Elliptical body fossils from the Fortunian (Early Cambrian) of Normandy (NW France)

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INTRODUCTION
The Fortunian biota (541-529 Ma) represents the first step of the Cambrian metazoan biodiversification referred as the "Cambrian explosion" (Conway Morris 1989;Erwin et al. 2011;Kouchinsky et al. 2012). The stratotype defined in Newfoundland, Canada, is located at the Fortune Head locality in the north of the Burin peninsula. The palaeontological content is mainly documented by ichnofossils (e.g. Mángano & Buatois 2017;Laing et al. 2019), especially treptichnids like Treptichnus pedum (Seilacher, 1955), an ichnospecies long considered as the stratigraphical marker of the Cambrian GSSP (Narbonne et al. 1987;Brasier et al. 1994). However, treptichnids lato sensu (Treptichnus ichnosp. indet.) are known in older levels (Buatois 2017) and the Precambrian/Cambrian boundary is not so easy to define. As far as body fossils are concerned, the end of the Fortunian is marked by an increase in Small Shelly Fossils or worm imprints in various areas worldwide (Landing et al. 2013), and especially well documented in China (e.g. Shao et al. 2015;Zhang et al. 2015Zhang et al. , 2017. Fortunian fossilized organisms are documented by several localities throughout the world in addition to the outstanding stratotype. Until now, limited data are available from the western part of Europe and no body fossils have been described from well-dated early Cambrian deposits from France. Only a few elliptical fossils, close to Nimbia Fedonkin, 1980, have been mentioned in the Brioverian deposits of Brittany, more likely late Ediacaran in age (Néraudeau et al. 2016(Néraudeau et al. , 2018Gougeon et al. 2019). However, Normandy (NW France) may be regarded as an optimal geological framework to record early Fortunian organisms. Indeed, the Cotentin peninsula, at the NE of the Armorican Massif, provides large exposures of marine early Cambrian strata (Jenkins 1984). On the one hand, body fossils (archaeocyathids, hyolithids, trilobites) were recorded in the southern and younger outcrops of the local Cambrian, especially near Carteret and Saint-Jean-De-La Rivière (Bigot 1925;Pillola 1993;Doré 1994;Pillola et al. 1994;Went 2020;Néraudeau et al. pers. obs. 2020) (Figs 1B;2). On the other hand, a lot of ichnofossils have been found in the northern and older part, of Fortunian age, at the Rozel Cape (Doré et al. 1984;Doré 1994;Went 2020).
This paper presents the first potential body fossils from the Fortunian of France, in deposits well dated by the abundance of the ichnostratigraphical index Treptichnus pedum and U-Pb zircon dating.

GeoloGical framework The Cambrian series from the Rozel Cape
The North of the Cotentin peninsula, at the NE of the Armorican Massif, provides large exposures of marine Paleozoic strata (Fig. 1A). On the western coast, the Cambrian series outcrop mainly at the northern and southern parts, Ordovician and Devonian deposits outcropping in the intermediate areas ( Fig. 1B). At the north, an Hercynian granitic intrusion has intersected the previous deposits and widely outcroups around Flamanville (Fig. 1B). The Cambrian series are deposited unconformably on top of a volcanic basement (Cadomian orogeny) from the Saint-Germain-le-Gaillard complex, outcropping near the Grosville village (Guerrot et al. 1992;Doré 1994) (Figs 1B ; 2). On the Rozel Cape cliffs and shore, the contact between the fossiliferous Cambrian strata and the volcanic rocks cannot be observed, the coast being 2 kms away from the basement outcrops.
