Taxonomy, functional morphology and palaeoecology of the Ordovician cystoid family Hemicosmitidae

Morphological aspects of the Middle-Upper Ordovician genus Hemicosmites are discussed. Most of the important features are related to respiratory functions, and the distribution and functions of the rhombs are evaluated. Five different patterns of rhomb distribution are outlined. Phyletic changes occur throughout the Ordovician in: (1) respiratory structures, (2) expansion of oral area, including addition of new plates, and (3) increased size of the arms. It is suggested that gonads were located in specialized appendages connected to the ambulacral furrows as previously suggested by Jaekel (1899), and that the genus, normally found associated with reefs and carbonate mud mounds, gave rise to Caryocrinites (Caryocrinitidae). Four new species are described, three from Norway and one from Sweden: Hemicosmites papaveris, H. sculptus, H. sphaericus, and H. variabilis. The type species of the genus, H. pyriformis, which is known from one specimen only, differs in many respects from all other known Hemicosmites species. It is either an aberrant specimen or the species is generically distinct from all others. This taxonomic question is left open due to lack of additional material.