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This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further information
regarding Elsevier’s archiving and manuscript policies areencouraged to visit:
journa l homepage: www.e lsev ier .com/ locate /precamres
The ‘string of beads’ fossil (Horodyskia) in the mid-Proterozoic of Tasmania
Clive R. Calvera,∗, Kathleen Greyb, Martin Laanc
a Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania 7018, Australiab Geological Survey of Western Australia, Department of Mines and Petroleum, 100 Plain Street, East Perth, WA 6004, Australiac PO Box 428, Smithton, Tasmania 7330, Australia
a r t i c l e i n f o
Article history:Received 23 September 2009Received in revised form 22 January 2010Accepted 4 February 2010
Keywords:HorodyskiaTasmaniaString of beads fossilRocky Cape GroupMesoproterozoic
a b s t r a c t
Horodyskia has been found at a single Tasmanian locality, in the Cassiterite Creek Quartzite (ca.1300–800 Ma), part of a thick Proterozoic, mildly deformed, low greenschist facies, marine shelfal silici-clastic succession known as the Rocky Cape Group. The rock hosting the fossils is thinly interbedded andinterlaminated dark grey slaty shale and quartzose siltstone. The sharp-based, graded siltstone layers areinterpreted as distal storm surge deposits on an outer marine shelf. The ‘strings of beads’ are mostly pre-served at the base of the siltstone layers, in concave hyporelief (external moulds) on the soles of the eventbeds, and as convex epirelief (casts) on the tops of the underlying shale beds. The casts comprise shaleidentical to the underlying bed. The beads average 1.7 mm in diameter, and the gap between the bordersof adjacent beads tends to be approximately equal to the bead diameter. Occasionally, the fossils arepreserved within shale, as wholly flattened beads delineated by a subtle darkened halo. The Tasmanian‘strings of beads’ have most of the morphological attributes of previously described Horodyskia, includ-ing regularity of size and spacing of beads in any one string, lack of branching, and in some instances,‘haloes’ and casts with apical depressions. The strings on at least one bedding plane have a strong N–Spreferred orientation of unknown origin. Tectonic deformation has resulted in 30% shortening in a SW–NEdirection. The morphologic similarity, but differing mode of preservation of the Tasmanian Horodyskiato the two previously described Mesoproterozoic species is strong evidence for a biologic origin for thestring of beads phenomenon. After morphological and morphometric comparisons with other species ofHorodyskia, the Tasmanian specimens are assigned to Horodyskia williamsii.
Horodyski (1982) was the first to describe a distinctive varietyof serial bedding plane markings in Mesoproterozoic sedimentaryrocks from Montana. The features, aptly named ‘strings of beads’,were considered by him to be of problematic origin. The markingsexhibited a striking regularity within a string, both of bead dimen-sions and the distance between adjacent beads. Similar structureswere described from the late Mesoproterozoic Bangemall Super-group of Western Australia by Grey and Williams (1990). Yochelsonand Fedonkin (2000) applied Linnaean nomenclature (Horodyskiamoniliformis) to the examples from Montana, and further detaileddescription of this material was given by Fedonkin and Yochelson(2002). The Western Australian examples are named Horodyskiawilliamsii by Grey et al. (2010), and details of their taphonomyand depositional environment were given by Martin (2004). Athird species, Horodyskia minor, was described from the upper Edi-
acaran of South China by Dong et al. (2008). This species is unusualin being preserved three-dimensionally in chert, and in its muchsmaller size and younger age than the two previously describedforms.
A biogenic origin for the ‘string of beads’ is now generallyaccepted, although biological affinities are still a matter of conjec-ture (Knoll et al., 2006). Grey and Williams (1990) concluded thestrings were most likely to be fossil phaeophytes (brown algae).Fedonkin and Yochelson (2002) suggested they were multicellular,tissue-grade, colonial eukaryotes, probably autotrophic. A spongalaffinity was suggested by Hofmann (2001), while Dong et al. (2008)suggested a comparison with uniseriate agglutinated foraminifers.The biogenicity and affinities of Horodyskia have been extensivelydiscussed by Grey et al. (2010) and previous workers, and will notbe further treated here.
The history of discovery of the Tasmanian Horodyskia began inthe mid 1990s when one of the authors (M. Laan) collected someslabs of Proterozoic slate with intriguing bedding plane impressionsthat he featured as flagstones in a concrete floor. On learning of the‘strings of beads’ fossils in discussions with Tasmanian governmentgeologists in early 2006, he sent photographs of the flagstones toC.R. Calver who recognised the fossils, having been shown the West
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Australian Horodyskia by K. Grey the previous year. The identifica-tion was confirmed by Grey, and Laan and Calver then relocated thesource of the fossiliferous slabs and collected additional material,described here.
