A re-investigation of the Rhabdinopora flabelliformis fauna from the early Tremadocian ‘Dictyonema Shale’ in Belgium

1. IntroductionThe Dictyonema Shale (‘Dictyonema oil shale’, ‘Dictyonema argillite’, etc.) is a largely used term, representing Tremadocian (the lowest stage of the Ordovician) shales that are rich in organic matter and generally contain graptolitic fossils (the Dictyonema flabelliforme fauna) (Veski & Palu, 2003). The name ‘Dictyonema’ derived from the graptolite species Dictyonema flabelliforme (Eichwald, 1840) (now Rhabdinopora flabelliformis) which was originally thought to be a benthonic root-bearing taxon, but was later considered to be a planktonic nema-bearing graptolite, reattributed to Rhabdinopora Eichwald 1855 (Erdtmann, 1982). Currently, the taxa that have been placed in the original graptolite genus Dictyonema are considered as either of benthonic type (Dictyonema s.s.) or of planktonic type (Rhabdinopora) (Erdtmann, 1982). Therefore most of the formations attributed to the ‘Dictyonema shales’ do actually not display any real Dictyonema, but contain the planktonic genus Rhabdinopora instead. However, the name has been retained because for more than one century and a half the term of ‘Dictyonema Shale’ has been largely used and is well known as a source rock for uranium and some other heavy metals (Althausen, 1992).Since the 1980s, much research was focused on the precise position of the Cambrian-Ordovician boundary (which is also the base of the Tremadocian stage), resulting in the detailed investigation of graptolites and conodonts from the ‘Dictyonema shales’