Scratch circles form when a tethered organism is rotated by currents, with the upper parts of the organisms dragged on the substrate surface around the attachment point, leaving arcuate to circular marks on the sediment—water interface [ 70 , 75 ]. Radial impressions can also be left by the stalk. However, the Hellefjord discs are unlikely to be scratch circles, as the sharp nature of the annuli would again be unusual in such an interpretation—scratch circles tend to have rings with smoother edges due to the erosional mode of formation. The apparent stalk of the Sørøy specimens is also far larger than the disc radius, with a scratch circle interpretation therefore requiring the stalk only to have been in contact with the substrate in the immediate vicinity of the attachment point, which is biomechanically unlikely. It is also worth noting that a scratch circle interpretation for the discs would indicate the presence of organisms with near-identical morphology and ecology to those envisaged by a biogenic interpretation. In our view a fully biogenic interpretation of the discs is the most parsimonious interpretation.

Microbial colonies can produce discoidal structures of similar size and shape to the Hellefjord discoidal fossils [ 71 , 74 ]. However, the widely-spaced and sharp nature of the annuli of the Sørøy specimens would be very unusual for a microbial colony, which tend to have multiple closely-spaced concentric annuli.

Vertical burrow trace fossils may be ruled out due to the lack of vertical pipes in cross-section ( Fig 2 ), as may fluid escape in concert with a microbial mat, which the lack of wrinkling and crack-fill also argues against. Gas escape through a microbial mat also appears unlikely due to the lack of such wrinkling and crack-fill [ 72 ], in addition to the presence of annuli and apparent stems.

Liquefaction or fluid escape structures such as load casts, or so-called sand volcanoes, are formed due to liquefaction [ 65 ] following rapid deposition of water-rich sediment [ 68 ] or during/after earthquakes [ 69 ]. However, the cross sectional morphology of the discoidal fossils in the Hellefjord Schist is inconsistent with a load cast or sand volcano, given that multiple concentric annuli are preserved that undulate to an extent greater than the central region. The narrow stem-like feature running towards the central bosses are dissimilar to sheet flow on the edge of a sand volcano, which would be expected to diverge away from its vent. Additionally, there is no interaction between discoidal fossils where one might expect irregularity to be developed when multiple sand volcanoes occur within a confined area, and there is no indication of a vertical fluid escape structure in the centre of the discs in cross section. In summary, an abiotic origin seems unlikely.

Raindrop impressions [ 63 ] may similarly be ruled out, as the discs characteristically do not include annuli within the pit formed by raindrop impact. The Sørøy discs, at up to 20mm, are also considerably larger than the maximum size of raindrop impressions. Likewise, gas escape structures, which are often commonly mistaken for raindrop impressions [ 64 ], are generally considerably smaller than the maximum size of the Sørøy specimens.

Salt pseudomorphs and pyrite rosettes may be immediately ruled out on morphological grounds, as they show neither the radial structures characteristic of pyrite rosettes [ 67 ], nor the collapse structures typical of salt pseudomorph pseudofossils [ 66 ].

The first priority must be to establish whether the Silurian discs are indeed biological in origin, as discoidal structures may be formed by inorganic processes, including raindrop imprints [ 63 ], fluid escape structures (sand volcanoes), gas escape structures [ 64 ], load casts [ 65 ], salt pseudomorphs [ 66 ], and pyrite rosettes [ 67 ].

Phylogeny and relationships.

Discoidal fossils are most commonly associated with fossil localities of Neoproterozoic age [43]. Initially regarded as jellyfish impressions [76–81], it is now understood that Neoproterozoic discoidal impressions can be formed by a wide range of benthic discoidal organisms [43], including—but not limited to—microbial colonies [71], fungi [82], and cnidarians [43,83]. Multiple lineages of epibenthic frondose Neoproterozoic organisms, such as rangeomorphs and arboreomorphs, also produced discoidal impressions through a basal flattened or bulbous disc which acted as a holdfast for an upper stem and petalodium [18,84–91].

