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‘Intrites’ from the Ediacaran Longmyndian Supergroup, UK: a new form of microbially-induced sedimentary structure (MISS)

LATHA R. MENON1*, DUNCAN MCILROY2 & MARTIN D. BRASIER1,2† 1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK 2Department of Earth Sciences, Memorial University of Newfoundland, St John’s, NL, A1B 3X5, Canada *Correspondence: [email protected]

Abstract: Simple discoidal impressions are the only evidence of complex life in some Ediacaran and older rocks, but their interpretation is notoriously difficult. We reassessed a puzzling discoidal form from the c. 560 Ma upper Burway Formation of the Ediacaran Longmyndian Supergroup, Shropshire, UK. The structures, previously described as Intrites punctatus Fedonkin, are found on both the bed tops and soles. They vary in morphology from mounds with central depressions to incomplete rings and pairs of short ridges. Examination of the purported Intrites documented from the Longmyndian in cross-section revealed a torus-shaped structure bounded by microbial mat layers and commonly containing white laminae. We interpret the ‘Longmyndian Intrites’as a product of microbial trapping, sediment binding and authigenic clay mineral and carbonate pre- cipitation on the flanks of small sediment volcanoes. Subsidence of the ring-like structure into muddy sediments resulted in a torus-shaped microstromatolite. Preferential stromatolitic growth parallel to the prevailing current produced the observed partial rings or parallel ridges and explains their preferential orientation as current alignment. This interpretation of ‘Longmyndian Intrites’ expands the known variety of microbially-induced sedimentary structures (MISS) and emphasizes the importance of considering microbially-induced structures and abiological processes when interpreting discoidal impressions in ancient rocks.

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The enigmatic, soft-bodied Ediacaran macrobiota, fossilized remnants of microbial filaments in thin which flourished from c. 580 to 541 Ma, is domi- section (Bland 1984; Peat 1984; McIlroy et al. nated by discoidal impressions. In the absence of 2005; Callow & Brasier 2009; Callow et al. 2011b). more diagnostic features, such as fronds, some Pre- The nature of the Longmyndian discoidal impres- cambrian successions with discoidal markings have sions has long been debated (e.g. Cobbold 1900), been used as evidence for the presence of an Edia- but they have latterly been regarded as probable evi- caran macrobiota and thus of probable complex dence for complex Ediacaran life. The discoidal life (e.g. Hofmann et al. 1990; Farmer et al. 1992; structures were formally described as Medusinites Arrouy et al. 2016). It is well known that round aff. asteroides, Beltanelliformis brunsae, B. minutae impressions may be produced in a variety of ways, and Intrites punctatus (McIlroy et al. 2005). The including abiogenically by fluid escape (e.g. Cloud first three of these structures have been reinterpreted 1960; Farmer et al. 1992; Menon et al. 2016). The as pseudofossils resulting from sediment injection careful, critical investigation of possible discoidal and loading produced by small-scale fluid escape body fossils is a fundamental prerequisite to an structures mediated by microbial mats (Menon accurate assessment of Ediacaran taxonomic diver- et al. 2016). These processes are insufficient to sity, palaeobiology and palaeoecology. explain the remaining disc, described as I. punctatus Discoidal impressions were first observed in Fedonkin by McIlroy et al. (2005). A re-examina- rocks of the Ediacaran Longmyndian Supergroup, tion and reinterpretation of the purported Longmyn- Shropshire, UK by the Victorian palaeontologist dian examples of Intrites is the subject of this John Salter (Salter 1856, 1857). Subsequent obser- paper. It should be emphasized that the discussion vations revealed widespread evidence for the herein refers purely to material from the Long former presence of microbial mats, including micro- Mynd. The type material from the White Sea Edia- bially-induced sedimentary structures (MISS) and caran successions (Fedonkin 1980) differs from the

†Deceased 16 December 2014.

