# Quantitative analysis of horizontal bioturbation from Brioverian (Ediacaran - Fortunian) deposits of Brittany (Armorican Massif, NW of France)

Baptiste Coutret, Didier Néraveau

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Baptiste Coutret, Didier Néraveau. Quantitative analysis of horizontal bioturbation from Brioverian (Ediacaran - Fortunian) deposits of Brittany (Armorican Massif, NW of France). Annales de Paléontologie, 2022, 108 (2), pp.102543. 10.1016/j.annpal.2022.102543. insu-03758696

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# Quantitative analysis of horizontal bioturbation from Brioverian (Ediacaran - Fortunian) deposits of Brittany (Armorican Massif, NW of France)

## Analyse quantitative de la bioturbation horizontale des dépôts du Briovérien (Ediacarien-Fortunien) de Bretagne (Massif Armoricain, NO France)

Baptiste COUTRET¹,²*, Didier NÉRAUDEAU¹

¹Univ. Rennes, CNRS, Géosciences Rennes, UMR 6118, F-35000, Rennes, France

²Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada.

*Corresponding author: baptiste.coutret@gmail.com

## ABSTRACT

Ediacaran-Cambrian bioturbation on bedding planes provides physical and chemical data about the environmental conditions and biological activities of early metazoans. We propose a quantitative method to estimate horizontal disruption of the substrate using bedding-plane trace fossils from Brioverian deposits. This methodology provides the first quantitative analysis of the trace fossil assemblage from the Armorican Massif (Brittany, NW of France). The Ediacaran-Cambrian transition within the Brioverian series is characterized by the abundance of simple and horizontal trace fossils such as *Helminthopsis*, *Helminthoidichnites*, and *Palaeophycus* as well as rare *Gordia* and *Spirodesmos*. Recent U-Pb dating has been carried out on detrital zircon grains from the upper Brioverian series suggesting a late Ediacaran to Fortunian age of the fossiliferous deposits (ca. 550 to 530 Ma). With Arc-Gis software, we use a semi-quantitative approach to estimate the relative 2D bioturbation rate recorded on bedding planes. The dataset is used to discuss the feeding strategies and how the seafloor ecospace was colonized by the early bilaterian metazoans. This approach combines new values such as: the length of a trace fossil, the surface of a trace, the cumulative length, and the cumulative surfaces from all of the ichnofossils recorded on the same slate surface, to finally suggest that the bioturbation rate of the microbial grazers impacted the use of the seafloor ecospaces and feeding strategies through the biomats.

**Keywords:** 2D analysis, Armorican Massif, Biometry, Bioturbation, Brioverian, Brittany, Ediacaran-Fortunian, Ichnofossils, NW France
## RESUME

La bioturbation visible en surface de banc dans les dépôts édiacariens-cambriens fournit des informations physiques et chimiques concernant les conditions environnementales et les activités biologiques des premiers métazoaires. Nous proposons une méthode quantitative pour estimer la bioturbation horizontale du substrat à partir des traces fossiles préservées sur des plaques de schistes provenant des dépôts briovériens de Bretagne, dans le Massif Armoricain (N-O France). Le passage Ediacarien-Cambrien au sein de la série briovérienne est documenté par l'abondance de traces fossiles horizontales simples telles que les ichnogenres *Helminthopsis*, *Helminthoidichnites* et *Palaeophycus* et la présence, plus rare, de *Gordia* et *Spirodesmos*. De récentes datations U-Pb réalisées sur des zircons détritiques du Briovérien supérieur ont révélé un âge s'étalant entre l'Ediacarien tardif et le début du Fortunien pour ces dépôts fossilifères (ca. 550 à 530 Ma). Avec une approche semi-quantitative, menée avec le logiciel Arc-Gis, nous avons estimé la bioturbation 2D relative visible en surface des niveaux schisteux. Les données obtenues permettent de discuter les stratégies d'alimentation et la colonisation de l'éco-espace benthique par les premiers métazoaires bilatériens. Cette approche combine des nouvelles données : la longueur d'une trace fossile et sa surface, la longueur et la surface cumulées des traces présentes sur une même surface de banc (ou plaques de schistes), et donne finalement une idée relative de la densité en organismes présents sur le substrat, tout en suggérant que le taux de bioturbation a impacté la répartition des organismes brouteurs et les stratégies d'alimentation à travers les biomats.

