Original articleThe sclerobionts of the Bajocian Oolithe ferrugineuse de Bayeux Formation from Calvados (Paris Basin, Normandy, France)Les sclérobiontes de l’Oolithe ferrugineuse de Bayeux bajocienne du Calvados (Bassin de Paris, Normandie, France)
Introduction
Since the mid-19th century, the Bajocian beds of the Calvados department (Normandy, Western Paris Basin) and especially the Oolithe ferrugineuse de Bayeux Formation (abbreviated as OFB Fm. herein) have been internationally famous for their palaeontological content, mainly molluscs including a high diversity of ammonites.
D’Orbigny (1850: 606) chose the type of his Bajocian stage in Normandy, and designed the localities around Bayeux (Latin Bajoce, hence the name of the stage) and between Bayeux and Caen (Calvados) as the typical sites of the Bajocian stage (Rioult, 1964). Although the I.U.G. S. recently selected the Portuguese locality of Cabo Mondego as the stratotype of the Bajocian (Pavia and Enay, 1997), the Calvados department remains the historical stratotype of the Bajocian. It provided a very important range of fossils studied and often figured by different authors such as d’Orbigny (1842–1851; 1850), J.-A. Eudes-Deslongchamps (1849), E. Eudes-Deslongchamps (1865), Brasil (1895), who provided a list of 66 species of ammonites from the OFB Fm. from one quarry, Sully near Bayeux, Calvados, in a 4-fold stratigraphical division still in use to-day; Oppel (1856–1858), Wetzel (1924), and Arkell, 1930, Arkell, 1956 (Rioult, 1964). Many French and foreign museums accommodate rich collections of Bajocian fossils from Calvados.
After the litho- and biostratigraphical studies of Rioult, 1964, Rioult, 1971, Rioult, 1980a, Rioult, 1980b, Gabilly and Rioult (1967), Rioult and Fily (1980), Rioult et al., 1991, Rioult et al., 1997, Gauthier et al. (1996), Dugué et al. (1998), the works of Pavia (1994), Pavia and Martire, 1997, Pavia and Martire, 2010, Pavia et al., 2013, Pavia et al., 2015 stressed the taphonomic history of the ammonites within the OFB Fm. to take into account the generalized reworking of fossils (resedimented and/or re-elaborated elements) and reconstruct “… their registratic succession and the chronological order of sedimentary events …” (Pavia et al., 2015:5). This taphonomic critical analysis is applied to ammonites only, given their biostratigraphical importance. They follow the methods developed by Fernández López, 1991, Fernández López, 1995, Fernández López, 2007, Fernández López, 2011a, Fernández López, 2011b and Fernández-López and Meléndez (1994). The taphonomic analysis is considered by these authors as an aid to a better stratigraphical approach. With the notable exception of the microperforations studied by Glaub (1994), the only mentions of trace fossils are those of Thalassinoides isp. or similar burrows at or under omission surfaces (Fürsich, 1971).
The aim of the present work is to introduce the Bajocian sclerobionts and their taphonomy, highlighting the trace fossils, mainly from the OFB Fm., the study of which has been somewhat neglected, receiving far less attention than the very abundant, highly diverse and stratigraphically significant body fossils.
Section snippets
Geological context, material and methods
The studied material has been collected from ploughed fields, in the vicinity of Gavrus, west of Caen (Calvados). The fossils, reworked in the soil, show that the beds of the subsoil extend stratigraphically from the Aalenian (Lower Mâlière.) to the Upper Bajocian (Calcaire à Spongiaires Fm.) (for a complete stratigraphical account, see Rioult et al., 1991 and Dugué et al., 1998). The upper part of the Mâlière Fm. (Lower Bajocian, discites and laeviuscula Biozones) is poorly represented in our
Remarks on the taphonomy and the mineralogy of the studied fossils
Because these fossils were collected from ploughed soil, and not in situ from the OFB Fm., no analyses of their relationships with the sedimentary environment is possible and thus no complete taphonomic analysis. Moreover, the taphonomic conditions may differ from one bed to another. Our remarks are therefore limited to those directly deduced from the fossil itself. Whatever their taxonomic identities, most (if not nearly all) fossils of the OFB Fm. are reworked, and the majority of them are
The encrusting sclerobionts
We consider here the epibenthic organisms (only animals in the studied material), which are fixed on organic hard substrates, for instance shells. Most of them can also dwell on lithic hard substrates but the latter are beyond the scope in our work, since we had no access to OFB hardgrounds.
Borings and bioerosional traces
Numerous attempts have been made to classify trace fossils at a supra-generic rank, including morphological, preservational or ethological criteria. To introduce the perforations made by endoskeletobionts in the Bajocian shells, we have followed the categories of architectural design as defined by Buatois et al. (2017), which provide a good framework, less formal but not more subjective than any other morphological-ethological based classification. The number given by Buatois et al. (2017) to
Discussion
We must stress again that the studied fossils were collected from ploughed fields, all being Bajocian in age and nearly all from the OFB Fm. Our conclusions may therefore reflect the diversity or the evolution of the palaeoenvironmental, sedimentological and taphonomical conditions in Normandy throughout the Bajocian. Moreover, most of the fossils are resedimented and/or re-elaborated (Pavia et al., 2013, Pavia et al., 2015), which complicates the understanding of the individual histories of
Conclusion
The vacuum cast-embedment with an epoxy resin is the most approved study method of fossil microborings under a SEM (Wisshak, 2012), together with micro-CT, non-destructive micro-computed tomography (Wisshak et al., 2017a, Wisshak et al., 2017b). The study of natural casts, when available, is a good additional method. Our work shows that naturally silicified borings, after HCl processing, can be studied under an optical stereomicroscope. A number of ichnotaxa were revealed both by this method
Disclosure of interest
The authors declare that they have no competing interest.
Acknowledgements
We want to convey our warm thanks to all those who helped us with bibliography, documentation, identification, work on the field, equipment loan: Marc Chesnier (Caen), Dr Chis Clayton (Southampton), Dr Franz Fürsich (Erlangen), Benoît Hérisson (Le Havre), Nicolas Morel (Le Mans), association Port Vivant (Le Havre), Patrice Rabœuf (Le Mans), Jean-Philippe Rioult (Caen), Michel Rioult (Caen), Simon Schneider (Cambridge). Our special thanks to Dr Max Wisshak (Senckenberg am Meer) who permanently
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