From:Brasier, A. T., McIlroy,D.&McLoughlin, N. (eds) 2017. Earth System Evolution and Early Life: A Celebration of the Work of Martin Brasier. Geological Society, London, Special Publications, 448, 271–283. First published online November 2, 2016, updated November 8, 2016, https://doi.org/10.1144/SP448.12 # 2017 The Author(s). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Longmyndian form in being found only as rows of small central depression to broad, low-relief ridges positive hyporelief impressions and is currently a forming an incomplete ring around a wide central valid taxon. flat area (Fig. 4). In some cases, the ILS are found as paired, almost straight, short ridges instead of arcs forming an incomplete ring (Fig. 4c, f; the ‘Are- Lithology and stratigraphic context nicolites didymus’ morph of Salter 1856). Some upper surfaces of beds exhibit counterparts of ILS The specimens of Longmyndian Intrites discussed as negative impressions with a central peak or ridge in this paper were collected from rocks of the upper (Figs 3 & 4d), presumably moulds of positive Burway Formation, Stretton Group, Longmyndian impressions on the sole of the originally overlying Supergroup and were exposed in a disused quarry bed. ILS are usually found in clusters on bedding in Ashes Hollow (Fig. 1). Intrites-like structures planes. Interestingly, they are not found in associa- (ILS) are found at Ashes Hollow in the Long tion with the ridges of the common Ediacaran Mynd, along with other discoidal impressions that MISS Arumberia (McIlroy & Walter 1997; McIlroy have been determined to be fluid-escape structures et al. 2005), a structure also seen in the Long Mynd. (Menon et al. 2016), preserved in thinly inter- ILS are sometimes seen in association with Medu- laminated units of mudstone, siltstone and fine- sinites-like pseudofossils preserved in positive grained sandstone. The fine laminae, ,0.2–1 mm hyporelief and, in such cases, the ILS differ only thick, show a strong colour contrast between the by virtue of having a central depression rather than grey-green of the chloritic mudstone and the dark a pinhead boss. More characteristically, however, brown of the hematitic sandstone, allowing a ILS occur in dense aggregations on bedding planes detailed study of the sedimentary structures in pol- as the only circular morphotype present. ished cross-section. Associated thin white laminae ,0.2 mm thickness, previously thought to host microbial filaments preserved in a white aluminosil- Methods icate mineral (Callow & Brasier 2009), were found in this investigation to consist entirely of the silici- In addition to field observations and the examination fied remains of microbial matgrounds (Fig. 2). of .40 hand specimens, the ILS from the Longmyn- Initial scanning electron microscopy–energy-dis- dian were examined in polished cross-section to persive X-ray spectrometry investigations of thin gain insights into their nature and as a test of bioge- sections through these silicified mats indicated a nicity. Fifteen hand specimens from the upper Bur- striking absence of sulphur in these rocks. The way Formation with ILS on either the upper or lower microbial layers appear to have been limited to a bedding surfaces were sectioned, serially ground few types of filamentous cyanobacteria without and photographed. The resultant photographic data- any evidence for underlying assemblages of either base enabled examination in cross-section of .30 sulphur-reducing or sulphur-oxidizing bacteria. Lit- examples of ILS on these hand specimens. ILS, tle phosphorus was evident, suggesting nutrient- along with other sedimentary features and microbial poor conditions. The Burway Formation has previ- mats, were also studied in 10 thin sections. Work on ously been interpreted as shallow marine deltaic the microbial mats, including scanning electron sediments and the overlying Cardingmill Grit as a microscopy–energy-dispersive X-ray spectrome- fluvial deposit (Pauley 1986, 1990, 1991). try, is ongoing, following which the thin sections The age of the upper part of the Burway Forma- will be placed in the Oxford University Museum tion is constrained by U/Pb geochronology of tuffs of Natural History, where specimens of ILS previ- from the underlying Stretton Shale Formation and ously collected by Richard Callow and examined the stratigraphically higher Lightspout Formation during this study are deposited (OUMNH A´ .02390– and is estimated to be c. 560 Ma (Compston et al. A´ .02394). 2002; Fig. 1; Menon et al. 2016). Longmyndian Intrites in cross-section Morphologies of Longmyndian Intrites The ILS on bedding planes can look similar to gas The discoidal ILS described from the Long Mynd domes or sediment volcanoes as they are raised are unusual in occurring with the same morphology structures with central depressions, although the on the soles and tops of beds (i.e. they occur in pos- crater flanks are more rounded than the conical itive epirelief and positive hyporelief; Fig. 3). There shape typical of sediment volcanoes. Given the is little size variation within the ILS material, which small-scale fluid escape that characterizes these is generally 2–3 mm in diameter, with rare exam- rocks (see Menon et al. 2016), ILS might be ples reaching 4 mm. This morphotype occurs in a expected to be simply sediment volcanoes. In verti- continuum of forms, from a raised mound with a cal cross-section, however, the ILS consist of two Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Fig. 1. Location and stratigraphic position of Ashes Hollow area of study (after Menon et al. 2016). (a) Simplified geological map of the study area with the site of the specimens indicated. Inset map shows the location of the Long Mynd. (b) Stratigraphy of the Longmyndian Supergroup following the interpretation of Pauley (1990, 1991), with the stratigraphic position of the Ashes Hollow site marked, together with dates measured by Compston et al. (2002). lobes formed from the pinching and swelling surfaces. Some examples only have upper lobes of laminae (Fig. 5). The symmetry of the lobes (e.g. Fig. 5e) and, in some cases, additional laminae above and below the horizontal plane explains the drape the upper lobes to form a dome-like structure identical appearance of the ILS on the sole and top (Fig. 5g). Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Fig. 2. White microbial matgrounds in upper Burway Formation rocks. (a) White crinkly lamina (arrowed) visible in polished cross-section of hand specimen, amid dark sandstone and green mudstone laminae. (b) Ground top of same specimen, viewed from above, showing exposed edge of white lamina seen in (a), revealing the lamina to consist of a mesh of microbial filaments. Scale bars: 1 mm.

