As such, they represent preservational morphologies that may or may not imply differences in physiology, but in many cases are likely to be morphological variants of the same species of organism. The exceptions are the structurally distinctive discoidal genera,

23 GEHLING ET AL.: EDIACARAN BODY FOSSIL 449 such as Eoporpita and Hiemalora, where tentacle- or root-like elements extend beyond the margin of the disc. As demonstrated for Aspidella from Newfoundland, the variable morphologies of Ediacaran discoidal fossils in assemblages from other continents probably re ect body size and sedimentary context more than generic differences. It follows that estimates of diversity based on named Ediacaran discoidal fossils, described from small numbers of specimens, should be regarded with caution. In particular, form-genera should not be used for palaeoecological analysis of communities, such as cluster analysis. PALAEOBIOLOGY Size distribution Inspection of bed soles and cross-sections in the fossiliferous facies shows that tiny specimens of Aspidella are overwhelmingly more common than larger specimens (Text- g. 9). A top surface, exhibiting a wide range of specimen sizes, was chosen as representing a census-preservation. In bed-section, the specimens in the larger size classes of the distribution could be seen to have originated at laminations a few millimetres below. However, they are considered to have been part of the same community prior to the event that preserved them. Ghosted outlines of small specimens were interpreted as probable print-through impressions of discs from overlying laminae, and excluded. The resultant distribution is strongly skewed toward small specimens (Text- g. 16). Juvenile specimens of Aspidella were likely to have been smothered by anything more than one or two millimetres of sediment. The few large individuals were those that survived previous smothering events to compete for space with subsequent spat-falls of juveniles. Thus the timing and scale of sedimentary events may have determined the modal size of specimens. If the frequency of sharply based, sandy event laminae is taken as a defacto measure of storm frequency, then thicker mud accumulation represented longer periods of background sedimentation. It is notable that the rare surfaces with numerous larger specimens (for example, Text- g. 15A) were moulded by substrates composed of relatively thick clay and silt. Well-spaced juveniles at one level that survived to grow to several centimetres in diameter eventually competed for space as epibenthic dwellers at a higher level (Text- g. 16). Once a specimen survived to be 15 mm or more in diameter, it was more resistant to smothering by frequent, small, sandy depositional events. Although Conway Morris (1989) presented similar data from the Fermeuse Formation he considered the skewing of the distribution toward small specimens as unrepresentative of animals, but did not elaborate. This kind of distribution is expected for a census of a community of sessile organisms subject to periodic sediment smothering. It resembles that of published benthic communities with high juvenile mortality (Levinton and Bambach 1970; Parker 1975). Aspidella biomass The density of specimens on the surveyed surface approached 1000 per m 2, other surfaces near Ferryland reached estimated population densities of 3000±4000 individuals per m 2 (e.g. Text- gs 8H, 9A±B). Such numbers of specimens equal some of the most densely populated benthic communities of invertebrates, algae and angiosperms known in modern shallow marine environments. Arguably, in the absence of predators, body-space and the effects of storm action were the main limiting factors to the accumulation of benthic biomass within macro-organisms. The existence of large numbers of closely spaced specimens of Ediacaran organisms seems to have been common in a number of assemblages around the globe. Beltanellifomis (`Hagenetta aarensis' Hahn and P ug, 1988) in the Kuibis Formation of Namibia, the Ukraine (`Medusinities palij' Gureev, 1987, in Ryabenko et al. 1988, pl. 8, gs 1±3), and in the Windermere Supergroup of north-western Canada (Narbonne and Hofmann 1987, pl. 75), Pteridinium (Crimes and Fedonkin 1996, pls 1±2), and Ernietta in the Kuibis Formation of Namibia (Dzik 1999), are examples of closely packed Ediacaran organisms preserved in late Neoproterozoic, shallow marine sandstone. In ecosystems where trophic activity by macro-organisms may have been limited to primary production, lter feeding and detritus feeding, high population densities should be expected where environmental conditions were suitable for growth. The diversity associated with the `Cambrian explosion' probably

