CONTRIBUTION OF MORPHOMETRICS TO THE SYSTEMATICS OF THE ORDOVICIAN GENUS NESEURETUS (CALYMENIDAE, TRILOBITA) FROM THE ARMORICAN MASSIF, FRANCE

The genus Neseuretus Hicks, 1873 is the most abundant trilobite of the Ordovician siltite succession of the Andouille and Traveusot Formations in the French Armorican massif. The systematics of some species of Neseuretus is still unclear. Armorican and Iberian domains formed part of a distinctive paleobiogeographical province in the Ordovician and, while five Neseuretus species were defined in Iberia that follow each other through time, from the Middle to the Upper Ordovician, only one Neseuretus species, N. tristani, has been identified in the Armorican massif so far. The discovery of new fossil deposits in the Menez-Belair syncline has led to the identification and re-description of three Neseuretus taxa: Neseuretus avus Hamman, 1977 from the early-middle Darriwilian, Neseuretus tristani (Brongniart in Desmarest, 1817) from the late middle Darriwilian to late Darriwilian and Neseuretus tardus (Hammann, 1983) from the Darriwilian-Sandbian boundary and early Sandbian. Morphological characters of the cephalon and pygidium were determined that can definitively distinguish the three identified taxa. These results stem from both traditional and geometric (outline and landmark-based) morphometric analyses. The three taxa follow each other through time and constitute a morphological sequence that contributes to improving the stratigraphy of the Middle and Upper Ordovician.