In three dimensions, the ILS form either a com- The ILS sometimes approximately overlie other plete, or partially open, torus that is sometimes so ILS at lower stratigraphic levels (e.g. Figs 5c & 6b). open that it forms linear, sub-parallel lobes. ILS The central depression of the torus commonly con- have such a characteristic appearance that they are tains sandstone where the ILS are underlain by a clearly identifiable when entirely within cross- white microbial matground (Fig. 6). The sediment sections, allowing them to be studied in full sedi- is fine-grained within the lobes, commonly paler mentological context (Figs 5c & 6). in colour than the adjacent mudstone laminae, and The ILS in the Longmyndian only occur in asso- may contain pale or white laminae (Figs 5d, g & ciation with mudstone laminae, with which they are 6a). In thin section, the pale areas within the lobes contiguous, and all are associated with white micro- appear dark, suggesting the presence of micrite bial mat laminae that may be above, below, or some- (Fig. 7). times both above and below the ILS (e.g. Fig. 5a). Forms with only upper lobes overlie a thick white microbial mat (Figs 5c, e & 6a). In some examples, Interpretation an indentation is observed on the bed sole, forming the lower halves of the lobes, but the formation of A variety of sub-millimetre-scale dark sandstone the upper halves of the lobes appears to have been features observed in cross-section in upper Burway impeded by an overlying white matground (Fig. 5h). Formation rocks have already been explained as

