First report of the mitrate Peltocystis cornuta Thoral (Echinodermata, Stylophora) in the Lower Ordovician of central Anti-Atlas (Morocco)

Abstract

The mitrate Peltocystis cornuta is one the best known and most abundant stylophoran echinoderms in the Lower Ordovician (upper Tremadoc - lower Arenig) of Montagne Noire (southern France). It is here documented outside this region for the first time, in coeval deposits of central Anti-Atlas (Morocco). This report confirms the strong faunal affinities between Montagne Noire and Moroccan assemblages in the Ordovician. Twenty-eight individuals of P. cornuta are described from two distinct localities of the Zagora region. Their overall morphology is very comparable to that of Montagne Noire specimens. However, the Moroccan Peltocystis differ from the French ones in their smaller mean size, and in the absence of abnormalities in thecal plating ( situs inversus is frequent in the Montagne Noire specimens). These differences are possibly related to the distinct palaeoenvironmental conditions, in which the Moroccan and French assemblages were living: in Montagne Noire, P. cornuta was found in deeper, quieter (less stressful) deposits than in the Anti-Atlas. Finally, the periproct is clearly identified for the first time in Peltocystis, in a more anterior position than previously thought.