In this paper, the Tasmanian examples are described and com-pared with the three previously named species. The Tasmanian‘strings of beads’ are morphologically similar to H. williamsii and H.moniliformis, although their mode of preservation is somewhat dif-ferent. They may be of similar age to H. williamsii, but are youngerthan H. moniliformis. We tentatively assign them to H. williamsii.Only one Tasmanian locality is so far known, although the fossilsare found in great profusion on some bedding planes. We usedGIS (Geographic Information Systems) software to spatially anal-yse the Tasmanian material, which has undergone a slight tectonicdistortion.
Figured specimens, numbered Z3701–Z3706, are lodged at theTasmanian Museum and Art Gallery, Hobart.
2. Geological setting and age
The single Tasmanian Horodyskia locality is in the CassiteriteCreek Quartzite, a formation in the Balfour Subgroup, in the lowerpart of the Rocky Cape Group, the oldest known succession innorthwest Tasmania (Figs. 1 and 2). The Rocky Cape Group consistsalmost entirely of siltstone, mudstone and quartzarenite, in totalover 10 km thick, deposited in shelfal environments ranging fromtidal flat to offshore below wavebase (Fig. 2; Gee, 1968; Everard etal., 2007). It is moderately deformed and weakly metamorphosed(lower greenschist facies). Age constraints are poor. The Rocky CapeGroup is thought to be younger than the Surprise Bay Formation (ca.1300 Ma) of King Island, although no stratigraphic contact is knownbetween the two units (Black et al., 2004). The Rocky Cape Groupis unconformably overlain by the Cryogenian to lower CambrianTogari Group (Calver, 1998; Everard et al., 2007). The lowermostpart of the Togari Group consists of a probable equivalent of the
Fig. 1. Generalised geology of north-west Tasmania, with ‘string of beads’ localityshown (star).
Fig. 2. Generalised stratigraphic column and palaeoenvironments of the Rocky CapeGroup. Star indicates stratigraphic position of Horodyskia.
Burra Group of South Australia (ca. 800–700 Ma: Calver, 1998;Hill and Walter, 2000). The youngest detrital zircon populationin the Jacob Quartzite (the youngest formation in the Rocky CapeGroup) is 1000 Ma, so this unit is early Neoproterozoic (1000–ca.800 Ma: Black et al., 2004). However, the Cassiterite Creek Quartzitecould be significantly older, given the great stratigraphic thick-ness separating it from the Jacob Quartzite (Fig. 2). Current ageconstraints therefore suggest that the Tasmanian Horodyskia –bearing horizon is late Mesoproterozoic to early Neoproterozoic(1300–800 Ma).
The Cassiterite Creek Quartzite, in its type section near Balfour,is described as interbedded quartzarenite, laminated siltstone andcarbonaceous shale, about 350 m thick. The formation thickens andincorporates a greater proportion of siltstone, and less quartzaren-ite, north of the type section (Everard et al., 2007).
The Blackwater Road Quarry, where Horodyskia is found 10 kmnorth-west of Balfour, exposes some 50 m stratigraphic thicknessof fresh (unweathered), thinly interbedded and interlaminated,quartz siltstone and dark grey, slaty shale, mapped as CassiteriteCreek Quartzite (Everard et al., 2003). Bedding dips and faces con-
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Fig. 3. Outcrop of interlaminated siltstone and shale in the Blackwater Road Quarry.
sistently north-east, and a weak tectonic cleavage (apparent only inthe shale layers) dips steeply to the WSW. The quartz siltstone lay-ers, 1–50 mm thick, have planar or gently undulose, sharp lowercontacts, and grade up into dark grey shale (Fig. 3). No cross-lamination is apparent. Rarely, soles of siltstone layers exhibit flutecasts and prod casts. Through most of the section, the rock is lam-inated or thinly laminated (siltstone-shale couplets <10 mm), andsiltstone is volumetrically subordinate. In thin section, siltstone lay-ers are dominantly quartz, <50 �m grainsize, with minor detritalwhite mica, and grade up into shale (pelite) composed of authi-genic, very fine-grained sericite, minor fine quartz silt and pyrite.The sericite is oriented parallel to bedding, comprising a weak, slatycleavage, and there is a weak, oblique crenulation cleavage. Thebedding-parallel slaty cleavage may be an overburden fabric, butmay also be partly of tectonic origin (S1), while the crenulationcleavage is the regional S3 of Devonian age (Everard et al., 2007). Inthe lowest 15 m of the quarry section, layering tends to be thicker,with couplets 10–100 mm thick, and siltstone and shale are sube-qual in proportion. Horodyskia appears to be more common in thislower unit.
