Cambrian-Ordovician Boundary beds at Tõnismägi, Tallinn, North Estonia

The Cambrian–Ordovician Boundary beds at the Tônismägi section in Tallinn, North Estonia, are represented by siltstones and sandstones with interlayers of clay and kerogenous argillites. In this section, three conodont zones – Cordylodus proavus, C. intermedius and C. rotundatus-angulatus – were established. Graptolites of the Rhabdinopora flabelliforme group were found in the upper part of the C. proavus Zone. This section demonstrates that in suitable facies, species of Rhabdinopora may appear earlier than usually considered.