New findings of trilobites and graptolites in the Ordovician of Bulgaria

Abstract

Darriwilian trilobites and graptolites from the Grohoten Formation (Svoge anticliorium, Sofia Stara Planina Mountain) are reviewed in the light of the new records and by direct examination of the original material reported by previous authors between 1934 and 1958 years. Middle Ordovician fossils are distributed in three stratigraphically successive assemblages. The oldest one derives from Lower Oretanian shales with nodules and consists of pendent and extensiform didymograptids (Didymograptus cf. artus, D. cf. spinulosus, D. s.l. ferrugineus), some biserials (Haddingograptus ?), a single benthic atheloptic trilobite (Placoparia balcanica n. sp.), and probably also of rare pelagic form Cyclopyge cf. kossleri. The middle assemblage which is dominated by pelagic trilobites such as Pricyclopyge binodosa binodosa, R binodosa prisca and Microparia sp., and sparse benthic elements (Ectillaenus sp.), is recorded from siltstones and silty shales that directly overlie a variously developed middle, quartzitic member. The presence both of the graptolite Didymograptus murchisoni and the rare biserial forms confirm an Upper Oretanian age for the association. Finally, the third and youngest Middle Ordovician palaeontological horizon is characterized by the occurence of the Dobrotivian cyclopygid trilobite Pricyclopyge binodosa longicephala, which derives from the settled shales and is located well below the latest assemblage of trilobites and brachiopods already of Berounian (Upper Ordovician) age. Closest comparisons of the Bulgarian trilobites are with the cyclopygid biofacies from the deep outer shelf settings of the peri-Gondwanan platform, shown by the record of some typical Bohemian taxa as P. binodosa prisca and P. binodosa longicephala.