Article: A new mitrate from the Upper Ordovician of Norway, and a new approach to subdividing a plesion

This paper reconstructs, describes, and places systematically the mitrate Barrandeocarpus norvegicus sp. nov. from the Upper Ordovician (Hirnantian stage, Ashgill series) of Rambergoya in Oslo Fjord, Norway. This new species is the first mitrate described from Norway and a stem-group craniate in the plesion of Mitrocystella. To locate the species within its plesion in an objective manner, without using traditional categorial ranks, a number of new terms are proposed: a scion is a monophyletic group comprising a crown group and an adjacent crownward part of a stem group. A scion ought to be named after its basal plesion. Within a plesion, a first order apical group is a small monophylum (ideally a pair of sister species) further removed from the stem lineage than are any other species of the plesion, i.e. separated from the stem lineage by a greater number of phylogenetic segments (= species). The first order apical lineage is the direct line of descent leading from the stem lineage to the apical group. A first order paraplesion comprises all those members of a plesion which are equally closely related to the first order apical group. A first order parascion is a monophylum containing the first order apical group and one or more adjacent paraplesions, and should be named after its basal paraplesion. When a first order paraplesion contains several known species, it should be possible to recognize a second order apical group, a second order apical lineage, etc. and so on with still higher orders as the cladogram becomes more complex.In these terms, Barrandeocarpus norvegicus is placed in the plesion of Mitrocystella in its own first order paraplesion. This is less apical than the parascion of Ateleocystites guttenbergensis (i.e. the Anomalocystitida in conventional terms) and more apical than the paraplesion of Barrandeocarpus jaekeli Ubaghs.The locomotory cycle of B. norvegicus, crawling rearwards through the mud pulled by its tail, is reconstructed. The internal features of the head of B. norvegicus are similar to those of Placocystites forbesianus de Koninck in most respects, but show indications never seen before in mitrates, of the ventral surface of the hypophysis.