24 450 PALAEONTOLOGY, VOLUME Frequency % N = Diameter Classes (mm) TEXT-FIG. 16. Histogram showing a frequency distribution of size classes of Aspidella specimens from an area of 0 28 m 2, on the top surface of a bed (inset) from a coastal exposure of the Fermeuse Formation, 2 km south of Ferryland. Specimens less than 10 mm in diameter may represent recruitment on the census surface; those less than 2 mm are rare or too small to measure; well spaced specimens, larger than 10 mm, probably represent survivors, recruited at a lower level. re ects the evolution of predators and the consequent selection for new niches, and limitation of saturation by single species of macro-organisms. CONCLUSIONS The fact that a common Precambrian fossil, named in 1872 from outcrops on the main street of a provincial capital, has languished as a `pseudofossil' for more than a century, provides valuable insights to the problems of Precambrian palaeobiology. Discoidal shapes can have a variety of origins. Our evaluation of the organic origin of Aspidella is based on the study of thousands of specimens for their taphonomy, distribution on bedding surfaces, size range, mutual contacts and associations with other recognized Ediacaran body fossil taxa. Clear similarities of form and preservation with discoidal taxa, from all the main Ediacaran assemblages around the globe, con rm the conclusion that Aspidella is a body fossil with a range of morphologies produced as a consequence of body size and sedimentary conditions. The fossil assemblage described by Cope (1977, 1983), from South Wales, is remarkably similar to that of the Aspidella assemblage of Newfoundland. The Welsh assemblage not only exhibits a similar range of discoidal morphs, but includes well-preserved examples of Palaeopascichnus (Cope 1983, pl. 2, g. 2), and un gured specimens of Yelovichnus and the tentaculate disc Hiemalora. The putative trace fossils gured by Cope (1983, pl. 2, gs 3±4) are extremely rare, and likely to be fragments of tubular fossils rather than furrow traces. Farmer et al. (1992) described a very similar assemblage of Ediacaran discoidal fossils, including specimens of Hiemalora, from the Innerelv Member of the Stappogiedde Formation in north-eastern Finnmark. However, they demonstrated that associated vertical tubes in the Innerelv Member, previously attributed to the ichnotaxa Skolithos and Arenicolites (Banks 1970), were dewatering pillars. Planolites, the earliest recognized trace fossil in the succession, occurs in the overlying Manndraperelv Member, succeeded by Treptichnus (Phycodes) pedum in the Brevik Formation (Farmer et al. 1992). The absence of clearly identi able trace fossils in the disc-bearing

25 GEHLING ET AL.: EDIACARAN BODY FOSSIL 451 strata in both South Wales and Finnmark is consistent with our observations in the Fermeuse Formation. Excluding Palaeopascichnus as a probable body fossil, the absence of unequivocal trace fossils in the older Ediacaran fossil-bearing formations of Newfoundland (Conception Group and St. John's Group), South Wales, and the Sheepbed Formation of north-western Canada, may indicate an earlier Ediacaran stage, before the evolution of benthic trace-making animals in the latest Neoproterozoic. The record of numerous trace fossils, in the form of non-branching, levee-bordered furrows, showing random paths and crossings, occurs in the Blue ower Formation of north-western Canada (Narbonne and Aitken 1990) and the Rawnsley Quartzite in South Australia (Glaessner 1969; Jenkins 1995), where they represent the rst unequivocal evidence of locomotion and sediment processing by animals on the sea oor. SYSTEMATIC PALAEONTOLOGY Class unknown Genus TRIFORILLONIA gen. nov. Derivation of name. Genus name is a compound of the French pre x `tri', and `Forillon', the French cartographer's name for the rocky point that became the settlement of Ferryland, the type locality. The species name (below) refers to the costellae present on all specimens, and also honours our hosts, Aidan Costello and family, who trace their ancestry to the rst permanent European settlers on the Avalon Peninsula. Type species. Triforillonia costellae sp. nov. Diagnosis. As for type species. Triforillonia costellae gen. et sp. nov. Text- gure 17A±H Diagnosis. A three lobed body, in positive hyporelief, with rounded spatulate lobes, radiating at equal angles from a central rosette or tripartite invagination; margins of lobes smooth with slightly raised, unmarked border; surface of lobes slightly raised, bearing uneven costellae parallel to lobe axis, stopping inside marginal rim. Body outline consists of three semicircles meeting at right angles at the join of the lobes. Holotype. GSC (Text- g. 17A±D). Material. Ten complete specimens preserved in positive hyporelief, including six also preserved as counterparts in negative epirelief. All specimens are preserved on facing partings of a split slab that has undergone cleavage shortening; slab is composed of shale and thinly laminated sand. Descriptions were made from the natural casts, in positive hyporelief. Dimensions. After retrodeformation, average body diameter is 20±26 mm, being composed of three sub-equal lobes 10±13 mm long by 8±11 mm wide. Description. The holotype and one paratype (Text- g. 17A, G2) are almost identical in outline and surface morphology; other paratypes vary in having unequal lobes or the depth of the central tripartate invagination. When retrodeformed, such that small specimens of Aspidella on the same surface are circular, the angles between lobe axes approximate 120 deg. (Text- g. 17C). The size and shape of lobes are not invariably equal in the paratypes. Two specimens (Text- g. 17E, G1) have relatively acute terminations on the lobes and inter-lobe angles that are obtuse, consistent with compression and escape of part of the soft body into the overlying sediment.