*Mots-clés:* Analyse 2D, Biométrie, Bioturbation, Bretagne, Briovérien, Ediacarien-Fortunien, Ichnofossiles, Massif Armoricain, NO France

## 1. Introduction

Trace fossils from the Ediacaran-Cambrian boundary provide crucial paleontological information to understand the evolution of early animal life, prior to the advent of mineralized skeletons related to the Cambrian Explosion (Mángano & Buatois, 2020). In addition, trace fossils and bioturbation analyses are used to better understand the paleoenvironmental conditions and the ecology of benthic metazoans. These studies are further complemented by deep analyses of the physical and the chemical depositional conditions and by the study of benthic biological activity (Miller & Smail, 1997; Marenco & Bottjer, 2010, 2011; Gingras et al., 2011a). To shed light on early bilaterian life, different studies have also emphasized the study of organism-sediment interaction and the evolution of benthic behavior (e.g. Liu et al., 2010; Rogov et al., 2012; Pecoits et al., 2012). In fact, many
trace fossils from this period are recorded worldwide (e.g. Crimes, 1987: fig. 4; Jensen, 2003) and mostly reflect feeding strategies on microbial mats (Buatois et al., 2014). For example, horizontal trace fossils related to microbial grazing are extremely common in Ediacaran-Cambrian deposits (Buatois & Mángano 2011; Buatois et al., 2014).

In northwestern France, upper Brioverian marine deposits are locally rich in ichnofossils and Microbially Induced Sedimentary Structures (MISS) (Gougeon et al., 2018a). In addition, U-Pb dating of zircons suggested an age between 550 to 530 million years for the fossiliferous beds and confirmed that the upper Brioverian series belongs to the Ediacaran-Fortunian time interval (Gougeon et al., 2018a, 2021). The trace fossil assemblage of these deposits was historically described by Lebesconte (1886) and placed within the ichnogenera *Montfortia* Lebesconte 1886. Later, Gougeon et al. (2018a) revised the Brioverian trace fossil assemblage and the associated microbial mats firstly named *Neantia* structures by Lebesconte (1886). According to this revision, the trace fossil assemblage comprises abundant *Helminthopsis* Heer 1877, *Helminthoidichnites* Fitch 1850, and *Palaeophycus* Hall 1847 as well as a few other ichnogenera such as *Gordia* Emmons 1844 or *Spirodesmos* Andree 1920 (Gougeon et al., 2018a, 2019, 2021). Despite this taxonomic work, no investigation on the bioturbation density nor of the behavior changes or benthic ecology has been proposed for French Brioverian deposits. This observation is surprising since numerous and well-preserved specimens were found in these deposits, which facilitates their studies and interpretations. In addition, this is particularly interesting because the timing and the tempo between the Ediacaran to the Cambrian ecosystems are apparently asynchronous, depending on the locality and paleoenvironmental deposit conditions (Wood et al., 2019; Bowyer et al., 2022). Therefore, to gain in a global view of these events, additional investigations on these deposits should be conducted.

To embrace this idea, we investigate the relative 2D bioturbation rate of upper Brioverian bedding plane surfaces using a quantitative analysis of the simple, horizontal ichnofossils (e.g. *Helminthoidichnites*, *Helminthopsis* and *Gordia*). A quantitative approach is used to provide a relative overview of the surface bioturbation density and at the same time, to compare and discuss the different behaviors of the early metazoan as recorded in our assemblage. Finally, by quantifying the bioturbation and the trace fossil-biomat associations from the Ediacaran-Cambrian transition, when fundamental geobiological changes occured (Seilacher, 1999; Bottjer et al., 2000), we aim to provide new information about early mobile life and the seafloor ecospace colonization from the Ediacaran-Cambrian transition in Brittany.