Fig. 3. Summary of different morphologies of Longmyndian Intrites. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Fig. 4. Various forms of Longmyndian Intrites.(a) On bed sole, both as raised rings with flat centre and as mounds with central depression. (b) On bed top, as raised mound with central depression or two slightly curved parallel ridges. (c) On bed sole as relatively narrow rings and parallel ridges. (d) Counterparts on top surface as shallow negative impressions with central peaks or ridges. (e, f) Close-ups of portions of specimen in (c): sole surface, with complete and near-complete ring-like forms (e); and approximately aligned ridges and pairs of arcs (f) (note different scale for (e) and (f)). Scale bars: (a–d) 1 cm; (e–f) 1 mm. small-scale fluid-escape structures (Menon et al. to produce surface features with rounded flanks, 2016). The common occurrence of such fluid-escape rather than the conical flanks expected of a sediment structures between the lobes of ILS, just above a volcano (e.g. Fig. 5f; Reineck & Singh 1973). The white microbial matground lamina (Fig. 6), may indentations on sole surfaces, with local distortion provide the key to understanding these ILS. Micro- of laminae (e.g. Fig. 5h), suggest failed injections bial mat growth and decay may result in gas effu- or the narrow seepage of fluids through the structure, sions that have the potential to escape upwards, with the indentation producing gentle lobes of dis- sometimes in association with fluidized sand placed muddy sediment on either side. Fluid-escape (Menon et al. 2016). Fluid-escape columns and dis- structures with entrained dark sand are sometimes turbances commonly originate at matground lami- observed to have disrupted the tops of lobes and nae. It is considered that gas, and pore fluids, may are inferred to have exploited the pre-existing sedi- seep through tears in the matgrounds without leav- ment feeder column that created the ILS (Fig. 8). ing any indication of a feeder column below. The It is likely that homogeneous mudstone within ILS may develop around small sediment volcanoes the lobes primarily accumulated by baffling and Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Fig. 5. Longmyndian Intrites in cross-section. (a) Cross-section through a double Longmyndian Intrites on bed sole. (b) Plan view of double Longmyndian Intrites shown in cross-section in (a). (c) Several Intrites-like structures (arrowed), including the edge of one on the top surface showing a single lobe, a full Intrites-like form within the rock and, at the bottom of the image, a shallow Intrites-like form. (d) Longmyndian Intrites on bed sole clearly showing whitish internal layers. (e, f) An unusually large and well-formed Intrites-like form on the bed top, shown in cross-section, illustrating paler colour and different texture within the structure (e) and the plan view (f). (g) Intrites-like structure on bed sole with dome-like laminae above the lobes. (h) Simple indentation on bed sole, compressing the overlying laminae, with some Longmyndian Intrites-like build-up of laminae within the gentle lobes on either side of the indentation. Scale bars: 1 mm. trapping by microbes and may also include some processes have been described as occurring within microbially-mediated authigenic clay mineral preci- days in cyanobacterial biofilms grown in siliciclas- pitation – a process that has been implicated in the tic environments with high seawater silica concen- exceptional preservation of fossils in lagersta¨tten trations and suspended clay (Newman et al. 2016). such as the Burgess Shale (see Orr et al. 1998; Gab- Trapping and binding, together with the pre- bott et al. 2001; Butterfield et al. 2007; Wacey et al. cipitation of authigenic clay minerals and some 2014) and has already been proposed to explain micritic carbonate on to the extracellular poly- the aluminosilicate preservation of Longmyndian meric substances of the microbial mats, may have microbes (Callow & Brasier 2009). Both these caused the build-up of laminae (Dupraz et al. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Fig. 6. (a) Longmyndian Intrites above white microbial mat layer (left, arrowed) showing internal layering and dark sandstone in the dip between lobes; the edge of a further, shallow Intrites-like form is seen to the right (right arrow). (b) Two Intrites-like structures, arrowed, in approximate vertical alignment. The upper specimen has associated sandstone features, whereas the lower specimen is unusual in having a thick white microbial layer above rather than below, although a thin white layer forms the lower bound. Scale bars: 1 mm.

Fig. 7. (a, b) Thin sections through Intrites-like structures in plane polarized light. Note dark areas within lobes in transmitted light, which appear pale in reflected light, suggesting some microbially-mediated carbonate precipitation as micrite. Scale bars: 1 mm.