Discoidal structures are also known from post-Ediacaran sediments. Concentrically structured discoidal fossils comparable to Nimbia and Tirasiana have been reported from the lower Cambrian of California [20] and from the Digermul Peninsula, northern Norway [19]. Younger discoidal fossils were produced by a wide range of organisms, arguably even wider than those of Neoproterozoic specimens, including the extinct fossil eldonids [92,93] and a number of incertae sedis organisms such as Patanacta pedina, Parasolia actiniformis, or Velumbrella bayeri [94–96] in addition to extant clades like cnidarians [97,98]. However, these are generally different in aspect to Neoproterozoic discoidal remains.

Other Phanerozoic discoidal structures are known to have been produced abiogenically [99,100]. Scratch circles in particular are known throughout the Phanerozoic, including specimens from the Cambrian of Ireland [18,70] originally assigned to Nimbia by Crimes et al. [101], and examples from the Paleocene of Italy produced by foraminifera [102].

The discoidal forms in the Hellefjord Schist do not have the complexity of phylogenetically determinate Phanerozoic discoidal organisms such as eldonids or cnidarians, nor do they resemble any of the incertae sedis material. Rather, they cannot be distinguished from Neoproterozoic discoidal taxa, and would be identified variously as Nimbia or Tirasiana if found in sediments of Ediacaran age.

Due to the morphological simplicity of discoidal structures, the range of discoidal organisms, and the potential for taphonomic processes to cause variation between the preserved forms of similar organisms, identifying the phylogenetic origin of discoidal fossils is commonly difficult. This is especially true in the Neoproterozoic, with numerous extinct lineages existing alongside the ancestors of extant discoidal organisms. In addition, structural elements within the water column, including more delicate frondose structures, are far more difficult to preserve than body parts within or on the substrate [51,103,104]. As a result, it may be impossible to identify whether or not discoidal structures are holdfasts of epibenthic frondose organisms.

The available evidence from the Hellefjord fossils is consistent with their genesis under taphonomic processes similar to those responsible for the preservation of discoidal gravity casts in Ediacaran sediments. Such Fermeuse-style assemblages preserve only gravity-cast fossils (in positive hyporelief on bed soles), such that only features that were on the base of the organisms, in contact with the substrate, can be preserved. Hence, attachments between a disc and its stem can never be recorded by this style of preservation. However, in some occurrences a stem impression may emanate from the margin of the disc, as potentially hinted at by the Hellefjord specimens. The Hellefjord organisms were therefore apparently at least similar in general morphology to Ediacaran-aged stem-holdfast organisms, with a stalk extending from a basal discoidal attachment to the substrate.

The absence of any biomineralisation in the Hellefjord discs suggests a soft-bodied nature for the producing organisms. The preservation of imprints or traces from soft-bodies necessitates a general lack of heavy bioturbation [92]. As in the case of many Ediacaran sites, certain body elements may not have been preserved due to removal in the water column or labile tissue destruction prior to complete lithification [104].

The observations from the Hellefjord Schist extends the stratigraphic range of similar fossils to the Silurian and cautions about the simple nature of certain discoidal forms. The Hellefjord Schist and correlative Juldagnes Formation were deposited in a deep marine slope setting as evidenced by the lithofacies, ichnofacies and fossil assemblage implying a relatively shore-distal environment, mainly receiving low velocity turbidity currents. Such a deep water setting is consistent with the habitat of frond-holdfast organisms of Ediacaran age, which are known ranging from deep-marine basinal contour-current and turbidite settings to shoreface environments above fair-weather wave base [47,105–107].