The Cambrian deposits are progressively younger from the North, at the Rozel Cape (with Treptichnus pedum), to the South, on the coast of Carteret and Saint-Jean-de-La Rivière (containing archeocyathids, hyolithids and trilobites) (Fig. 2). At the Rozel Cape, the deposits are mainly sandstones and siltstones, showing many sedimentary structures (ripple marks, syneresis cracks, diagenetic nodules) and sometimes conspicuous microbial mats (Fig. 1C). The elliptical fossils studied herein come from the older Cambrian layers, outcropping on the Rozel Cape coast. The fossiliferous facies The fossiliferous layer (Figs 1C; 3B) is located on a fine micaceous siltstone surface (Fig. 1C) extremely rich in well preserved exquisite and various ichnofossils (mainly Treptichnus pedum), typical of the early Cambrian (Fig. 4). This layer is about 2 m above two key horizons with large (c. 10 cm) diagenetic nodules   Néraudeau D. et al. 2023 and 2 m under 5 m of deposits especially rich in syneresis cracks (Fig. 1C). The fossiliferous surface shows a few sedimentary structures due to sedimentary transport processes: -ripples are present, but of small wavelengths (about 150 mm) and width (about 20 mm), inconspicuous with a very faint relief (Fig. 4A); large and conspicuous ripple marks can be observed on younger (3 m above) and older (2 m under) beds only (Fig. 1C); -all the elliptical fossils have the same orientation, their alignment by the current being approximately SW-NE and crossing the previous slight ripples with an angle about 45°; -most of the elliptical fossils have been tilted by the current, their "northern" edge being in relief on the substrate when their "southern" edge is hidden by sediment; -the cluster of elliptical structures located at the NE of the fossiliferous surface includes several overlappings or repeated slippings of elliptical imprints; specimens have been accumulated either displaced on the substrate surface; -two isolated elliptical specimens seem out of current effects and are more horizontal, with a more visible complete outline: the first one at the northern part of the shale bed surface ( Fig. 4A: 1) and the other one at the southern part, near a treptichnid trace fossil ( Fig. 4B: 2); -two parts of the fossiliferous surface can be distinguished: a part with a lot of ichnofossils and elliptical structures and a part where fossils are rarer but load cast cup imprints very abundant (area in the lower mid part of Fig. 3B, beginning in the left lower side of Fig. 4B).
Except for the ripple marks present in various levels of the section, above the fossiliferous layer the sediments are rich in syneresis cracks (Fig. 1C). Thus, sedimentary and ichnological characteristics are compatible with a very shallow platform environment, periodically subjected to unidirectional current on a tidal flat area. The activity of the burrow makers and the elliptical fossils preservation took place during mud deposition at low energy levels.

MATERIAL AND METHODS
The elliptical fossils are preserved on a single shale surface, observed at the base of a cliff at the Rozel Cape, during field trips in February 2021 (Figs 1C; 3). The precise location of the fossiliferous outcrop is 49°28'19.46"N, 1°50'42.06"O. The shale surface shares approximately 80 specimens. However, it is difficult to precise their real number because they are more or less complete and covered by sediments (Fig. 4). The specimens are gathered in two clusters, with a first one on the cliff side of the outcrop (Fig. 4A), and the other one on the seaside (Fig. 4B). The total size of the shale bed is about 50 cm long and 30 cm width. The shale bed was collected and is housed at the University of Rennes, in the Collections of the Geological Institute, with the number IGR-23261A. A thin section of the fossiliferous shale (IGR-23261B) has been realized, crossing three elliptical fossils, to understand if they are surface reliefs only or 3D structures.
A sample (GRO21-2) from the Saint-Germain-le-Gaillard ignimbrite was collected in a small quarry along the road D367 (49°31'02.1"N/1°44'16.6"O), near the Grosville village (overlapping or rebound) and are associated to slight ripples, and trace fossils, some burrows "piercing" previous body fossils imprints (specimen 3), a single elliptical structure being almost complete (specimen 1); a load cast from a younger layer deform the elliptical fossils bearing surface; B, cluster on the sea side where several elliptical structures are associated to trace fossils, some burrows "piercing" or crossing previous body fossils imprints (specimens 4, 5, 6), a single elliptical structure being almost complete (specimen 2); a load cast from a younger layer deform the elliptical fossils bearing surface. Abbreviations: CD, current direction; He, Helminthopsis Heer, 1877 or Helminthoidichnites Fitch, 1850 segments; MLC, load cast; R, slight ripples; Tr, Treptichnus Miller, 1889. Scale bars: 20 cm. Photo credit: Didier Néraudeau. Néraudeau D. et al. 2023 ( Fig. 1). After the crushing and the mineral separation, magmatic zircon grains were mounted in an epoxy puck, polished to expose their inner parts and imaged by cathodoluminescence. The U-Pb dating was performed in the GeOHeLiS analytical platform (Université de Rennes) by laser ablation ICP-MS (LA-ICP-MS). More information on the analytical protocol can be found in Manzotti et al. (2015) and in Appendix 1.