Fig. 8. Example of Longmyndian Intrites on bed sole, with pale layers within the lobes and disruption by fluid escape above, resulting in the distortion of the still soft and unconsolidated mud laminae at the top of the lobes. Scale bar: 1 mm. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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2009; Noffke & Awramik 2013). Larger micro- The ILS of the Longmyndian are inferred bially-mediated sediment domes and related struc- to result from a combination of stromatolite-like tures without the central depression would be microbial trapping and binding of sediment and called stromatolites. The ILS are far smaller than microbially-mediated clay mineral and carbon- stromatolites – consisting only of several rather ate precipitation with the effusion of fluid and than tens of laminae – and appear to lie on the boun- fine-grained sediment from a sediment volcano. dary between the predominantly two-dimensional The raised topography of a shallow sediment vol- MISS and the mainly three-dimensional stromato- cano would attract cyanobacteria competing for lites (see Noffke & Awramik 2013). light (Gerdes et al. 1994) and give greater access

Fig. 9. A model for the formation of Longmyndian Intrites by the build-up of stromatolitic laminae on the raised topography of a small sediment volcano produced by injection from below with seepage of fluid through compressed laminae (a, c) or effusions of gas through tears in the matground (b). Scale bar: 1 mm. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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Fig. 10. Preferential build-up of current-parallel sides of a low-relief sediment volcano crater due to trapping and binding by cyanobacteria produces two arcs or parallel ridges on the bedding surface. Equivalent structures on sole surfaces result from the sinking of the stromatolitic structure into soft muddy sediment below. Scale bar: 1 mm.

to sediment particles sweeping past on a current, thereby enhancing the microbial trapping and bind- ing of sediment together with possible authigenic clay mineral and carbonate precipitation on the cra- ter flanks, resulting in the growth of a torus-shaped ILS around the central vent (Fig. 9). The drawing up of nutrients from below the mat by fluid escape could also attract higher concentrations of cyano- bacteria around a sediment volcano (Gerdes et al. 1994; Gerdes 2007). However, such a chemotactic response may be less significant in this example because the environment appears to have been nutrient-poor. A combination of the sinking of the ILS sedi- ment cone into the underlying muddy sediment and the displacement of sediment at the injection site is inferred to produce the typical full double- lobed form of the ILS in vertical cross-section, with the lower half of the lobes producing Intrites- like rings on bed soles (Fig. 9a). Effusion through small tears in a thick microbial mat may result in the formation of only the upper lobes (Fig. 9b). In some cases, after the cessation of sediment effusion, the build-up of laminae by microbial trapping and binding continued, producing a more typical stro- matolite-like dome shape, which overarched the original lobes (Figs 5g & 9c). Trapping and binding by cyanobacteria tends to cause stromatolitic laminae to preferentially build up parallel to a current (Logan et al. 1964; Garlick 1988; Schieber 1998, 1999). Thus the portions of the Intrites-like ring parallel to the current are more Fig. 11. (a) Salter specimen BGS GSM 49160 likely to aggrade, resulting in incomplete rings or, in showing microbial surface texture and paired pits extreme examples, parallel ridges (Fig. 10). This (example arrowed). (b) Close-up of Salter specimen influence of the current may explain the alignment shown in (a), showing double pits and microbial of the double-ridge form of ILS observed on some filaments (arrowed); note the close similarity with the positive Intrites-like structures on bed sole in bedding planes (e.g. Fig. 4c). Figure 3b. Scale bars: (a) 1 cm; (b) 1 mm. Images of Currents may also cause the erosion and re- Salter’s specimen BGS GSM 49160 reproduced with deposition of parts of a raised ring, but erosion the permission of the British Geological Survey # does not appear to be a major factor in the formation NERC. All rights reserved. of the incomplete rings of ILS. Had they been purely Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

