MORPHOLOGIC CHARACTER DEFINITION OF THE ROOT-STOCK OF CYCLIDA (ARTHROPODA, CRUSTACEA?)

The oldest representatives of the enigmatic group of presumed crustaceans, the Cyclida, are known from England, Scotland, and Ireland. Examination and illustration of type and non-type material of the thirteen species currently arrayed in two genera, Cyclus de Koninck, 1841, and Americlus Dzik, 2008, make it possible to define the morphological features of the dorsal carapace of the cyclid rootstock. The two genera are referred to different families. Cyclidae tend to be vaulted and display dorsal thoracic segmentation, whereas Americlidae are flattened and lack dorsal thoracic segmentation. Features of Cyclus radialis, the type species, suggest that it and a few other Carboniferous species are clearly distinct from other taxa currently referred to Cyclida. Carapace characters exhibited in common by species of Cyclus and Americlus from the British Isles include possession of a distinct cephalic region generally with an array of bosses, a thoracic region with well-defined axial structures, often vaulted flanks, and a discrete lateral rim. Many specimens, whether in dorsal or ventral aspect, are preserved with complete margins, suggesting that a marginal linea facilitated molting. A single specimen from the Carboniferous of Montana shows a molt in "Salter's position." The only portion of the margin that is generally incomplete is that of the front. Some Mesozoic taxa will likely be assigned to groups other than Cyclida; this must be tested by development of a meaningful phylogeny for all forms currently considered to be cyclids. Defining a detailed set of morphological characters based upon the basal members of Cyclida will provide a firm foundation for testing relationships with presumed descendant taxa. In addition, these characters will provide a basis to determine the overall affinities of the Cyclida within Crustacea and Arthropoda.