Shale laminae were evidently deposited from suspension,and interrupted periodically by weak, waning bottom currentsand silt deposition. The likely palaeoenvironment is mid-shelf,below storm wave base but within reach of periodic storm surgecurrents.
3. Materials and methods
About 15 fossiliferous slabs were collected, from outcrop andloose boulders. Several slabs comprise matching part and counter-part. Several thin sections were made to examine the sediments andsearch for internal bead structure. Photography was undertakenwith low-angle incident light on dry slabs.
Spatial analysis was undertaken on one loose slab, an upper bed-ding plane surface with a particular abundance of the fossils, asfollows. A 300 × 500 mm area was photographed at high resolu-tion, with low-angle incident light. The digital image was importedinto ArcGIS 8.0 and given an arbitrary (but correctly scaled) datum,and a selection of the strings of beads was digitised as a polygonshapefile of bead outlines (Fig. 4). Not all strings were digitised, butan attempt was made to digitise strings of the full range of sizes andorientations. A total 1019 beads belonging to 151 strings were digi-tised. The area of each polygon, and the co-ordinates of its centroid,were calculated using ArcGIS functionality. The calculated diame-ter of each bead is taken to be that of a circle of the same area,i.e. diameter = 2
√(area/�). The distances between the centroids
of each adjacent pair of beads on the same string, and the orien-
Fig. 4. Part of digitised array of strings of beads.
tations of the lines joining adjacent centroids, were calculated in aspreadsheet from the co-ordinates of the centroids.
Graded lamination in this slab and its matching counterpartallowed facing to be determined. Because bedding and the weak,inclined S3 cleavage remain roughly constant in orientation in thequarry, the slab can be given its correct outcrop orientation andrestored to horizontal using the standard stereographic method(Ragan, 1973, p. 100), so that directional features can be relatedto present-day north (Figs. 4 and 11).
4. Description
The strings of beads are usually preserved at the surface betweenan underlying shale lamina and an overlying, graded lamina or thinbed (1–10 mm thick) of quartz siltstone. The rock tends to spliteasily along these bedding planes, although the great majority ofsuch surfaces are unfossiliferous. Abundance varies from isolatedsingle strings (Fig. 7) to a great profusion (Figs. 5 and 6). Thereis no obvious reason why only some horizons are fossiliferous,although it is possible that preservation may be favoured by rel-atively thicker (ca. >5 mm) overlying siltstone layers. The beads atshale-siltstone contacts are preserved in convex epirelief on upperbedding plane surfaces (Figs. 5A and 6), and as pits (moulds) in con-cave hyporelief on the counterpart sole of the overlying siltstonebed (Fig. 5B). On the bedding plane surfaces the beads are low sub-rounded hemispheres, the larger ones tending to be flat-topped,roughly 0.2–1 mm in height, and tend to be subcircular to gentlyovoid, rarely more irregular, in outline. Some strings have beads instrong relief; in others the beads are barely perceptible. In somecases, splitting of the rock at a slightly deeper level exposes beadsof lower relief (Fig. 6B). Diameters, as calculated above, range from0.6 mm to a maximum 4.3 mm (mode 1.6 mm, average 1.7 mm,n = 1019).
The typical slightly elliptical outlines of the beads are probablythe result of tectonic distortion of originally circular outlines. A fewmore irregular outlines on the digitised slab appear to be the resultof beads fortuitously touching or overlapping. Some beads have acentral depression or dimple. These are relatively few in number,but tend to be present in several adjacent beads in a string, or inan entire string (Figs. 5A and 7). The material comprising the beadsappears to be pelite identical to the adjacent rock. No composi-tional difference or internal structure is apparent in thin sections,which show the beads to be merely topographic prominences onthe bedding plane.
On the counterpart of the bedding plane surfaces (the sole of theoverlying siltstone layer) the impressions (external moulds) faith-fully reflect the shape of the underlying beads, including, wherepresent, the central depression (preserved as a small prominence
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Fig. 5. (A) Upper bedding plane surface with beads preserved in positive relief.Illumination from bottom right. Note string at top centre in which beads possesscentral pits. Sample Z3701. (B) Corresponding sole of overlying bed (counterpart)with beads preserved as molds; illumination from top left. Sample Z3702.
on the counterpart). There is no evidence for a void or former spacefilled with a secondary mineral. Nothing resembling a thin filamentlinking adjacent beads was noted in any of the Tasmanian material.No current crescents (strongly developed in some of the West Aus-tralian material) were seen. No convincing examples of holdfastsor of branching of strings were observed.