GEOCHRONOLOGICAL CONSTRAINTS
Twenty-three analyses were acquired on 20 different zircon crystals (Appendix 2). Reported in a Wetherill's concordia diagram (Fig. 5), they all plot in a concordant position yielding a concordia age of 550.6 ± 5 Ma (MSWD (conc+Equiv) = 0.67, n = 23) that we interpret as the emplacement age for this volcanic flow. This age is within error of the poorly constrained evaporation age of 531 ± 20 Ma (Guerrot et al. 1992), demonstrating that the volcanic activity in the region took place during the late Ediacaran. As the fossiliferous sediments from the Rozel Cape were unconformably deposited on top of these ignimbrites, their oldest possible deposition age is therefore late Ediacaran.
The suite of the sedimentation of the Rozel Cape outcrops 10 kms to the south, in the Carteret coast (Fig. 1B) where it has been dated to the lower and mid-Cambrian by stratigraphical fossil index such as hyolithids and trilobites (Pillola et al. 1994).
Thus, the Cambrian deposits from the Rozel Cape are older than Cambrian series containing hyolithes or trilobites and younger than 550 Ma ignimbrites basement. Consequently, the Rozel Cape fossils can only be the earliest Cambrian in age, probably Fortunian.
Biostratigraphically, complex burrows such as Treptichnus pedum (Dzik 2005), typical of the base of the Cambrian, are abundant in the Fortunian strata from Rozel Cape and are associated to simple horizontal trails on the shale bed surface with the elliptical fossils studied in this paper.

ICHNOFOSSILS AND MICROBIAL MATS
ichnoloGy Trace fossils are associated with the elliptical fossils. The first part of the ichnofossils corresponds to elongated, straight (Helminthoidichnites Fitch, 1850)   ("He" on Fig. 4). A few horizontal trails with lateral levees have a larger, plurimillimetric, diameter (Archaeonassa Fenton & Fenton, 1937). Other ichnofossils correspond to varying typical treptichnids, with a rectilinear ("Tr" on Fig. 4A) or an arched arrangement ("Tr" on Fig. 4B). When arched, their alignment constitutes an oval arrangement of 2.5 by 2 cm. It can be noticed that the two areas where elliptical fossils are numerous and make clusters are close to Helminthopsis and Helminthoidichnites when the areas rich in treptichnids are very poor, but not devoid, of elliptical specimens, a complete elliptical structure being close to Treptichnus Miller, 1889 ( Fig. 4B: specimen 2), and a few other ones being crossed by treptichnid burrows (Fig. 4A: specimen 3; B: specimens 4-6). As treptichnids "pierce" the elliptical fossils, it can be concluded that these burrows were realized after a first step of body fossil preservation.  Néraudeau D. et al. 2023 fossiliferous layer, crossing three elliptical fossils (Fig. 6A, B) and a treptichnid, reveals a triple layer mat, with a clear lamina between two dark, brownish laminae ( Fig. 6C-F). The mat covers the elliptical fossils and could have played a key role in the preservation of these structures very uncommon in the Cambrian series from the Rozel Cape. The elliptical fossils are not microbial mat folds or fragments, nor elliptical holes in a microbial mat due to gas bubbles, like the ones illustrated by Davies et al. (2016: fig. 10G). Finally, no conspicuously wrinkled mats, observed in overlaying layers (Fig. 1C), can be distinguished in association with the elliptical fossils.