26 452 PALAEONTOLOGY, VOLUME 43 TEXT-FIG. 17. Triforillonia costellae gen. et sp. nov., from coastal section in the Fermeuse Formation, parallel to the main road, Ferryland (Avalon Peninsula), Newfoundland. A, holotype (GSC ). B, counterpart of holotype in negative epirelief. C, retrodeformed image of the holotype. D, sketch of the holotype. E, paratype (GSC ). F, paratype (GSC ). G±H, pair of paratypes and sketch (GSC , ). A±F, 1. G±H, Comparisons and interpretation. T. costellae shares three-fold symmetry with four other Ediacaran taxa: Tribrachidium heraldicum Glaessner, 1959, Skinnera brooksi Wade, 1969, Anafesta stankoviskii Fedonkin, 1984, and Albumeres brunsae Fedonkin (Keller and Fedonkin 1976). However, in the cases of Tribrachidium, and Anafesta, the outlines are composites of three lobes. Tribrachidium is distinguished by spiralling arms. The three petal-like lobes in Anafesta meet as ridges, but are never as lobate as in Triforillonia. The costae in the lobes of Triforillonia do not appear to bifurcate as they do in the other Ediacaran taxa with three-fold symmetry. The style of preservation, lobes, and ornamentation of Triforillonia resemble those of Inaria karli Gehling, However, Inaria has a dorsal tubular extension, and varies from circular to lobate with eight or more lobes, but has no trace of tripartite symmetry. Even though Triforillonia is not preserved with evidence of a dorsal opening, it appears to have been a soft, sacklike body with the dorsal surface extruded into the overlying sediment. Evidence of whether it was the holdfast of a larger organism or perhaps a polyp-like form must await discovery of more material. The taphonomic analysis of Aspidella demonstrates the dif culty of assigning particular structures to original body geometry, let alone regarding them as representing internal organs. Acknowledgements. Research was supported by an NSERC (Natural Sciences and Engineering Research Council of Canada) research grant to GMN, and a William E. White Fellowship to JGG. A preliminary study by JGG was nanced by A. Seilacher, from his 1992 Crafoord Prize. R. W. Dalrymple assisted with eldwork and provided helpful comments on the sedimentology. We thank B. Bland for drawing our attention to the variety of discoidal forms on the Avalon Peninsula. Access to the Welsh assemblage of Ediacaran fossils, in the Cardiff Museum, was kindly provided by J. C. W. Cope. We are grateful to the people of Ferryland, Renews, Portugal Cove South, Trepassey, and other communities on the Avalon Peninsula for their interest and encouragement. S. Conway Morris and J. Dzik are thanked for their advice in review. REFERENCES ANDERSON, M. A. and MISRA, S. B Fossils found in the Precambrian Conception Group in southeastern Newfoundland. Nature, 220, 680±681. ÐÐ and KING, A. F Precambrian tillites of the Conception Group on the Avalon Peninsula, southeastern