The distance between adjacent bead centroids varies from 1.3 to8.0 mm (average 3.2 mm, n = 868). In any one string there is a looserelationship between distances and diameters such that distancesbetween bead centroids tend to be approximately twice the beaddiameter; in other words, the gap between the borders of adjacentbeads tends to approximate the bead diameter (Fig. 10). There is noconsistent tendency for beads to become larger or smaller along astring.
A few strings – notably those on one particular, sparsely popu-lated slab – show a flat, slightly depressed border or halo extendingaround the string, to a distance of some 2 mm from the margins ofthe beads (Fig. 7A and B). On sole surfaces, if present, the haloescomprise slightly raised rims around bead moulds – a feature com-mon in the West Australian H. williamsii (Grey et al., 2010). Possiblyequivalent structures are seen in some of the Montanan material,but are there ascribed to scour-and-fill (e.g. Fig. 11c of Fedonkinand Yochelson, 2002).
Fig. 6. (A) Upper bedding plane surface with beads preserved in positive relief. Illu-mination from top. Sample Z3703. (B) Detail of two strings. Low relief of some beadsin middle of larger string, left of centre, due to flaking away of shale exposing slightlydeeper surface. Sample Z3704.
The maximum observed number of beads in a string on the digi-tised slab was 36 (this string was 157 mm long). A poorly preserved,longer string, not collected, had approximately 63 beads in a totallength of 250 mm (Fig. 7C). Strings are gently curved or sinuousin plan view; longer ones may be U-shaped. On the digitised slabthe strings have an obvious preferred orientation (Fig. 4) which canbe shown as a rose diagram of the frequency versus orientation ofthe line segments joining adjacent pairs of beads (Fig. 11A). Theorigin of this strong N–S orientation is uncertain. It is not due totectonic distortion, as the bedding-cleavage intersection lineationtrends NW. No palaeocurrent indicators are discernible on the bed-ding surface or in the overlying siltstone layer. Two siltstone bedswere observed with basal flutes and tool marks that show NW-directed palaeocurrents, but as these surfaces lack Horodyskia, noinferences can be drawn regarding current influence on orientation.
A minority of fossils display a style of preservation that is dif-ferent from the samples described above. In this case the exposedbedding plane is within a shale lamina, not at the shale-siltstoneinterface. The bedding plane surface is flat and the beads lack anyrelief. The beads are visible on both part and counterpart as ovoidpatches, about 2 mm in diameter, delineated by a diffuse, slightlydarker halo 1–2 mm wide (Fig. 8). The differentiation of the beadsand haloes from the surrounding surface is subtle, and often onlyvisible under certain angles of illumination. No obvious mineralog-ical or compositional variation between beads, halo and matrix isvisible under the binocular microscope. The beads are more closelyspaced than normal, with inter-bead gaps considerably less thanbead diameter. This style of preservation (flat bead surface, darkhalo) appears similar to some of the H. moniliformis from ApikuniMountain, Montana (Fedonkin and Yochelson, 2002), except thatthe Tasmanian beads are closer together and not notably siliceous.
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Fig. 7. (A) Beads on upper bedding plane surface, showing central depressions andslightly depressed border area (‘halo’) surrounding beads. Sample Z3705. (B) Detailof A. (C) Longest observed string (between arrows) (Coin is 28 mm in diameter).
Fig. 8. String of beads preserved as subtle colour variation in grey shale. Wettedslab with high-angle illumination. Sample Z3706.
5. Tectonic overprint
The weak S3 crenulation cleavage is inclined at about 50◦ tobedding and is also evident as a faint lineation on bedding planes(S0/S3 lineation, Figs. 4 and 11). Tectonic strain in the bedding planewas quantified by analysing the shapes of the beads in outline, andthe distances between bead centroids.
The digitised outlines of 130 beads are superimposed on a com-mon centroid in Fig. 11B, while maintaining their orientations. A‘best-fit’ ellipse, estimated by eye, has an axial ratio of 0.7:1 andits long axis almost exactly parallel to the northwesterly trendingS0/S3 lineation (Fig. 11B). Making the (reasonable) assumptions ofno competence contrast between the beads and the surroundingrock, and that the bead outlines were originally subcircular, thisshould reflect the tectonic strain ellipse in the bedding plane.
