New specimens of Lingulocystis Thoral, 1935 (Eocrinoidea, Blastozoa) from the Arenig (Lower Ordovician) of Montagne noire (southern France): intraspecific morphological variability, stratigraphic, and palaeoecological implications.
Résumé
The genus Lingulocystis Thoral, 1935 is a preponderant component of the echinoderm fauna of the Lower Ordovician of Montagne noire (France), as suggested by its abundant remains. Its morphology is characterized by a flattened theca, composed of a frame enclosing numerous small polygonal central plates. The new specimens have been determined partly as Lingulocystis elongata Thoral, 1935 and partly as Lingulocystis aff. deani Ubaghs, 1994. This study is the first report of Lingulocystis in the Foulon Formation. New stratigraphic ranges have been defined for both species: L. elongata from the early Tremadoc (Saint- Chinian Formation) to the middle Arenig (lower part of the Landeyran Formation), L. aff. deani from the early Arenig (Cluse de l'Orb Formation and Foulon Formation), and L. deani from the middle Arenig (upper part of the Landeyran Formation). The new specimens of L. elongata highlight its intraspecific variability, mainly concerning the size and arrangement of the central plates, the size of the theca, and the type of stem columnal articulations. Sedimentological studies in Montagne noire currently use the presence of Lingulocystis remains as taphonomic and palaeoecological indicators. The presence of two types of columnal articulations enhances the palaeoecological use. The development of the synostosial articulation seems to be correlated with a proximal platform palaeoenvironment, whereas development of the symplexial type seems to be related to a deeper palaeoenvironment or relatively high hydrodynamic conditions.