Original articleQuantitative 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)
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 and 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 and Smail, 1997, Marenco and Bottjer, 2010, Marenco and Bottjer, 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 and 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, Gougeon et al., 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, Gougeon et al., 2019, Gougeon et al., 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 occurred (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.
Section snippets
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 90180.
The trace fossils are present in slate shales from the upper Brioverian, corresponding to greenish shales at
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
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, Gougeon et al., 2021) and some large sub-horizontal burrows with variable multi-millimeters
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 and Bottjer, 2010 or Knaust, 2012). For example, Hofmann
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
Disclosure of interest
The authors declare that they have no competing interest.
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)
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