Abstract

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? circularis is synonymized with O. moncereti; 5) Chauvelia discoidalis and Mitrocystites riadanensis are both assigned to the genus Aspidocarpus; 6) Mitrocystella barrandei is assigned to the new genus Promitrocystites. The original reconstruction of Vizcainocarpus dentiger proposed by Ruta, (1997a) is modified, as a result of the identification of two additional plates in the posterior portion of the theca. The new species Vizcainocarpus rutai from the Tremadoc (Lower Ordovician) of Shropshire (England) is described and represents the oldest record of mitrocystitidan mitrates. V. rutai differs from V. dentiger in 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 a Ceratocystis-like ancestor; 2) Peltocystida and Mitrocystitida are sister-groups; 3) Lagynocystida is sister-group of (Peltocystida + Mitrocystitida); 4) Lobocarpus is 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.