Deformation should also have the effect of decreasing the dis-tance between adjacent beads, if the line joining them is nearnormal to the S0/S3 lineation, relative to distance between beadpairs aligned subparallel to the S0/S3 lineation. Distances werebinned in 10◦ sectors, averaged, and plotted as a rose diagram(Fig. 11C). The resultant, roughly elliptical distribution has an axialratio of 0.7:1 and a long axis between north and the S0/S3 lineation.This is likely to approximate the tectonic strain ellipse, and hasthe same axial ratio to that derived from bead outlines. The NNWorientation of the long axis may reflect a tendency for beads tohave been spaced slightly further apart in the direction of preferredorientation (Fig. 11A), prior to deformation.
The tectonic distortion has implications for size comparisonwith the other two species. The bead diameters, as calculated above,will only be equal to the original (undeformed) diameters if area hasbeen conserved in the bedding plane during deformation. However,Devonian deformation in Tasmania is commonly characterised byplane strain, with the long axis of the strain ellipsoid orientedapproximately down the dip of cleavage if the folds are shallowlyplunging (Williams and Seymour, 1984). If that is the case here, thenthe shortening in the bedding plane took place without any exten-sion in the S0/S3 lineation direction, and the calculated diameterwill be an underestimate of the pre-deformation bead diameter byup to approximately (1.0/0.7) or 42%, in which case the Tasmanianbeads may have been significantly larger than those of H. williamsii,though still smaller than the type H. moniliformis (see below).
The strong preferred orientation of the strings in the digitisedslab is clearly unrelated to tectonic strain. The preferred orientationis not parallel to the S0/S3 lineation, and given that shortening isonly 30%, is too well developed to be a result of passive tectonicrotation of a previously existing random array of strings.
6. Depositional environment
In Western Australia, H. williamsii is found in a spectrum of envi-ronments ranging from intertidal (Stag Arrow Formation) to belowstorm wavebase (Backdoor Formation: Martin, 2004; Grey et al.,2010); while the palaeoenvironment of H. moniliformis is describedas quiet water lagoonal or estuarine, with some aeolian supply ofsediment (Fedonkin and Yochelson, 2002). The palaeoenvironmentof the Tasmanian Horodyskia was probably similar to the Back-door Formation. The dark, pyritic shale probably reflects an anoxicseafloor which would have been inhospitable to eukaryotes; conse-quently we suggest that the fossils are allochthonous. The sedimentgrainsize in the Tasmanian host succession (shale/silt) is generallyfiner than in the case of the other two species (largely fine-grainedsand/silt). This has probably had an important bearing on the con-trasting modes of preservation.
7. Taphonomy
The mode of preservation of the Tasmanian material is stronglyreminiscent of that of certain soft-bodied Ediacara fauna (Gehling,1999). In both cases, the fossils are preserved at the base of anevent bed, in concave hyporelief (external mould) on the sole ofthe event bed, and as convex epirelief on the top of the underlyingbed. Possibly, the Tasmanian beads were preserved when rapidly
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buried by a sufficiently thick (>5 mm) silt lamina. As inferred forthe West Australian and Montanan examples (Grey and Williams,1990; Fedonkin and Yochelson, 2002), the beads were sufficientlyrigid to maintain their shape during initial burial. “Apical dimples”(central depressions) are probably a result of partial compactionalcollapse (Grey and Williams, 1990). As inferred for the Ediacarafauna (Gehling, 1999), decay of the organic matter comprising thebeads may have resulted in partial cementation of the overlyingsiltstone bed immediately adjacent to the beads soon after burial,preserving the mould against obliteration by further compaction;and once the bead had fully decayed, the still-plastic mud of theunderlying bed flowed into the remaining void to form the cast.Occasionally, the Tasmanian Horodyskia is preserved in shale with-out a superposed siltstone layer, as wholly flattened beads with norelief on the bedding plane (Fig. 8). In this instance, the relativelysmall gaps between adjacent beads suggest the beads have spreadlaterally by a relatively large degree of soft-bodied compaction.
In the case of the Western Australian H. williamsii, it is sug-gested that coarser enclosing sediment allowed the underlying aswell as overlying layers to be cemented, resulting in preservationof the original void. H. moniliformis in general seems to be consid-erably more flattened by compaction than the Tasmanian or WestAustralian forms, but a remanent, flattened void probably survivedcompaction to become filled with silica.