Description
The elliptical fossils from the Rozel Cape have been deposited on a fine-grained mud, without load effect deforming the sediment, and cannot be confused with load casts (Fig. 6C-F). They have the same SW-NE orientation by currents, but are not lengthened by elongated impressions that characterize flute casts and tool marks (Figs 4; 6A; 7A, B). They correspond to both reliefs on the shale surface and 3D structures within the sediment. They have a "pancake" shape in section view, with an unconspicuous inner wall and a clear sedimentary content different from the surrounding sediment, darker and richer in phyllosilicates with a sub-horizontal arrangement (Fig. 6C-F). The zoom observation of the thin section, both in natural and polarized light, shows that the phyllosilicates are present with various orientations within the elliptical fossils and correspond to a re-crystallization and not to a sedimentary filling. The elliptical fossils are relatively irregular in size, ranging between 5 mm and 12 mm in length. However, their precise size is sometimes difficult to measure, when their right end (SE, according to the orientation on Fig. 6A; 7A, B, D) slightly dives or disappears in the sediment (Fig. 6B). They are often slightly tilted by the currents in the mud, like the similar specimens from the Brioverian of Brittany described by Néraudeau et al. (2018) (Fig. 7G, H). Their thickness, about 0.8-1 mm, generally increases and forms a larger bulge on the upstream side (on the left of the section) ( Fig. 7D-F). The bulge crosses partly the microbial mat covering (Fig. 6C, D). On the shale surface, the peripheral bulge is 1 to 2 mm in width, and surround a large central disc, generally flat or slightly depressed (Figs 6; 7C, E). The elliptical fossils stand on a single sediment surface, but are not regularly spaced, with two main clusters and a few isolated specimens. The first cluster, located at the NW of the shale bed surface, is composed of specimens with a conspicuous peripheral bulge, some imprints seeming to overlap other ones (Fig. 7A, B, E, F). It is difficult to distinguish between two possibilities: first, it corresponds to the real overlapping of several specimens, gathered by the current; secondly a single elliptical fossil produced several imprints, after repeated slipping. The second cluster, located at the SE of the shale bed surface, is composed of specimens with a less conspicuous peripheral bulge and a more variable shape. The more conspicuous and complete specimens are relatively small (6 mm), uncommon and isolated near treptichinid trace fossils ( Fig. 4: 1, 2). Apart from these small, isolated specimens, the elliptical fossils have the same orientation and the same NW-SE axis of elongation in their clusters, with always the northern part conspicuous with a crescent of marginal bulge on the upstream side.

DISCUSSION
According to their conspicuous elliptical outline and "pancake" profile, their permanent peripheral bulge, their little variable size, and their overlappings, the fossils of the Rozel Cape cannot correspond to sedimentary structures such as load cast or groove casts, or even raindrop imprints, and no more to surface openings of burrows (e.g. Monocraterion Torell, 1870;Schlirf & Uchman 2005).
According to their single peripheral bulge, without ornamentation, elliptical fossils from the Rozel Cape clearly differ from typical Ediacaran discoid fossils such as Aspidella Billings, 1872, Cyclomedusa Sprigg, 1947, Liaonanella Liu & Yang, 1988, Marsonia Raghav et al., 2005and Medusinites Sprigg, 1949 which show strong concentric lines or regular radial structures (Wade 1972;Narbonne & Hofmann 1987;Narbonne & Aitken 1990;McCall 2006;Zhang et al. 2006;McGabhann 2007;Kumar & Ahmad 2012;Burzynski & Narbonne 2015;Tarhan et al. 2015). The "pseudo-concentric" lines observed on few specimens from Le Rozel correspond to overlapping or repeated slipping of their elliptical imprints (Fig. 7A, B, E, F). The overlappings and slippings group generally two (Fig. 7E, F) to four or five specimens (Fig. 7A, B), with 220 to 280° of they elliptical outline being conspicuous. It is clearly different from the linear series of numerous arches or lobes forming the fossil Palaeopascichnus Palij, 1976(Palij 1976Dong et al. 2022). The fossils from the Rozel Cape more likely correspond to elliptical organisms accumulated on fine sediment surface. The unconspicuous radial lines observed between two consecutive concentric bulges (overlapping or repeated slipping) of a single specimen are probably linked to slipping and taphonomic folds of its body (central specimen of Fig. 7A, F).
