Manuscrit accepté / Accepted manuscript Arumberiamorph structure in modern microbial mats: implications for Ediacaran palaeobiology A.V. KOLESNIKOV, T. DANELIAN, M. GOMMEAUX, A.V. MASLOV and D.V. GRAZHDANKIN Reçu le /Received date: 25/06/16 Accepté le /Accepted date: 17/01/2017 Prière de citer l’article de la façon suivante / Please cite this article as: KOLESNIKOV A.V., DANELIAN T., GOMMEAUX M., MASLOV A.V. and GRAZHDANKIN D.V. (2017). – Arumberiamorph structure in modern microbial mats: implications for Ediacaran palaeobiology. – Bulletin de la Société géologique de France, 188, n° thématique (sous presse). Arumberiamorph structure in modern microbial mats: implications for Ediacaran palaeobiology Présence de structure arumbériamorphe dans des tapis microbiens modernes: implications pour la paléobiologie à l’Édiacarien A. V. Kolesnikov,1, 2 T. Danelian,2 M. Gommeaux,3 A. V. Maslov4 and D. V. Grazhdankin1, 5 1Trofimuk Institute of Petroleum Geology and Geophysics of Siberian Branch Russian Academy of Sciences, prospekt Akademika Koptyuga 3, Novosibirsk 630090, Russia, [email protected] 2Université de Lille 1 – Sciences et Technologies, CNRS, UMR 8198 Evo-Eco-Paleo, F 59655 Villeneuve d’Ascq, France, [email protected] 3Groupe d’Étude sur les Géomatériaux et les Environnements naturels Anthropiques et Archéologiques EA 3795 (GEGENAA) – Université de Reims Champagne-Ardenne, 2 esplanade Roland Garros, F 51100 Reims, France, [email protected] 4Zavaritsky Institute of Geology and Geochemistry of Ural Branch Russian Academy of Sciences, ulitsa Vonsovskogo 15, Yekaterinburg 620016, Russia, [email protected] 5Novosibirsk State University, ulitsa Pirogova 2, Novosibirsk 630090, Russia, [email protected] Keywords. – Microbial mats, microbiallyBSGF induced sedimentary structures, Guérande salinas, Ediacaran, Arumberia Mots-clés. – Tapis microbien, structures sédimentaires microbiennes, salines de Guérande, Édiacarien, Arumberia Abstract. – In the course of studying modern halotolerant microbial mats in salterns near the village of Kervalet, western France, we observed fanning-out and curved series of macroscopic ridges on the surface of a newly formed biofilm. The structure resembles the late Ediacaran fossil Arumberia which is globally distributed in Australia, Avalonia, Baltica, Siberia and India, always confined to intertidal and delta-plain settings subject to periodic desiccation or fluctuating salinity. Although the origin of the structure observed in modern microbial mats remains enigmatic, wrinkled and rugose variants of microbial biofilms in general exhibit increased levels of resistance to several environmental stresses. By analogy, the fossil Arumberia !1 ACCEPTED MANUSCRIPT could be interpreted as a microbial mat morphotype (the “Arumberia” morph) developed in response to environmental perturbations in terminal Ediacaran shallow marine basins. If environmental conditions are likely to be responsible for the formation of Arumberia, it is not that a specific biological community has survived since the Ediacaran – it is that the biological response of microbial communities that manifested itself quite commonly in certain terminal Ediacaran and early Cambrian environments can still be found (seemingly in much more restricted settings) today. Résumé. – Dans le cadre d'une étude des tapis microbiens halotolérants actuels, dans des salines situées près du village de Kervalet (France), nous avons observé des ensembles de rides curvilignes disposées en éventail, à la surface d'un tapis microbien nouvellement formé. La structure actuelle observée rappelle Arumberia, fossile de l'Édiacarien supérieur, qui a une très large distribution (Australie, Avalonia, Baltica, Sibérie et Inde), présent uniquement dans des contextes intertidaux ou de plaines deltaïques, sujets à une dessication périodique et à une salinité variable. Même si l'origine de la structure observée dans les tapis microbiens actuels demeure inconnue, les tapis montrant une structure froncée ou rugueuse sont généralement dotés d'une résistance supérieure à différents stress environnementaux. Par analogie, le fossile Arumberia pourrait être interprété comme un morphotype (dit “arumbériamorphe”) de tapis microbien développé en réponse à des perturbations de l'environnement de l'Édiacarien terminal, dans des bassins marins peu profonds. Si le facteur causant la formation d’Arumberia est vraisemblablement les conditions environnementales, on ne peut pas conclureBSGF qu’une communauté biologique particulière ait survécu depuis l’Édiacarien – mais plutôt que certaines communautés microbiennes répondent de la même façon aujourd’hui qu’à l’Édiacarien ou au début du Cambrien mais que ceci se manifeste dans des contextes bien plus restreints. INTRODUCTION