280 L. R. MENON ET AL. the result of the erosion of an originally complete These markings are otherwise similar to ILS in ring-like build-up based on a sediment volcano, size and appear in association with them. If the then indications should remain below the surface suggestion that microbial build-up may occur of the sunken parts of the complete ring. This is not preferentially in the direction of the current due borne out by grinding through incomplete ring ILS, to the greater trapping and binding of sediment is however. The structure below the bed top matches correct, then cyanobacteria in neighbouring areas the arc-shaped ridges above, thereby favouring up-current are likely to be drawn to the existing localized microbially-mediated sediment aggrada- ridge. The ridge may then expand in an up-current tion rather than the partial erosion of a ring. direction and merge with other current-aligned John Salter’s description of paired pits in these ILS. This would explain the occasional extended rocks, impressions that he interpreted as the twin double-ridge forms and negative counterparts holes of worm burrows and named Arenicolites observed in the Long Mynd. The presence of such (Salter 1856, 1857), has subsequently been refuted extended double ridges also argues against micro- because close examination indicates that a number bial chemotaxis towards nutrients brought up from of the impressions are not paired nor connected below by fluid escape as the main cause of stro- by a U- or J-shaped burrow (Callow et al. 2011b; matolitic growth on the flanks of sediment volca- the genus name is retained as a nomen conser- noes in these rocks. vandum for reasons of nomenclatural stability). Thus ‘Longmyndian Intrites’ can now be under- The impressions on one of Salter’s specimens are stood as a form of torus-shaped microstromatolite unquestionably paired, however (Fig. 11; Callow that grows from a small, fine-grained sediment vol- et al. 2011b). These markings, named by Salter as cano. It can be considered as a new form of MISS. Arenicolites didymus (Salter 1856, 1857), were sub- As microbial mats are understood to have been sequently described as a form of Rusophycus arthro- widespread in the Ediacaran Period, in the absence pod resting trace (R. didymus;Ha¨ntzschel 1975). of grazing, and the physical processes involved in They may, however, now be interpreted as double- the formation of ILS are ubiquitous, it may seem ridge Longmyndian ILS, preserved in negative epi- surprising that ILS and the other discoidal features relief. (R. didymus is also a nomen conservandum observed in the upper Burway Formation (Menon still in use, despite being first used for this abiogenic et al. 2016) are not commonplace in Ediacaran structure.) rocks. However, the environmental conditions under Pairs of longer parallel ridges or counterpart which these Longmyndian markings arose seem pits that cannot be explained as the current-parallel to have been unusual, involving the build-up of sides of an originally ring-like structure are occasio- fine-grained silt layers rich in pore waters and colo- nally observed in the Longmyndian rocks (Fig. 12). nized by filamentous cyanobacteria, which formed

Fig. 12. Elongated paired pits (white arrow), which may be counterparts of merged Intrites-like structures on the sole of the overlying bed, together with typical paired pits (black arrow) and microbial texture, upper bedding surface, Synalds Formation, Carding Mill Valley, Long Mynd. Scale bar: 1 cm. Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