## 2. Material & Methods
*Geographical framework.* - All ichnofossils used for this study were sampled by the authors and their collaborators during the last seven years in two localities from Brittany (NW France): Le Bois du Buisson quarry at Montfort-sur-Meu and La Lammerais quarry at Saint-Gonlay (Fig. 1). The samples are housed in the Geological Institute of the University of Rennes 1, with the collection numbers IGR 90174 to IGR 90184.

The trace fossils are present in slate shales from the upper Brioverian, corresponding to greenish shales at Montfort-sur-Meu and to grey-blue shales at Saint-Gonlay. The average size of fossiliferous slate samples is between 15-20 cm by 10-15 cm. Ichnofossil abundance varies on the slate surface, and both microbial mats (MISS) and grazing trace fossils are observed on bedding planes. MISS are generally preserved as wrinkle surfaces, occasionally slightly oxidized (Gougeon *et al.*, 2018a).

*Methods.* - The bioturbation rate (in %) from the upper Brioverian fossiliferous deposits was determined by using the Arc-Gis software calculating the percentage area of the horizontal trace fossils substrate disruption. The material from the two different sites (Saint-Gonlay and Montfort-sur-Meu) was screened to choose the most relevant slates containing the most visible traces were sampled – the choice was not influenced by the shape of the traces fossils and the sampling was randomized by several persons external to the study – then each slate was cleaned using a thin brush to remove the remaining dust and facilitate imaging process.

The analytical process includes 4 steps:

- step 1: after sampling, photographs were taken with a Canon Eos 1200 D digital camera equipped with a 50 mm focal length camera lens; the slates were placed on a black background containing a scale bar; an oblique light source was used to bring out the traces (Fig. 2A).

- step 2: the photographs were imported into ESRI's Geographic Information System (GIS) software (ArcGIS Desktop - ArcMap 10.8); once the slate was scaled, its outline was scanned into a polygon feature (shp) file; its total surface area (in dm²) was then determined (Fig. 2B);

- step 3: the traces were scanned into a polyline feature (shp) file; the aim was to reconstruct each trace outline in the form of distinct lines; the length of each trace was calculated with ArcGis; following the treatment, the total accumulated lengths for each slate were calculated (Fig. 2C);

- step 4: to obtain the surface disruption of each trace, a 2 mm buffer feature based around the previously defined lines was created (Fig. 2D); it was determined to be 2 millimeters in thickness, which represents the maximum width observed in the trace fossil assemblage studied.

This operation enabled us to determine the percentage of disruption and the bioturbation density of for each slate with the following formula: [cumulative surface area of the trace fossils / total surface area of the slate × 100]. This method does not differentiate the ichnofossils themselves
i.e., *Helminthoidichnites*, *Helminthopsis*, and *Gordia*, but allows for the creation of indices indicating the presence or absence of trace fossils.

### 3. Results

The majority of the Brioverian trace fossils studied in this paper have a constant diameter between 1 to 2 mm. They are principally related to the *Helminthoidichnites* and *Helminthopsis* ichnogenera (Gougeon *et al.*, 2018a), and sometimes to less abundant thin trace fossils of the *Gordia* ichnogenus. There are two types of Brioverian trace fossils excluded from this study: *Spirodesmos* (Gougeon *et al.*, 2019, 2021) and some large sub-horizontal burrows with variable multi-millimeters diameter which are related to the *Palaeophycus* ichnogenus. These are uncommon and not related to trace fossils – biomat associations. The *Helminthoidichnites* trace fossil corresponds to straight to slightly curved, smooth, and sub-cylindrical trails, resting sub-horizontally on the bedding-plane surface, and preserved in both positive and negative epi- and hypo-reliefs (Gougeon *et al.*, 2018a). When preserved, the infill differs from the surrounding sediment, with coarser grain size, sometimes enriched in red iron oxides. Gougeon et al. (2018a) identified the Brioverian ichnospecies as *Helminthoidichnites tenuis* Fitch, 1850, and have noticed that these traces often incise microbial mats surface. The *Helminthopsis* trace fossils are irregularly winding to meandering, unbranched, horizontal trails that tend to form unguided meanders. The ichnogenus *Helminthopsis* differs from *Helminthoidichnites* by rotations conspicuously greater than 20 degrees. Gougeon et al. (2018a) identified the Brioverian ichnospecies as *Helminthopsis tenuis* Ksiazkiewicz, 1968. The *Gordia* trace fossils are horizontal, non-branching, mostly winding, and curving grazing trails that diagnostically cross over themselves, forming one or several loops. The ichnogenus *Gordia* strongly differs from the previous *Helminthoidichnites* and *Helminthopsis* discussed above by the presence of the self-crossing pattern, displaying a trail with loop (Buatois et al., 1998). In addition, Gougeon et al. (2018a) identified the Brioverian ichnospecies as *Gordia marina* Emmons 1844.