Dong et al. (2008) describe three-dimensionally preserved fos-sils from an Ediacaran chert in south China, in which beads, beadspacing, and haloes are proportionally similar to those of H. monil-iformis and H. williamsii, but the fossils are only about one-fifth thesize of the Mesoproterozoic forms. Dong et al. assigned them toa new species (H. minor) and compared their quartz-rich haloesto the tests of agglutinated foraminifers. However, the haloes ofthe Mesoproterozoic forms, whether preserved as impressions (e.g.Fig. 7, this paper; Fig. 5 of Grey and Williams, 1990) or subtle colourdifferentiation (Fig. 8, this paper; Figs. 3 and 4A of Fedonkin andYochelson, 2002), are not quartz-rich and there is no evidence thatthey comprised an agglutinated test that completely enclosed thebead. Rather, the thin integument of the bead itself (though not pre-served) appears to have behaved semi-rigidly in compaction, whilewhatever comprised the halo appears to have been either highlycompressible – perhaps a gelatinous sheath enclosing the beads –or was a blade-like ribbon (often not preserved) in which the beadswere embedded (Grey et al., 2010). For this reason (and becauseof its much smaller size) we doubt that H. minor can be assignedto Horodyskia, and consider it unlikely that H. moniliformis and H.williamsii were agglutinating foraminifers.
8. Systematic palaeontology
Domain Eukaryota; Kingdom, Phylum, Class and Family IncertaeSedis
Horodyskia Yochelson and Fedonkin 2000
1982 ‘String of beads’ Horodyski,Journal of Palaeontology,56, p. 882–889
1990 ‘String of beads’ Grey andWilliams, PrecambrianResearch, 46, p. 307–327
2000 Horodyskia Yochelson andFedonkin, Proceedings ofthe Biological Society ofWashington, 113, p. 844,fig. 1
Holotype: Horodyskia williamsii Grey, Yochelson, Fedonkin andMartin (new species in this volume, GSWA F 51187, GSWA samplenumber 169534 OO, Geological Survey of Western Australia, FossilCollection, locality 2, Stag Arrow Formation, Bangemall Super-group, Western Australia (Grey et al., 2010 , Fig. 2A–D).
New material: Sample registration numbers Z3701–Z3706, fromBlackwater Road Quarry, Balfour, Tasmania. Specimens housed atthe Tasmanian Museum and Art Gallery, Hobart.
Diagnosis: As for Horodyskia williamsii Grey, Yochelson,Fedonkin and Martin (2010).
Description: Strings of serial, subrounded, bead-like structures ofapproximately uniform size and spacing on any one string. Stringsgently curved or sinuous in plan view. In some cases, a preferredorientation is demonstrated. Maximum length of a string is 250 mmand the maximum observed number of beads in a string is 63. Beadson bedding planes have positive relief and are low subroundedhemispheres, with the larger ones having flattened tops and tend-ing to be subcircular to gently ovoid in outline. Beads on soles ofoverlying siltstones consist of impressions reflecting the shape ofunderlying beads. Many beads have a small, central depression(preserved as a small prominence in the sole mark impressions).There is no evidence of an internal void or of infilling of a for-mer space by a secondary mineral. A few strings show, on beddingplanes, a flat, slightly depressed border or halo extending aroundthe string, to a distance of some 2 mm from the margins of thebeads. On sole surfaces, these haloes comprise slightly raised rimsaround the bead moulds
Measurements: Beads have variable relief and are up to 1 mmin height. Diameter ranges from 0.6 mm to a maximum 4.3 mm(average 1.7 mm, n = 1019). The distance between adjacent beadcentroids varies from 1.3 to 8.0 mm (average 3.2 mm, n = 868)
Comparison: The Tasmanian Horodyskia has most of the mor-phological attributes previously noted in H. moniliformis and H.williamsii: uniserial strings of bead-like elements, regularity of beadsize and spacing in any one string, lack of branching, and in someinstances, haloes and apical dimples (central depressions). Themuch smaller H. minor (Dong et al., 2008), whose assignation toHorodyskia we find doubtful (see above), is not further consideredhere. No connecting strands were seen in the Tasmanian mate-rial, but these are apparently only rarely preserved in the otheroccurrences. The Tasmanian beads are not significantly different insize distribution to H. williamsii (Fig. 9), although because the Tas-manian beads are tectonically deformed they may have originallybeen larger. Likewise, the distances between beads are not signif-icantly different, but may not be directly comparable because ofthe deformation of the Tasmanian material. Notwithstanding theuncertainties bestowed by tectonic deformation, H. moniliformisstands apart from the Tasmanian fossils and H. williamsii in a largermaximum size and spacing of beads. This is evident in Table 1 ofFedonkin and Yochelson (2002), where twelve out of a total of six-teen measured strings (representing selected paratype material)have an average bead diameter that is greater than the maximum
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Fig. 9. Frequency distribution of diameters of five bead populations. Populations1–3 based on bead diameters; populations 4 and 5 based on diameters averagedwithin strings. The paratype material of Fedonkin and Yochelson (2002) is signifi-cantly larger than the beads of the other documented populations.