The low thickness, the peripheral bulge and the lack of a deep central depression differentiate the Fortunian specimens of Normandy from the Intrites punctatus of Ediacaran deposits of United Kingdom and from the very variable and unconspicuous "Medusinites aff. asteroides" from the same outcrops (McIlroy et al. 2005;Liu 2011).
The elliptical outline, the thick peripheral rim and the lack of both fine concentric lines and dome shape challenge an affinity with most of the convex rounded fossils corresponding to Beltanelloides-like structures such as the Beltanelliformis, Beltanelloides, and The flat elliptical fossils observed at the Rozel Cape strongly differ from the lower Cambrian plug-shape burrow Bergaueria, such as Bergaueria perata Prantl, 1945or B. langi (Hallam, 1960 Fig. 4) with its peripheral bulge, close to a Helminthoidichnites trace fossils; D, specimen with a single crescentic bulge (no. 8 in Fig. 4); E, F, specimens with overlapping or slipping showing pseudo-concentric arched lines; the specimen of picture F (no. 7 from Fig. 4) shows unconspicuous radial lines (pointed by the arrows) on its left side; G, H, Nimbia sp., Brioverian of Saint-Gonlay, Brittany, shale IGR-PAL-2778: G, specimen IGR-PAL-2778a; H, specimen IGR-PAL-2778b; each specimen size is about 10 mm; I-K, specimens of Nimbia published in previous works (their sizes vary from a few millimeters to almost two centimeters): I, type specimen from Siberia; J, specimen from West Africa; K, specimen from Kazakhstan. Abbreviations: He, Helminthoidichnites; Tr, treptichnids. Photo credits: A-H, Didier Néraudeau;I, Fedonkin 1980;Fedonkin et al. 2007;J, Bertrand-Sarfati et al. 1995;K, Meert et al. 2011 Néraudeau D. et al. 2023 Seilacher, 1990, corresponding doubtfully to the same ichnogenus, is rather similar to the specimens from the Rozel Cape according to their subcircular disc shape displaying laterally a repeated p attern of crescent-shaped impressions (Hofmann et al. 2012), but without a conspicuous peripheral bulge. Bergaueria sucta has probably to be considered as an ichnological species belonging to another ichnogenus than Bergaueria, considered as a short, plug-shaped, vertical sea anemone burrow (Prantl 1945;Pickerill 1989;Lima & Netto 2012), and not as the imprint of the disc shape base of an actinian cnidarian, as suggested by Seilacher (1990), the laterally repetitive crescentic pattern recording the sideway-migration of the trace-maker.
Finally, according to their elliptical shape, their conspicuous peripheral rim, their size, and the lack of central depression, the Fortunian elliptical fossils from Normandy are similar to the Cambrian discoidal structures from New Brunswick (Hagadorn & Miller 2011: fig. 4g, h), and the elliptical fossils from California (Sappenfield et al. 2017: fig. 3h). In addition the fossils described herein are close to Nimbia occlusa from the White Sea, in Russia (Fedonkin 1980;Fedonkin et al. 2007; Fig. 7A (Crimes et al. 1995). The Fortunian specimens from Normandy are finally close to the elliptical structures recently described from the Ediacaran-Fortunian beds (Brioverian) from Brittany, putatively referred to "Nimbia-like" fossils ( Fig. 7E, F) (Néraudeau et al. 2018).
The biological attribution of Nimbia is not consensual and different interpretations are possible. Nimbia was generally viewed as a cnidarian (Fedonkin 1980;Sepkoski 2002;Meert et al. 2011) or as a microbial mat structure (Grazhdankin & Gerdes 2007;Liu et al. 2013). Sometimes Nimbia as been interpreted as a scratch circle formed by the site of attachment of organisms (Jensen et al. 2002). However, various elliptical fossils were named Nimbia and the genus is probably a catchall term. Consequently, Erwin et al. (2011) removed the three classical species of the genus (N. dniestrei, N. occlusa, N. paula) from their phylogenetic analysis of the Ediacaran biota because they consider the taxa poorly or incorrectly described.