The Brioverian slates show different grazing behaviors of primitive organisms. For instance, figure 3A-D shows two slates including elongate and straight *Helminthoidichnites*, which corresponds to the lowest bioturbation rate. In contrast, figure 4A-D shows two slates including a network of shorter *Helminthopsis* with unguided meanders as well as curves or loops, which corresponds to a high bioturbation rate. The grazing trace fossils are clearly associated with the MISS (Fig. 4B). Then, a *Gordia* ichnofossil showing the diagnostic self-crossing pattern is also recorded on the same slate (Fig. 4). In this experiment, the horizontal trace fossils occupation of organisms serves as input data for the statistical analysis to estimate the trace fossil density. Table I
includes the data collected with the Arc-Gis software, which includes the trace length, the trace
fossils occupation (in dm²), the cumulative length of ichnofossils for each slate, and the cumulative
area of occupation by trace fossils from the same slate surface. An average surface area of 46 dm²,
of the selected 19 slate samples, was determined to obtain the maximum number of significant
results. In total, over one hundred trace fossils were selected, corresponding to a cumulative length
of 3.187 meters. The bioturbation rate (in %) of trace fossils varies from one slate to another. For
example, the lowest trace fossil density is 2.4% (Fig. 5A) whereas the highest trace fossil density is
26.3% (Fig. 5B). In addition, the maximum trace length on the slates ranges from 5 cm to almost 16
cm for *Helminthoidichnites*, the longest trace fossil.

To illustrate the relationship between the bioturbation rate and the maximum trace length recorded on the slates, the values of bioturbation rate (in %) and the maximum length of a trace (in cm) from the slate were related and presented in the graph below (Fig. 6). Each red dot illustrates a rock sample that has been analyzed. A fully bioturbated slate surface that yields a high density of trace fossils statistically tends to show a general decrease in the largest trace size with shorter *Helminthoidichnites* and *Helminthopsis* (Fig. 4A-D; Fig. 5B). Conversely, a poorly bioturbated slate shows only elongate *Helminthoidichnites* (Fig. 3A-D; Fig. 5A). A general trend is observed: when the bioturbation rate increases, the length of the largest trace decreases. For instance, slate sample 5 which has the highest occupancy rate (26.3%), is associated with a maximum trace length of 5 cm. On the other hand, slate sample 18 with the lowest bioturbation rate (2.4%) has a trace fossil length of 12.3 cm.

In addition, to provide credibility and to discuss the correlation between the bioturbation
rate and the maximum length of the trace, a linear regression line has been performed (Fig. 6). The
graph shows a group of red dots that are following the general trend and could support the
correlation between the bioturbation and the trace length. However, 3 red dots representing the
samples 4, 9, and 17 do not seem to follow the general trend. They can also be considered as
extreme values.

**4. Discussion**

*Quantitative analysis.* - Several approaches to quantitative ichnology have been conducted in studies of traces fossils. After the pioneering work of Raup and Seilacher (1969) on computer simulation of fossil foraging behavior, further computer simulations of bioturbation have been realized, but rarely with a quantitative measurement of the bioturbated substrate surface, except from the semi-quantitative estimations of the degree of bioturbation (e.g. Marenco & Bottjer, 2010 or Knaust, 2012). For example, Hofmann (1990) proposed a computer simulation method on Late
Proterozoic trace fossils with random patterns using goniograms, although quantitative approaches to measure the traces were not developed. Moreover, the few semi-quantitative or quantitative methods previously used to study bioturbation were mainly 3D methods using 10 × 10 cm grids which were superimposed in Adobe Photoshop software (e.g. Cribb *et al.*, 2019). The intersection grid technique for quantifying the extent of 2D bioturbation on bedding planes was used by Marenco & Bottjer (2010, 2011) and Knaust (2012). Therefore, other studies have developed semi-quantitative methods using bioturbation index to evaluate bedding plane bioturbation (see Miller & Smail, 1997) or ichnofabric disruption (see Droser & Bottjer, 1986; Taylor & Goldring, 1993) to quantify changes in trace fossils disruption (e.g. Gougeon *et al.*, 2018b; Cribb *et al.*, 2019).

