Cryptic Disc Structures Resembling Ediacaran Discoidal Fossils from the Lower Silurian Hellefjord Schist, Arctic Norway

The Hellefjord Schist, a volcaniclastic psammite-pelite formation in the Caledonides of Arctic Norway contains discoidal impressions and apparent tube casts that share morphological and taphonomic similarities to Neoproterozoic stem-holdfast forms. U-Pb zircon geochronology on the host metasediment indicates it was deposited between 437 ± 2 and 439 ± 3 Ma, but also indicates that an inferred basal conglomerate to this formation must be part of an older stratigraphic element, as it is cross-cut by a 546 ± 4 Ma pegmatite. These results confirm that the Hellefjord Schist is separated from underlying older Proterozoic rocks by a thrust. It has previously been argued that the Cambrian Substrate Revolution destroyed the ecological niches that the Neoproterozoic frond-holdfasts organisms occupied. However, the discovery of these fossils in Silurian rocks demonstrates that the environment and substrate must have been similar enough to Neoproterozoic settings that frond-holdfast bodyplans were still ecologically viable some hundred million years later

An enigmatic large discoidal fossil from the Pennsylvanian of County Clare, Ireland

A rare and unusual large solitary discoidal fossil has been discovered on a paving slab quarried from the cyclothems of the Central Clare Group (Kinderscoutian, Pennsylvanian, Carboniferous), western Ireland. The fossil impression consists of a smooth raised inner discoidal area, surrounded by a slightly lower relief outer ring, ca. 130–135 mm in diameter, with eight prominent equidistant ovoid raised nodes towards the outermost margin. The octoradial body plan of this enigmatic specimen suggests a cnidarian connection and, as it is preserved as a positive hyporelief cast, it is tentatively interpreted as the resting trace of a large benthic anemone, which was either partially or fully infaunal. The discoidal fossil is interesting palaeoecologically; it occurs within the well-known Liscannor flagstone, which consists of thinly bedded, fine-grained sandstone that is extensively covered by prominent, sinuous to meandering, horizontal grazing trails attributed to Psammichnites plummeri. This sedimentary facies likely represents mouth-bar sedimentation on a delta front of a river-dominated delta. The discoidal impression occurs on a portion of the slab where these trace fossils are relatively scarce. Uncertainty surrounds the classification and interpretation of the disc due to its relatively simple morphological form, coupled with a lack of unequivocally diagnostic features — a problem commonly encountered in studies of discoidal fossils from both the Ediacaran and the Phanerozoic

Images of the Hellefjord fossils.

<p>Left: photograph of discoidal fossils and sinusoidal tube casts, Hellefjord Schist. Right: sketch of salient features of fossils. Inset: photogrammetric surface reconstruction with radiance scaling shader. Planes A-A’ and B-B’ reflect eastward looking tilted section through discoids.</p

Examples of preservation styles found in Ediacaran (and one early Cambrian) sites compared to fossils from Sørøy, northern Norway.

<p>A: Cluster of flat-convex discs from Newfoundland [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0164071#pone.0164071.ref084" target="_blank">84</a>]; note similarity in contact between discs and those in B. B: Southern Sørøy discs of varying size [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0164071#pone.0164071.ref055" target="_blank">55</a>]. C: <i>Primocandelabrum</i> 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 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0164071#pone.0164071.ref019" target="_blank">19</a>]. Note similarity of central boss to E. E. Discs and tube cast (stem) from Hellefjord Schist Sørøy—see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0164071#pone.0164071.g002" target="_blank">Fig 2</a>. F: Small discs (Type morph of <i>Aspidella</i>) showing central invagination with recessed bosses [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0164071#pone.0164071.ref084" target="_blank">84</a>] 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 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0164071#pone.0164071.ref020" target="_blank">20</a>] note similarity to E. H: Positive rimmed disc impressions associated with <i>Aspidella</i>, Newfoundland [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0164071#pone.0164071.ref083" target="_blank">83</a>], note similar edge morphology to E. Scale bars are 1 cm.</p

Boss area versus disc area with linear regression fits to all data and by geographic location.

<p>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 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0164071#pone.0164071.ref108" target="_blank">108</a>]). − = negative epirelief; + = positive epirelief.</p

Locality and stratigraphy of the fossils.

<p>Geological map of the Langstrand—Hellefjord area, Sørøy (modified after [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0164071#pone.0164071.ref021" target="_blank">21</a>]). Fossil locality indicated by star. Inset left: main tectonic units of Finnmark with overview map of Norway. Inset right: simplified tectonostratigraphy of KNC and overlying Hellefjord Schist.</p

Inverse concordia diagrams for zircon grains analysed by SIMS.

<p>CK011 and CK004 are detrital material. The age of the youngest analysis is shown. CK231 is interpreted to reflect both inheritance and a magmatic population. U—Pb data are plotted as 2 σ error crosses.</p