A new mitrate (Echinodermata, Stylophora) from the Tremadoc of Shropshire (England) and the origin of the Mitrocystitida

Plate homologies are identified and discussed in primitive representatives of cornute and mitrate stylophorans. Comparative morphological analysis suggests that: 1) Lagynocystida are digital-bearing mitrates; 2) Peltocystida are glossal-bearing mitrates; 3) in Mitrocystitida, glossal and digital are incorporated into a closed marginal thecal frame or modified into articulated posterior spines (Anomalocystitidae); 4)Ovocarpus? circularisis synonymized withO. moncereti; 5)Chauvelia discoidalisandMitrocystites riadanensisare both assigned to the genusAspidocarpus; 6)Mitrocystella barrandeiis assigned to the new genusPromitrocystites.The original reconstruction ofVizcainocarpus dentigerproposed by Ruta, (1997a) is modified, as a result of the identification of two additional plates in the posterior portion of the theca. The new speciesVizcainocarpus rutaifrom the Tremadoc (Lower Ordovician) of Shropshire (England) is described and represents the oldest record of mitrocystitidan mitrates.V. rutaidiffers fromV. dentigerin the relatively broader size of its zygal and marginals and in the presence, on the lower thecal surface, of a peripheral fringe of fibrillar stereom. A cladistic analysis of selected stylophoran taxa based on the proposed plate homologies indicates that: 1) cornutes and mitrates are sister-groups, both deriving from aCeratocystis-like ancestor; 2) Peltocystida and Mitrocystitida are sister-groups; 3) Lagynocystida is sister-group of (Peltocystida + Mitrocystitida); 4)Lobocarpusis not a cornute but a primitive Cambrian mitrate belonging either to the stem-group of Mitrocystitida or to the stem-group of (Peltocystida + Mitrocystitida); 5) Anomalocystitidae represents a family of the suborder Mitrocystitida.