diameter (4.3 mm) seen in the Tasmanian beads. The average beaddiameter in Table 1 of Fedonkin and Yochelson is 4.0 mm, comparedto 1.7 mm for the Tasmanian beads and 1.6 mm for H. williamsii(Grey et al., 2010). The measurements of Horodyski (1982, repro-duced here in Fig. 9) appear to be significantly smaller, but it isclear that the type material of H. moniliformis comprises signifi-cantly larger fossils (in terms of both bead diameter and spacing)than the type H. williamsi, or the Tasmanian material. Even if theTasmanian beads were originally 42% larger (as permitted by theanalysis of tectonic strain), they would still be closer to H. williamsiithan H. moniliformis in size (Fig. 9).
There is a strong morphological similarity between the typeH. williamsii and the Tasmanian strings of beads, putting asidetaphonomic differences. Moulds and casts of the Tasmanian beadsapproach a hemispherical shape (with flatter shapes attributable
Fig. 10. Average bead diameter versus average distance between bead centroids for151 strings of the Tasmanian Horodyskia. The data for the 19 strings of H. williamsii(from Grey et al., 2010) uses average bead ‘spacing’ as defined by Horodyski (1982)and Grey and Williams (1990), an equivalent measure to ‘distance’ as used in thisstudy.
Fig. 11. (A) Rose diagram showing preferred orientation on digitised slab. (B) Super-imposed outlines of 130 beads and inferred strain ellipse. (C) Rose diagram showingaverage distance versus orientation relationship, resulting from tectonic strain.
to compaction); suggesting an originally subspherical bead, as pro-posed for H. williamsii. The ‘apical dimple’ on moulds is found inboth cases. If due to compactive deformation, the dimple attests to asimilarity in the mechanical properties of the beads in both cases. Bycontrast, the upper bead surface of H. moniliformis tends to be flat-ter, with no dimple (with the possible exception of an interpreted‘slightly invaginated’ area on some beads: Fedonkin and Yochelson,2002, p. 16), and the lower surface is reconstructed as a bluntly con-ical shape (Fedonkin and Yochelson, 2002). The diameter-distancerelationship is similar (Fig. 10). Size and morphology thereforesuggest a provisional assignment of the Tasmanian material to H.williamsii.
9. Conclusions
The fortuitous discovery documented here brings to three thenumber of regions (or basins) in which Horodyskia is now known(discounting the Ediacaran occurrences of Mathur and Srivastava(2004), Shen et al. (2007) and Dong et al. (2008), whose assignationto Horodyskia we find doubtful). Possibly this inconspicuous fossilis much more common than its published record suggests.
The fossils are found in dark pyritic shale with thin graded quartzsiltstone laminae, interpreted as a marine shelfal environmentbelow storm wave base. The fossils are probably allochthonous, andit is suggested that they were either pelagic or transported from ashallower environment where they may have been attached to thesea floor.
The Tasmanian ‘strings of beads’ have a similar size distributionand morphology to H. williamsii Grey et al. (2010) of the BangemallSupergroup (Western Australia), and are provisionally assigned tothat species. The mode of preservation differs from the type H.williamsii in that the Tasmanian beads tend to be preserved onupper bedding plane surfaces as convex epirelief (casts). This isthought to be merely a taphonomic difference, resulting from thefiner-grained (originally muddy) sediment underlying the beddingplanes on which the beads are preserved. Soon after burial the mudflowed into the void left by the decayed bead; while in the type H.williamsii, found in coarser (silty/sandy) sediment, the void spaceleft by the bead was preserved.
GIS software is effective in rapid 2-D morphometric, spatial andstrain analysis of these fossils and its use is recommended in furtherstudy of these and other morphologically simple and numerousforms.
The presence of H. williamsii in the lower part of the Rocky CapeGroup suggests these poorly dated rocks may be of similar age to the1465–1070 Ma Bangemall Supergroup, and thus significantly olderthan the lower Neoproterozoic, uppermost Rocky Cape Group.
Author's personal copy
C.R. Calver et al. / Precambrian Research 180 (2010) 18–25 25
Acknowledgements
We have had useful discussions with Malcolm Walter and LisaGershwin. The paper was improved by the reviews of Jim Gehlingand an anonymous reviewer. David Seymour assisted with strainanalysis and photography. Calver publishes with the permission ofthe Director, Mineral Resources Tasmania. Grey publishes with thepermission of the Executive Director, Geological Survey of WesternAustralia.