Even though our results are significant, we should not exclude that these biometric experiments are based on analyses made with computer software and that a potential margin of error in accuracy, based on the digitalization of polyline features and interpretation of ichnological data, is to be considered.

*Trace fossils and microbial mats association.* - First, our new method allows to measure the length of each 2D trace fossil. Secondly, the total length of traces from a single bedding surface is calculated. Then, it is possible to deduce the bioturbated surface percentage by a surface unit (for instance 1 dcm²), to discuss the relation between the maximum or the average lengths of traces according to the density of 2D bioturbation. Finally, concerning the Brioverian ichnofauna from Brittany (NW France), the experiments carried out suggest that the density of bioturbation impacts the length of the tracks: the maximum length of sub-straight ichnofossils (*Helminthoidichnites*) decreases when the density of horizontal bioturbation increases. When the density of traces is high, meandering (*Helminthopsis*, *Cochlichnus*) and looped (*Gordia*) ichnofossils are more developed than straight ones. However, it can be noticed that slate surfaces providing body fossils such as small "medusoids" are more or less devoid of ichnofossils and when present, they are limited to very short traces near body fossils imprints and moulds (Néraudeau *et al.*, 2018). Conversely, slate surfaces rich in microbial mats (MISS), named *Neantia* in Brioverian deposits, are generally overlapped by several meandering trace fossils such as *Helminthopsis*. Association of *Helminthopsis* ichnofossils and *Neantia* biomats are present on a part of the slate surfaces of our samples, especially on Fig. 2A, Fig. 4B, and Fig. 5B. They correspond to a slightly wrinkled surface, with arched, meandering, or sigmoid wrinkles. This association of simple horizontal grazing trails with microbial mats is typical of both shallow and deep-marine deposits during the Ediacaran-Cambrian time frame (Carbone & Narbonne, 2014). Collectively, this grazing behavior within the widespread microbial mats has been included in the *Helminthopsis* ichnoguild (see Buatois & Mángano, 2003, 2011; Buatois *et al.*, 2014) and identified by Gougeon et al. (2018a).
concerning the Brioverian trace fossil assemblage. In this case, trace fossils are in the uppermost
surface of the sediment and are locally preserved through the biomats (see grazing trace in Gougeon
et al., 2018: fig. 4, 7; and Fig. 5B), but also preserved as a transitive form of positive to negative
trails on the same course (Fig. 5B). Therefore, herein the traces and associated microbial mats can
be split with the overlying (top surface as a negative trail) or underlying (bottom surface as a
positive trail) into the MISS (Fig. 5B). Currently, this relationship is interpreted to be the product of
mat-grazer vermiform animals surfing throughout the microbial mats (Buatois & Mángano, 2003,
2011; Carbone & Narbonne, 2014). Finally, MISS structures are well known in Precambrian and
Cambrian deposits and are generally named *Arumberia* (e.g. Kolesnikov *et al.*, 2012; Davies *et al.*,
2016; McMahon *et al.*, 2021). In the Armorican Massif, this kind of structure has recently been
noted by Davies et al. (2016) and Went (2021) in the lowermost Cambrian of Normandy, and also
by Gougeon et al. (2018a) and Néraudeau et al. (2018) in the Brioverian of Brittany.

