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 sug gests 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 articula ted posterior spines (Anomalocystitidae); 4) Ovocarpus? circularis is synon ymized with O. moncereti: 5) Chauvelia discoidalis and Mitrocystites riadan ensis are both assigned to the genus Aspidocarpus; 6) Mitrocystella barrand ei is assigned to the new genus Promitrcystites. The original reconstructio n of Vizcainocarpus dentiger proposed by Ruta, (1997a) is modified, as a re sult of the identification of two additional plates in the posterior portio n of the theca. The new species Vizcainocarpus rutai from the Tremadoc (Low er Ordovician) of Shropshire (England) is described and represents the olde st 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 pl ate homologies indicates that: 1) cornutes and mitrates are sister-groups, both deriving from a Ceratocystis-like ancestor; 2) Peltocystida and Mitroc ystitida are sister-groups: 3) Lagynocystida is sister-group of(Peltocystid a + Mitrocystitida); 4) Lobocarpus is not a cornute but a primitive Cambria n mitrate belonging either to the stem-group of Mitrocystitida or to the st em-group of(Peltocystida + Mitrocystida); 5) Anomalocystitidae represents a family of the suborder Mitrocystitida.