References
Black, L.P., Calver, C.R., Seymour, D.B., Reed, A., 2004. SHRIMP U-Pb detrital zirconages from Proterozoic and Early Palaeozoic sandstones and their bearing on theearly geological evolution of Tasmania. Australian Journal of Earth Sciences 51,885–900.
Calver, C.R., 1998. Isotope stratigraphy of the Neoproterozoic Togari Group, Tasma-nia. Australian Journal of Earth Sciences 45, 865–874.
Dong, L., Xiao, S., Shen, B., Zhou, C., 2008. Silicified Horodyskia and Palaeopascichnusfrom upper Ediacaran cherts in South China: tentative phylogenetic interpreta-tion and implications for evolutionary stasis. Journal of the Geological Society,London 165, 367–378.
Everard, J.L., Reed, A.R., Seymour, D.B., Calver, C.R., 2003. Digital Geological Atlas,1:25,000 Series, Sheet 3243. Dempster. Mineral Resources Tasmania.
Everard, J.L., Seymour, D.B., Reed, A.R., McClenaghan, M.P., Green, D.C., Calver, C.R.,Brown, A.V., 2007. Regional Geology of the southern Smithton Synclinorium:Explanatory Report for the Roger, Sumac & Dempster 1:25,000 map sheets, farnorthwestern Tasmania. 1:25,000 Scale Digital Geological Map Series Explana-tory Report 2: Mineral Resources Tasmania.
Fedonkin, M.A., Yochelson, E.L., 2002. Middle Proterozoic (1.5 Ga) Horodyskia monil-iformis Yochelson and Fedonkin, the Oldest Known Tissue-Grade ColonialEucaryote. Smithsonian Contributions to Paleobiology 94, 29.
Gee, R.D., 1968. A revised stratigraphy for the Precambrian of north-west Tasmania.Papers and Proceedings of the Royal Society of Tasmania 102, 7–10.
Grey, K., Williams, I.R., 1990. Problematic Bedding-Plane Markings from the MiddleProterozoic Manganese Subgroup, Bangemall Basin, Western Australia. Precam-brian Research 46, 307–327.
Grey, K., Yochelson, E.L., Fedonkin, M.A., Martin, D.McB., 2010. Horodyskia williamsiinew species, a Mesoproterozoic macrofossil from Western Australia. Precam-brian Research 180, 1–17.
Hill, A.C., Walter, M.R., 2000. Mid-Neoproterozoic (∼830–750 Ma) isotope stratigra-phy of Australia and global correlation. Precambrian Research 100, 181–211.
Hofmann, H.J., 2001. Ediacaran enigmas, and puzzles from earlier times. Geologi-cal Association of Canada - Mineralogical Association of Canada, Joint AnnualMeetings, Abstracts 26, pp. 64–65.
Horodyski, R.J., 1982. Problematic bedding-plane markings from the Middle Pro-terozoic Appekunny Argillite, Belt Supergroup, northwestern Montana. Journalof Paleontology 56, 882–889.
Knoll, A., Javaux, E., Hewitt, D., Cohen, P., 2006. Eukaryotic organisms in Proterozoicoceans. Philosphical Transactions of the Royal Society B 361 (1470), 1023–1038.
Martin, D.McB., 2004. Depositional environment and taphonomy of the ‘stringsof beads’: Mesoproterozoic multicellular fossils in the Bangemall Supergroup.Western Australia. Australian Journal of Earth Sciences 51, 555–562.
Mathur, V.K., Srivastava, D.K., 2004. Record of tissue grade colonial eucaryote andmicrobial mat associated with Ediacaran fossils in Krol Group, Garhwal syn-cline, Lesser Himalaya, Uttaranchal. Journal of the Geological Society of India63, 100–102.
Ragan, D.M., 1973. Structural Geology. An Introduction to Geometrical Techniques,2nd ed. John Wiley & Sons, 208 pp.
Shen, B., Xiao, S., Dong, L., Zhou, C., Liu, J., 2007. Problematic macrofossils from Edi-acaran successions in the North China and Chaidam blocks: implications for theirevolutionary root and biostratigraphic significance. Journal of Paleontology 79,1396–1411.
Williams, E., Seymour, D., 1984. Short Course: Stratigraphy and Structural Character-istics of the North Coast, Tasmania. Tasmania Department of Mines UnpublishedReport 1984/87.
Yochelson, E.L., Fedonkin, M.A., 2000. A new tissue-grade organism 1.5 billion yearsold from Montana. Proceedings of the Biological Society of Washington 113,843–847.
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