*Implications for Ediacaran to Early Cambrian ecosystems* - During the Ediacaran and Fortunian periods, microbial mats can serve as main food sources for early communities (e.g. Ediacaran biota), as well as produce oxygen through oxygenic photosynthesis (Gingras *et al.*, 2011b). It is suggested that biomats covered the shallow seabed before the Cambrian ecosystems evolved (Seilacher 1999; Bottjer *et al.* 2000; Buatois *et al.*, 2014; Herringshaw *et al.*, 2017). At the same time, bioturbation, a process of burrowing within the microbial mats by worms, led to the reduction of widespread microbial communities (see Seilacher, 1999; Mangano & Buatois, 2014; Herringshaw *et al.*, 2017; Darroch *et al.*, 2021). For instance, Herringshaw et al. (2017) applied the term "bioturbators as ecosystem engineers" to illustrate the impact of the bioturbation on Ediacaran to Early Cambrian ecosystems. In this context, even though horizontal grazing trace fossils were restricted to the sub-surface interface (Fig. 3, 4 and 5), we assume that the bioturbating animals have significantly affected the living conditions of microbial mats by altering resource flows from the biomats and mixing the sub-surface layer of sediment. In fact, microbial mats are dominated by patchy or wide wrinkled surfaces associated with grazing horizontal trails (Gougeon *et al.*, 2018a). In addition, different grazing behaviors have been interpreted from the Brioverian slates (Fig. 5A, B). Therefore, the resource flows from the biomats and the mixing of upper sediment likely influenced the colonization of the seafloor ecospace and controlled the behavioral patterns recorded in the slates (e.g., straight, unguided meanders, curves, or loops). These different behaviors are the results of both locomotion and feeding strategies (i.e. mat grazers) through the upper sedimentary surface.

From Ediacaran-Fortunian ecosystems, early mobile animals such as worm-like organisms,
which were potential trace-makers of the Brioverian trace fossils, have used both nutrients and O₂
from the microbial mats (Carbone & Narbonne, 2014). Modern examples of biomats also comprise
oxygen-producing cyanobacteria exploited by mobile animals (Gingras *et al.*, 2011b).

**5. Conclusion**

We provide quantitative analyses of the percentage of bioturbation using the trace fossil-
biomat associations from Ediacaran-Cambrian deposits (Brioverian slates) of northwestern France.
The use of a quantitative method with Arc-Gis software is illustrated herein. Our data suggest that
the bioturbation rate of the microbial grazers impacted the use of the seafloor ecospaces and the
feeding strategies. Depending on the intensity of the bioturbation, strongly supported by the
availability of food sources limited to flat microbial mats, the length and the shape of the trace
fossils change significantly. Finally, the lifestyles of microbial grazers producing
*Helminthoidichnites*, *Helminthopsis*, and *Gordia* could represent three different behavioral
responses among benthic worm-like organisms grazing on microbial mats.

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## Acknowledgments

The authors thank: the owners of the quarries for the free access to the outcrops and R. Gougeon, F. Noirit, and R. Vullo for their participations to the field trips; Yannick Rantier for training and assistance with Arc-Gis software; Abderrazak El Albani (University of Poitiers) and Jean-Paul Saint-Martin (National Museum of Natural History Paris) for reviewing the manuscript; Corentin Jouault (National Museum of Natural History Paris), Maya LaGrange Rao and Weiduo Hao (University of Alberta) and for help with language proofing and additional discussions.

## Figure captions

**Fig. 1.** Location of the Brioverian outcrops and the main studied localities in northwestern France. *Localisation des affleurements du Briovérien et des localités étudiées dans le nord-ouest de la France.*

**Fig. 2.** A. Photography of a typical slate (IGR 90174) with trace fossils from the Brioverian deposits (*Step 1*). B. Digitalization of a polygon, obtaining the total area of the slate (dm²) (*Step 2*). C. Digitalization of traces. Obtaining the lengths of the lines/tracks (*Step 3*). D. Creation of buffer zones around the lines. Obtaining the occupation surface of the traces (dm²) (*Step 4*). Scale bar: 2 cm. Abbreviation: MISS: Microbially Induced Sedimentary Structures.