Few Ediacaran fossils pose more problems in terms of pattern recognition and fabricational constraints than does Arumberia. The iconic Arumberia represents series of subparallel, curved or fanning- out ridges preserved on upper bedding plane surfaces of sandstone beds. Although originally interpreted as cup-shaped epibenthic organisms [Glaessner and Walter, 1975], an alternative view would be that these structures are either microbially mediated or even inorganic [Brasier, 1979; Jenkins, 1981; Jenkins et al. 1981; McIlroy and Walter, 1997; Gehling, 1999; Seilacher, 2007]. The regular arrangement of ridges in !2 ACCEPTED MANUSCRIPT Arumberia is akin of the quilted body plan of vendobiont organisms (Arumberia was originally compared to Natalia villiersiensis, Nasepia altae, and Baikalina sessilis), although the paucity of distinctive margins to individuals calls for a broader comparison with dissipative systems (to which the vendobiont construction may seemingly belong). With the rare exception of two lower Neoproterozoic localities, one in the Diabaig Formation of northwest Scotland [Callow et al. 2011] and another in the Zilmerdak Formation of South Urals (pers. obs.), and one early Palaeozoic locality in the Port Lazo Formation of France [Bland, 1984; Davies et al. 2016], Arumberia appears to be restricted to the uppermost Ediacaran strata, which presents a serious obstacle to the inorganic interpretation. Furthermore, Ediacaran fossil occurrences of Arumberia in Australia, Avalonia, Baltica, Siberia and India are all confined to intertidal or delta-plain settings subject to periodic desiccation or fluctuating salinity [Bland, 1984; Gehling, 1999; McIlroy et al. 2005; Mapstone and McIlroy, 2006; Kumar and Pandey, 2008; Kolesnikov et al. 2012, 2015]. The prospect of discovering relict elements of Ediacaran ecosystems that survived in modern biosphere has been tremendously appealing, but very few convincing analogues have been found. Xenophyophores, a group of giant deep-sea multinucleate rhizopodial protists are sometimes regarded as model organisms for vendobiont palaeobiology and even as possible living counterparts of Ediacaran palaeopascichnids [Zhuravlev, 1993; Seilacher et al. 2003; Seilacher and Mrinjek, 2011]. Another example are the detached cormidial bracts of siphonophore organisms [O’Hara et al. 2016] that have been compared to Ediacaran fossils Albumares brunsae, Anfesta stankovskii, and Rugoconites enigmaticus [Just et al. 2014]. In addition, concentric ring structures occasionallyBSGF observed in modern microbial mats resemble some of the Ediacaran discoidal fossils, specifically Cyclomedusa davidi and Ediacaria flindersi [Grazhdankin and Gerdes, 2007; Banerjee et al. 2014]. Here we describe fanning-out and curved series of macroscopic ridges on the surface on a newly formed microbial mat and discuss their similarities with the enigmatic Ediacaran fossil Arumberia banksi. FIELD OBSERVATIONS In the summer of 2015, we studied modern microbial mats growing in salterns of the Salines de Guérande Cooperative of Loire-Atlantique, Region of Pays de la Loire, western France (fig. 1A, B). Each saltern, called “salina” (fig. 2A–B) by local residents and cooperative workers, represents a system of reservoirs that are fed by seawater flowing through narrow channels and, occasionally, tubes. During spring tides the turbid seawater from a tide input channel (étier) passes through a gate (controlled by a salt worker) !3 ACCEPTED MANUSCRIPT and enters a special first decantation pond (vasière) where the suspended particles settle down as water salinity increases due to evaporation. Salinity has been measured by refractometer in each pond along the entire water circuit in three salinas (fig. 2A). The water becomes progressively more clear and salty (and hence of higher density) as it flows through an optional secondary filtering decantation pond (cobier) and a chain of fares ponds (salinity values vary between 5.5% and 17%) until it reaches the maximum clarity and salinity in brine storage adernes ponds (salinity up to 34%). The salt is collected in œillets ponds where salt workers (paludières) regularly sweep the salt crystals with a wooden spade and pile it on a round platform called the ladure. The floor in the salina (except for the œillets ponds) is covered with halotolerant microbial mats which stabilise the sediment and prevent it from further suspension by meteoric water and wind action [Giani et al. 1989; Gerdes et al. 1993, 1994; Gerdes, 2007]. The uppermost layer in microbial mats is dominated by cyanobacteria and diatom algae. The layers underneath record a steep gradient in redox potential. The fares and adernes ponds are cleaned, on average, once every 10 years when the