‘LONGMYNDIAN INTRITES’ AS MISS 281 effective seals that may have encouraged and added Supergroup can now be explained by a combination to millimetre-scale fluid escape. This set of condi- of microbial activity and physical processes. There tions is similar to that evoked for the formation of is therefore no longer any evidence for Ediacaran ‘synaeresis cracks’ (Harazim et al. 2013; McMahon macrofossils in these rocks. et al. 2016). Several features of the Longmyndian As the Longmyndian markings have long suite of discs and, in particular, the double-lobed been regarded as evidence for the presence of a pattern of the ILS, result from load structures for- depauperate Ediacaran macrobiota (Salter 1856; med by sinking into muddy sediments below. This Darwin 1859; McIlroy et al. 2005), the discs of combination of very soft muds bound by microbial the Long Mynd represent a cautionary tale. Micro- mats, so well preserved, may be uncommon. bial matgrounds and activity, in combination with Longmyndian sediments are considered to have physical processes, can produce a variety of discoi- been deposited in shallow water (McIlroy et al. dal morphotypes, on bed soles as well as bed tops, 2005). Suites of discoidal markings similar to that may be mistaken as evidence for the presence those of the Long Mynd have been observed in of complex life in ancient rocks. lacustrine Torridonian rocks containing MISS (Cal- The occurrence of several types of MISS in the low et al. 2011a; A. T. Brasier et al. 2016) and ILS Longmyndian succession, including Arumberia, can be seen in possibly fluvial rocks of the Signal elephant-skin texture, Medusinites-, Beltanelli- Hill Group of the Ediacaran succession of the formis- and Intrites-like forms (McIlroy et al. 2005; Avalon Peninsula, Newfoundland, Canada. The Menon et al. 2016), along with direct evidence discoidal markings described in Ediacaran rocks for matgrounds and filamentous microfossils (Peat from South America (Arrouy et al. 2016) may also 1984; Callow & Brasier 2009), demonstrates the represent a suite of discoidal features associated presence of a microbe-dominated ecosystem in a with MISS and ILS should be looked for in such siliciclastic system of late Ediacaran age devoid sediments. The lack of obvious Ediacaran macrofos- of macrobiota that is now in need of significant sils such as rangeomorphs in such assemblages – sedimentological and palaeoenvironmental reas- perhaps because of the shallow water depositional sessment. environments from which they are commonly reported – means that they have received relatively We thank R. Callow, A. Liu, J. Matthews and P. Toghill for little study. It is therefore possible that ILS and the assistance at field sites and in the collection of samples; other discoidal pseudofossils observed in the Long P. Shepherd of the British Geological Survey for access Mynd may be reported with greater frequency in to John Salter’s specimens in the BGS Palaeontological the future. However, the upper Burway Formation Collection; D. Wacey for helpful discussions; and J. Hyde, O. Green and J. Wade for technical assistance. We are rocks of the Long Mynd, with their excellent pre- grateful to D. Grazhdankin and S. McMahon for valuable servation and strong colour contrast, have given feedback which helped improve the paper. DM acknowl- an unprecedented opportunity to study the inter- edges the support of the NSERC and a Canada Research nal architecture of such discoidal markings. Discs Chair. from less heterolithic facies are unlikely to have such clear evidence when cross-sectioned, so that establishing a similar origin for comparable discs References in other successions may have to rely on the mor- phology of surface structures alone. Arrouy, M.J., Lucas, V.W., Quaglio, F., Poire´, D.G., Simo¨es, M.G., Rosa, M.B. & Go´mez Peral, L.E. 2016. Ediacaran discs from South America: probable soft bodied macrofossils unlock the paleogeography Concluding remarks of the Clymene Ocean. Scientific Reports, 6, 30590, https://doi.org/10.1038/srep30590 Our examination of ILS in cross-section demon- Bland, B.H. 1984. Arumberia Glaessner & Walter, a strates that the structures are microbially-modified review of its potential for correlation in the region of sediment volcanoes that developed above, or be- the Precambrian–Cambrian boundary. Geological tween, aluminosilicate-rich microbial matgrounds. Magazine, 121, 625–633. The ILS are always associated with microbial Brasier, A.T., Culwick, T., Battison, L., Callow, matgrounds and fluid-escape processes in a manner R.H.T. & Brasier, M.D. 2016. Evaluating evidence similar to that inferred for forms previously des- from the Torridonian Supergroup (Scotland, UK) for Brasier cribed as Medusinites and Beltanelliformis (McIlroy eukaryotic life on land in the Proterozoic. In: , A.T., McIlroy,D.&McLoughlin, N. (eds) Earth et al. 2005), but now reinterpreted as pseudofossils System Evolution and Early Life: A Celebration (Menon et al. 2016). Our interpretation of ILS as of the Work of Martin Brasier. Geological Society, a new form of microstromatolite built up around London, Special Publications, 448, First published sediment volcanoes means that all the discoi- online October 27, 2016, https://doi.org/10.1144/ dal impressions in the Ediacaran Longmyndian SP448.13 Downloaded from http://sp.lyellcollection.org/ by guest on October 1, 2021

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