The relationship between disc size and central boss size has been investigated for discoidal fossils of Ediacaran age in Newfoundland by Burzynski and Narbonne [108]. They observed a positive relationship between disc diameter and boss diameter, consistent with biological dimensions where a larger holdfast would be required to support a larger stem and other appendages (Fig 4). Using the Burzynski and Narbonne [108] dataset along with that from the Hellefjord Schist indicates a statistically significant relationship between boss area versus disc area (Boss Area = 26.2 + 0.041 Disc Area), that accounts for 19% of the observed variability. Although there is significant scatter within the dataset the relationship is greater than would be expected by chance alone (Fig 4). Linear regressions when separated on geographic basis generally result in better linear regression fits. The Hellefjord disc and boss dimensions closely match the relationships seen in discoidal fossils at Ferryland, within the Ediacaran Fermeuse Formation, consistent with a similar positive hyporelief preservation style. These Fermeuse Formation fossils are dominantly smaller than those from other fossil bearing surfaces from the Ediacaran of Newfoundland (Fig 5). Hence, the morphology of the Hellefjord discoidal fossils, size, distribution and relationship to stem like features, are similar to descriptions of holdfasts from the Ediacaran System (Fig 6). However, we note that successful bodyplans, able to remain structurally stable, can only have a limited range of stem to holdfast dimensions dependant on an array of factors, including but not limited to substrate stability, current velocity, stem length, and size of frondose element. Based on the disc and boss dimensions, we suggest that a bodyplan with general similarity to forms of Ediacaran age developed in the Hellefjord Schist under comparable environmental conditions, given the analogous depositional setting.

PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 4. Boss area versus disc area with linear regression fits to all data and by geographic location. The adjusted R2 value is shown as a percentage for each fit and indicates the degree of scatter accounted for by the regression. Upper plot shows 0–2000 and 0–200 mm2 region only. Lower plot is enlargement of dashed region (Newfoundland data from [108]). − = negative epirelief; + = positive epirelief. https://doi.org/10.1371/journal.pone.0164071.g004

PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 5. Plot of disc area versus number of measurements from Ediacaran sites in Newfoundland compared to those from Sørøy, northern Norway. Each symbol represents up to two observations. Measurements include results from Burzynski and Narbonne [108] and Holland and Sturt [55]. The disc area for the Sørøy sites is most similar to Fermeuse-style positive hyporelief fossils at Ferryland. https://doi.org/10.1371/journal.pone.0164071.g005

PPT PowerPoint slide

PowerPoint slide PNG larger image

larger image TIFF original image Download: Fig 6. Examples of preservation styles found in Ediacaran (and one early Cambrian) sites compared to fossils from Sørøy, northern Norway. A: Cluster of flat-convex discs from Newfoundland [84]; note similarity in contact between discs and those in B. B: Southern Sørøy discs of varying size [55]. C: Primocandelabrum from Newfoundland showing holdfast and branching stem which may have shared some similar morphological elements to the Hellefjord Schist forms. D: Disc feature from Digermul Peninsula, Norway [19]. Note similarity of central boss to E. E. Discs and tube cast (stem) from Hellefjord Schist Sørøy—see Fig 2. F: Small discs (Type morph of Aspidella) showing central invagination with recessed bosses [84] note similarity to areas on B. G: Early Cambrian fossil from California, previously compared to discs of Ediacaran age, highlighted region with “burrow” abutting disc [20] note similarity to E. H: Positive rimmed disc impressions associated with Aspidella, Newfoundland [83], note similar edge morphology to E. Scale bars are 1 cm. https://doi.org/10.1371/journal.pone.0164071.g006

It is important to note that this interpretation should not be taken as evidence of any direct or meaningful close biological relationship between Silurian and Neoproterozoic forms. Frondose morphology independently evolved at least three times in the Ediacaran, in the rangeomorphs, arboreomorphs, and erniettomorphs, and perhaps more often if other attached epibenthic taxa such as Thectardis are considered [109,110]. In the Phanerozoic, several additional groups independently evolved a similar morphology, most notably the octocorallid cnidarian ‘sea pens’ (previously suggested as an affinity for some fronds of Ediacaran age, though subsequently ruled out; see [111], and also graptoloids, pelmatozoan echinoderms (blastoids and crinoids), poriferans, actinians, algae, and others. Whilst not all of these organisms are unmineralized, and whilst some are adapted for hard rather than soft substrates, this still strongly indicates that frondose morphology is relatively easy to attain through convergent adaption to similar environments.

