Original articleReassignment of Pentamerus davyi Oehlert to Zdimir robustus (Barrande) (Brachiopoda, Devonian): Stratigraphic and palaeogeographic implicationsAttribution de Pentamerus davyi Oehlert à Zdimir robustus Barrande (Brachiopoda, Dévonien) : implications stratigraphiques et paléogéographiques
Introduction
The Devonian reefal limestones of Chalonnes, in the southern Armorican Massif (western France), display an abundant fauna (Le Maître, 1934) essentially consisting of stromatoporoids, tabulate and rugose corals as well as a few brachiopods. The latter were first described by Oehlert (1881), who recognized three large-sized species, namely Uncites galloisi Oehlert, 1881 (up to 7 cm long), Pentamerus davyi Oehlert, 1881 (max. 6 cm long) and the giant Amphigenia? bureaui Oehlert, 1881, which can reach 17 cm in length.
Despite a revision by Le Maître, 1932, Le Maître, 1934, who was conservative in keeping the same nomenclature as Oehlert (1881), the brachiopods have not been re-investigated since their first description. This poses major problems when putting them in a larger-scale context, i.e. the Early to Middle Devonian faunas of western Europe. The three large-sized species described in the Chalonnes Limestone have not been found in any other place, except perhaps in the Palaeozoic from the Graz area in Austria (Heritsch, 1915, Heritsch, 1935). This might be due to the description of the same species under different names in other places, or to endemism of the Chalonnes species. A taxonomic revision should help solve this issue, as well as the exact age of the fauna, because the Chalonnes Limestone is considered to be either Emsian, or Eifelian or even Givetian.
The aim of this paper is to revise one of three large-sized brachiopod species from the Chalonnes Limestone, namely Pentamerus davyi. Because we will show that it is a junior synonym of Zdimir robustus Barrande, 1879, this enables to suggest an Upper Emsian age for the Chalonnes Limestone based on its brachiopod fauna, and once again points its biogeographic links with the classic Barrandian area in central Bohemia.
Section snippets
Geological setting
The stratigraphy of the Palaeozoic sediments in the Ancenis to Chalonnes area has recently been revised by Ducassou et al. (2011). Two well-preserved (i.e. faulted and tilted but not folded or cleaved) Palaeozoic successions are distinguishable, in the Châteaupanne and the Tombeau Leclerc units (Fig. 1). We are only concerned with the former succession, which rests unconformably over a Proterozoic basement (Mauges micaschists). Ordovician sediments (dated from the Floian to the Katian) are
The brachiopod Pentamerus davyi Oehlert, 1881
The distribution of Pentamerus davyi in the Chalonnes Formation has been reported by Le Maître (1934), who mention its occurrence in two localities, namely Saint-Charles quarry in Chaudefonds-sur-Layon (about 150 m thick) and Châteaupanne quarry in Chalonnes-sur-Loire (about 120–130 m thick). In both quarries, brachiopods are unevenly distributed within the limestone sequence. In the Châteaupanne quarry, Pentamerus davyi has been found in the northernmost (i.e. uppermost) layers, where it is
Stratigraphic distribution of Zdimir in western and central Europe
Previous syntheses have pointed out that Zdimir is known from Europe to China (Chen and Liao, 2006, Lu et al., 2017) and Japan (Tazawa, 1988), and that its stratigraphical range is limited to the Upper Emsian (serotinus and patulus conodont Biozones) and Early Eifelian (partitus Biozone) (Godefroid in Brice et al., 2000).
Conclusions
Reassignment of Pentamerus davyi Oehlert, 1881 from the Chalonnes Formation (Armorican Massif) to Zdimir robustus (Barrande, 1879) has two major consequences. Firstly, it allows to propose an Upper Emsian age for the Chalonnes Limestone based on its brachiopod fauna, an age consistent with other estimates. Secondly, the occurrence of Zdimir robustus suggests close links between the Chalonnes area and central Bohemia, possibly also with the source of the “exotic slices” now found in the Wetzlar
Acknowledgments
This work could not have been done without the painstaking efforts from our predecessors in collecting fossils (especially Edouard and Louis Bureau, Daniel and Pauline Oehlert, Joseph Péneau, Pierre Cavet), and in keeping them in the various collections at the Université Catholique de l’Ouest in Angers (Gérard Beaulieu), Musée des Sciences de Laval (Jérôme Tréguier), Muséum des sciences naturelles d’Angers (Benoît Mellier) and of the Muséum d’histoire naturelle de Nantes (Serge Régnault). The
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