| The recent discovery of Radiograptus stratigraphically below a series of morphological predecessors to the planktic siculae 'Dictyonema' flabellforme-sociale groups suggests a phyletic derivation of these forms from the anisograptid Radiograptus (Bulman 1950) and thus requires a taxonomic revision of the planktic species of Dictyonema, D. retiforme (Hall 1843, is a Silurian benthic member of the family Dendrograptidae. The genus Dictyonema must be restricted to represent the approximately 140 sessile species of cone-shaped, dissepimentous, dendroid graptolites (upper Cambrian - Carboniferous), whereas the homeomorphic planktic forms, now belonging to the family Anisograptidae, require a new generic name. The names Rhabdinopora (Eichwald 1855), Graptopora (Salter 1858) and Dictyograptus (Hopkinson &Lapworth 1875) are valid nomina oblita for this group of graptolites, with Rhabdinopora having priority. Dictyograptus would be a much favoured alternative name because of its long historical application to the 'flabelliforme-sociale grop" and its euphonic semblance to Dictyonema, but being a junior name, its preference over Rhabdinopora would require suppression of the rule of Priority and is not recommended. The history of the generic status of Rhabdinopora flabelliformis (Eichwald 1840) and of R. socialis (Salter 1858) is briefly reviewed. |