A. Photographie d'un schiste à traces fossiles (IGR 90174) typique du Briovérien (Etape 1). B. Digitalisation d'un polygone et obtention de l'aire totale de l'échantillon (dm²) (Etape 2). C. Digitalisation des traces et obtention des longueurs des lignes/traces (Etape 3). D. Création des
zones tampons autour des lignes et obtention de la surface d'occupation par les traces (dm²) (Etape 4). Barre d'échelle: 2 cm. Abbreviation: MISS: Structures sédimentaires d'origine microbienne.

**Fig. 3.** A-D. Overview of four slates (A: IGR 90175; B: IGR 90176; C: IGR 90177; D: IGR 90178) with low bioturbation rates scanned with Arc-Gis software. For each slate, visualization of the trace fossils before and after the drawing. Scale bar: 2 cm. Abbreviations: *Helminthoidichnites* (He); Microbially Induced Sedimentary Structures (MISS).

Vue générale de quatre plaques de schiste avec des densités faibles de bioturbation, scannées avec le logiciel Arc-Gis (A : IGR 90175 ; B : IGR 90176 ; C : IGR 90177 ; D : IGR 90178). Pour chaque schiste, visualisation des traces de fossiles avant et après le dessin. Barre d'échelle : 2 cm.
Abbréviations: Helminthoidichnites (He); Structures sédimentaires d'origine microbienne (MISS).

**Fig. 4.** A-D Overview of four slates (A: IGR 90179; B: IGR 90180; C: IGR 90181; D: IGR 90182) with high bioturbation rates scanned with Arc-Gis software. For each slate, visualization of the trace fossils before and after the drawing. Scale bar: 2 cm. Abbreviations: *Helminthoidichnites* (He); *Helminthopsis* (Hl); *Gordia* (Go); Microbially Induced Sedimentary Structures (MISS).

Vue générale de quatre plaques de schiste avec des densités élevées de bioturbation, scannées avec le logiciel Arc-Gis (A : IGR 90179 ; B : IGR 90180 ; C : IGR 90181 ; D : IGR 90182). Pour chaque schiste, visualisation des traces de fossiles avant et après le dessin. Barre d'échelle : 2 cm.
Abbréviations: Helminthoidichnites (He); Helminthopsis (Hl); Gordia (Go); Structures sédimentaires d'origine microbienne (MISS).

**Fig. 5.** Different ichnogenera related to organism density preserved on the bedding-plane surface.
A. Slate IGR 90183 with a low density of trace fossils including straight grazing trace of *Helminthoidichnites* (He). B. Slate IGR 90184 with a high concentration of trace fossils including various curves of *Helminthopsis* (Hl) and a self-crossings *Gordia* (Go). Dotted arrows: grazing trace fossils preserved through the biomats (MISS). Double white arrows: grazing trace fossils showing instantaneous switch between an overlying and underlying position into the MISS (from a positive to negative trail). Scale bar: 2 cm.

Les différents ichnogenres présents sur une surface de banc en fonction de la densité de bioturbation (et d'organismes). A. Plaque de schiste (IGR 90183) avec une faible densité de traces fossiles, avec les broutages subrectilignes de type Helminthoidichnites (He). B. Plaque de schiste (IGR 90184) avec une forte densité de traces fossiles, incluant les sinueux Helminthopsis (Hl) et les boucles de Gordia (Go). Flèches pointillées : traces fossiles de broutage préservées à travers les
tapis microbiens (MISS). Flèches blanches doubles : traces fossiles montrant un changement instantané entre une position sus-jacente puis sous-jacente dans le MISS (trace à relief successivement positif et négatif). Barre d'échelle : 2 cm.

**Fig. 6.** Graph showing the correlation between substrate occupancy rate and maximum trace length. The red rods represent the 19 samples analyzed.

Graphe montrant la corrélation entre le taux d'occupation du substrat et la longueur maximale des traces. Les points rouges représentent les 19 échantillons analysés.

**Table 1**

Table I: Data collected with the Arc-Gis software; from left to right: the sample numbers, the slate surface (in dm²), the trace fossils occupancy (in dm²), the cumulative lengths of ichnofossils for each slate (in cm), the occupancy (in %) and the maximum trace length. Asterisk symbol: La Lammerais quarry at Saint-Gonlay.
<figure><img src="image_2.png" /></figure>

Mesozoic deposits

Paleozoic deposits

Triassic deposits

Magmatic and metamorphic rocks
A

<figure><img src="image_3.png" /></figure>

B

<figure><img src="image_4.png" /></figure>

C

<figure><img src="image_5.png" /></figure>

D

<figure><img src="image_6.png" /></figure>
A.

