Biostratigraphy and paleoecology of the decapod fauna from the Jurassic of the Vaches Noires cliffs (Calvados, France).
Résumé
Crustaceans from the Jurassic of Normandy (France) were described in the 19th and at the beginning of the 20th century (Bassompierre, 1938; Morière, 1882; Raspail, 1901) and later revised by Hée (1924) and Van Straelen (1925). In Calvados, the Vaches Noires cliffs expose interesting layers for the study of decapods from the Upper Callovian to Middle Oxfordian, which can be compared to the equivalent English Oxford Clay Formation. Before our work, only two taxa assigned to erymids, were described in the Callovian-Oxfordian of Villers-sur-Mer: Eryma villersi MORIÈRE, 1882 (Upper Callovian, Lamberti Zone) and E. morieri HÉE, 1924 (Lower Oxfordian, Mariae Zone); another was cited as “small crustaceans not yet described” (Callovian-Oxfordian).
A first preliminary study was published by Charbonnier & Gendry (2014) to synthesize the knowledge about Jurassic crustaceans of the ‘Vaches Noires’. Here, we provide an updated synthesis of the biostratigraphy and add new informations about taxa found in the Jurassic of the ‘Vaches Noires’ with the evidence of erymids (Eryma villersi MORIÈRE, 1882 - Upper Callovian; Erymidae indet. - Lower Oxfordian), mecochirids (Mecochirus socialis (VON MEYER, 1841) - Upper Callovian to Lower Oxfordian), glypheids (Glyphea regleyana (DESMAREST, 1822); Glypheopsis sp. – Upper Callovian) and pagurids (Goniochirus cristatus CARTER, 1886 – Upper Callovian; indeterminate propods – Middle Oxfordian).
Eryma villersi
Photograph: Jean-Philippe Pezy
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This locality appears to be the richest decapods deposit of Normandy. Of particular interest is pyritised specimens of M. socialis preserved in small calcareous nodules. We analyzed about thousand of them found in place in several layers during our prospects in the foreshore and cliffs of Villers-sur-Mer (France) and Weymouth (England). Preliminary results show that M. socialis and E. villersi occupied different environments. Erymids are found as isolated fingers or cephalothoraxes at the surface of condensed-levels in mudflat above storm-wave base (indurated marls); articulated chelipeds are found in quieter environment with high sedimentation rate but without mecochirids. Episodic accumulations of mecochirids in nodules at several beds (until 72% of nodules with rests), mostly articulated, are indicating regular mass-decay. These levels globally poor in fossils are highly disturbed. Also, some episodic accumulations rich in Mesosaccella CHAVAN, 1946 (an infaunal deposit-feeder with some specimens in life position) found at the same level argue for a burrowing taking place later, when more oxygenated conditions have settled on the bottom, some centimeters above mecochirids. Unfortunately, almost all nodules are crossed by small burrows probably attributable to worms, leading to a cracked-aspect of carapaces. All of this indicate dysoxic bottom-water conditions in the Marne de Dives Formation.
The presence of Glyphea regleyana and Glypheopsis sp. is documented for the first time in the Upper Callovian of France (Charbonnier & al., 2013) and fill the gap between Bathonian and Oxfordian occurrences for G. regleyana. Both are usually found in the same beds as erymids but their remains are very rare in proportions and suggest a different habitat for glypheids. Curiously, no isolated chelipeds were found, only cephalothoraxes and an incomplete molting, all in nodules. Finally, epibionts (serpulids) have been found on seven specimens of Mecochirus that highlight new biotic interactions.
References
Bassompierre P. & Cardinet, J. 1938. Présence de petits décapodes et d'ophiures fossiles dans la falaise des Vaches Noires à Villers-sur-Mer. Bulletin de la Société Linnéenne de Normandie, série 9, 1 (séance du 4 juillet 1938) : 82.
Charbonnier, S. & Gendry, D. 2014. Les Crustacés. In Les falaises des Vaches Noires, un gisement emblématique du Jurassique à Villers-sur-Mer, Normandie. Fossiles, revue française de paléontologie, HS 4 : 74-75.
Charbonnier S., Garassino A., Schweigert G. & Simpson M. 2013. A worldwide review of fossil and extant glypheid and litograstrid lobsters (Crustacea, Decapoda, Glypheoidea). Mémoires du Muséum national d’Histoire naturelle, 205 : 1-304.
Hée, A. 1924. Catalogue critique des Crustacés jurassiques du Calvados et de l'Orne. Bulletin de la Société Linnéenne de Normandie, 6: 126-166.
Morière, J. 1882. Première note sur les crustacés de l'Oxfordien trouvés dans le Calvados. Bulletin de la Société Linnéenne de Normandie, série 3, 6: 161-170.
Raspail, J. 1901. Contribution à l'étude de la falaise jurassique de Villers-sur-Mer. La feuille des jeunes naturalistes, série 4, 366 : 145-149.
Van Straelen, V. 1925. Contribution à l’étude des crustacés décapodes de la période Jurassique. Mémoires de l’Académie Royale de Belgique, série 2, 7(1) : 1-462.