Morphology, growth and taxonomy of the Ordovician rhombiferan Caryocystites

Caryocystites has five peri-oral plates, as in other members of the superfamily, and a primitive stem with five pentameres. Evolutionary trends include addition of ambulacra, greater regularity in plate distribution and increasing complexity in the development of respiratory pores. In Caryocystites thorslundi n. sp. young specimens have only four primary circlets of thecal plates; the peri-oral, peri-gonal, periproctal and basal. Thecal enlargement was accomplished by accretional growth of primary plates and the addition of new thecal plates intercalated between the original four plate circlets. Caryocystites evolved in the early Middle Ordoviciap and became extinct at the end of the Ordovician. The morphology and evolutionary trends are similar to those of the other caryocystitids. Seven species from Sweden are described morphologically and taxonomically, including three new ones (C. valdari, C. rugosus, C. thorslundi).