The Nimbia occlusa from Eire, interpreted as scratch circles by Jensen et al. (2002), were previously interpreted as transported rigid-bodied fossils by Crimes et al. (1995) and correspond to two possibilities linked to body fossils. But generally, the alternative is between a "medusoid" body fossil option and a microbial mat one.
Grazhdankin & Gerdes (2007) have considered that many enigmatic discoïdal fossils from the Neoproterozoic, such as Cyclomedusa or Paliella, are microbial mats only, when Young & Hagadorn (2020) have concluded that medusan preservation is attested for Cambrian strata but not for Neoproterozoic ones. Unfortunately, these two last publications do not illustrate their conclusion by any microstructural observation provided by thin sections or Microtomographic Imaging of the fossils. Even if their conclusions are questionable, they refer to Ediacaran circular or elliptical fossils very different in shape and size of the small, centimetric and non-concentric typical Nimbia sensu Fedonkin (1980). On the other hand, the big non-concentric Ediacaran Nimbia published by Liu et al. (2013) as microbial mats do not correspond to the centimetric elliptical fossils from the Brioverian of Brittany and the Fortunian of Normandy. Overall, by combining both observations from thin sections or microtomography, the elliptical fossils reveal 3D structures with a "pancake shape", clearly covered by a microbial layer, but not corresponding to a fold, a fragment, or a development of a microbial mat, nor a sedimentary structure as a load cast. We tentitavely consider the elliptical fossils from the Rozel Cape as body fossils, possibly ctenarian comb jellies such as the modern Pleurobrachia pileus (O.F. Müller, 1776), also named "sea gooseberry", a centimetric and ovoid gelatinous animal.
The deposit environment of the Nimbia-like fossils from the Rozel Cape was coastal and shallow, according to the abundance of ripple marks and syneresis cracks at different levels in the geological formation. It is difficult to compare the palaeoenvironmental characteristics of these body fossils with the ones of the previously published specimens because various fossil structures have been named Nimbia (see above). Nimbia-like fossils are known generally in shallow water sequences, but Crimes et al. (1995), mentioned deep-water habitat for the Eire specimens with a central tubercle. However, at the scale of the Armorican Massif, the Fortunian Nimbia from Normandy had probably a slightly shallower habitat than the ones from the Ediacaran of Brittany, found in sedimentry series poor in ripple marks and devoid of syneresis cracks (Néraudeau et al. 2018).

CONCLUSIONS
The body "pancake" fossils from the Rozel Cape are slightly elliptical with a pancake profile, a peripheral bulge and a centimetric size. In addition they have a dominant orientation and the composition of their filling is like the surrounding sediments. According to the characteristics previously described, we assume that the elliptical fossils from the Rozel Cape cannot be interpreted as surface sedimentary structures (e.g. load-cast imprints), nor as burrow openings. Therefore, we suggest that they could be interpreted as body fossils close to the genus Nimbia, previously related to elliptical fossils found in late Ediacaran-Fortunian deposits from Brittany, approximately 200 km away from the Rozel outcrop. U-Pb dating on magmatic zircon grains extracted the Saint-Germainle-Gaillard ignimbrite yields a late Ediacaran (c. 550 Ma) maximum deposition age for this sample, a result compatible with an early Cambrian age for this fossiliferous shale. This new discovery reveals the interest of the Cambrian series from Normandy to investigate the biodiversity evolution around the Ediacaran-Cambrian boundary in NW Europe. Néraudeau D. et al. 2023

Data Processing
Gas blank 20 seconds on-peak Calibration strategy GJ1 zircon standard used as primary reference material, Plešovice used as secondary reference material (quality control) Common-Pb correction, composition and uncertainty No common-Pb correction. Ages are quoted at 2 sigma absolute, propagation is by quadratic addition according to Horstwood et al. (2016). Reproducibility and age uncertainty of reference material are propagated. Quality control / Validation Plešovice: concordia age = 339.9 ± 4.5 Ma (n = 6; MSWD (conc + Equiv) = 0.94) appenDix 1. -Operating conditions for the LA-ICP-MS equipment. appenDices