The question of the biological affinities of these fossils is impossible to answer with the material presented herein. Without a well-preserved upper part that can be definitively linked to a specific group, it is not possible to assign these fossils with any degree of confidence. Additionally, given the significant age difference of these fossils to other comparable forms, it is entirely feasible that they represent a different, previously unknown group, which evolved independently to attain a similar form due to evolutionary convergence. Hence, no primarily biological conclusions should be drawn.

Instead, we contend that these fossils are primarily indicative of environmental and ecological conditions. The frond-holdfast nature of these Silurian fossils is not of particular significance in isolation, given that numerous groups have independently evolved such a bodyplan. Rather, the significance of the Hellefjord fossils, and the justification for comparison to frondose specimens of Ediacaran age, lies in the combination of the unmineralized nature, the frond-holdfast morphology, and perhaps most importantly, in the nature of attachment of these unmineralized frond-holdfast organisms to the substrate. Phanerozoic frondose organisms are generally attached to hard substrates by means of root-like structures, or anchored in soft substrates by means of a deep, bulbous peduncle. Frondose forms of Ediacaran age, by contrast, were anchored on soft firmground substrates by means of a discoidal holdfast, a feature that has not previously been described in any subsequent frondose organism. The unmineralized Hellefjord frond-holdfast fossils similarly appear to have anchored by means of a discoidal holdfast; by far the youngest example of such a bodyplan.

Many factors have been proposed to control the Ediacaran-Cambrian diversification of animals, along with the origin of biomineralisation and the substrate changes in the early Cambrian, referred to as the Cambrian Substrate Revolution [112] or the Agronomic Revolution [100]. Some factors link to the importance of environmental and preservational change, others support animal developmental innovations, while another suite of explanations focuses on the growth of new ecological relationships [113]. It is likely that the events of the Ediacaran and Cambrian involved all of these factors [114]. A particular concern with regard to discoidal fossils has been to find a satisfactory explanation for their apparent restriction to the late Neoproterozoic. Proposals to address the apparent stratigraphic restriction included suggestions that some organisms during the Ediacaran were constructed from unusually tough biological materials to account for their preservation [115]. Specifically, such robust construction was seen as a means for the preservation of forms like Dickinsonia recorded as positive epirelief moulds of negative hyporelief casts. More recently, burrowing was proposed to have expunged the microbial mats necessary for the preservation of soft bodies in marine environments [103,116]. Specifically, vertical burrowing, which may have evolved as a defence against predation, has been widely proposed to have opened up new ecological niches beneath the sea floor as water and oxygen could now get into deeper sediment layers. At the same time, and consequentially, microbial mats were progressively destroyed and forced into more restricted habitats, in environments unfavourable for animals. This change in substrate is thought to be partly responsible for the demise of the ecological niches that the frond-holdfasts organisms (and, others) of the Ediacaran occupied [112, 117].

Importantly, the observations in this work indicate that discoidal impressions with forms ostensibly identical to some biological structures of Ediacaran age occur in Llandovery sediments, rendering the stratigraphic requirement for such explanations moot, while supporting the nature of substrates as a primary environmental and ecological control on the distribution of organisms with particular morphologies. We consider the most likely explanation for the similarity of the Hellefjord discs in bodyplan to organisms of Ediacaran age is convergent adaptation of both the overall unmineralized frond-holdfast bodyplan, and the attachment to the substrate by means of a discoidal holdfast, to similar environmental and ecological (including substrate) conditions.