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

C.

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

B.

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

D.

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>
<figure><img src="image_15.png" /><figcaption>B</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>C</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>D</figcaption></figure>

<figure><img src="image_18.png" /></figure>
A

<figure><img src="image_19.png" /></figure>

B

<figure><img src="image_20.png" /></figure>
Lengt of the longest trace fossils (in cm)

18,0

16,0

14,0

12,0

10,0

8,0

6,0

4,0

2,0

0,0

5,0

10,0

15,0

20,0

25,0

30,0

Bioturbation rate (in %)

R² = 0,3527

<figure><img src="image_21.png" /></figure>
<table><caption>Table I</caption><thead><tr><th>Samples</th><th>Slate surfaces (in dm²)</th><th>Trace fossils occupancy (in dm²)</th><th>Cumulative lengths (in cm)</th><th>Occupancy (in %)</th><th>Maximum lengths (in cm)</th></tr></thead><tbody><tr><td>1*</td><td>2.75</td><td>0.462</td><td>208.65</td><td>16.83</td><td>6.99</td></tr><tr><td>2*</td><td>4.08</td><td>0.460</td><td>230.03</td><td>11.28</td><td>10.20</td></tr><tr><td>3*</td><td>1.93</td><td>0.364</td><td>176.02</td><td>18.72</td><td>7.81</td></tr><tr><td>4*</td><td>1.81</td><td>0.094</td><td>41.90</td><td>5.18</td><td>15.84</td></tr><tr><td>5</td><td>0.94</td><td>0.246</td><td>122.04</td><td>26.25</td><td>5.02</td></tr><tr><td>6*</td><td>2.09</td><td>0.194</td><td>100.64</td><td>9.28</td><td>10.41</td></tr><tr><td>7</td><td>5.04</td><td>1.076</td><td>539.62</td><td>21.35</td><td>7.57</td></tr><tr><td>8</td><td>2.23</td><td>0.323</td><td>181.22</td><td>14.49</td><td>7.22</td></tr><tr><td>9*</td><td>1.83</td><td>0.443</td><td>213.70</td><td>24.28</td><td>10.78</td></tr><tr><td>10*</td><td>1.50</td><td>0.241</td><td>121.00</td><td>16.08</td><td>5.84</td></tr><tr><td>11</td><td>1.30</td><td>0.186</td><td>96.00</td><td>14.32</td><td>7.63</td></tr><tr><td>12</td><td>2.10</td><td>0.353</td><td>178.41</td><td>16.95</td><td>9.38</td></tr><tr><td>13</td><td>1.92</td><td>0.275</td><td>152.67</td><td>14.33</td><td>7.36</td></tr><tr><td>14*</td><td>1.63</td><td>0.285</td><td>148.75</td><td>17.45</td><td>7.87</td></tr><tr><td>15*</td><td>3.24</td><td>0.522</td><td>276.44</td><td>16.09</td><td>8.61</td></tr><tr><td>16*</td><td>2.93</td><td>0.23</td><td>130.23</td><td>8.50</td><td>11.68</td></tr><tr><td>17*</td><td>2.80</td><td>0.164</td><td>96.31</td><td>5.86</td><td>6.77</td></tr><tr><td>18*</td><td>2.79</td><td>0.066</td><td>35.73</td><td>2.37</td><td>12.27</td></tr><tr><td>19*</td><td>3.73</td><td>0.211</td><td>121.20</td><td>5.66</td><td>10.60</td></tr><tr><td>Min.</td><td>0.94</td><td>0.066</td><td>35.73</td><td>2.37</td><td>5.02</td></tr><tr><td>Max.</td><td>5.04</td><td>1.076</td><td>539.62</td><td>26.25</td><td>15.84</td></tr><tr><td>Total</td><td>46.60</td><td>6.195</td><td>3170.56</td><td></td><td></td></tr></tbody></table>