Middle Devonian coccosteid (Arthrodira, Placodermi) biostratigraphy of Scotland and Estonia

Abstract

A coccosteid nuchal bone from the Middle Devonian Tamme Cliffs locality in Estonia is described as Dickosteus cf. threiplandi, D. threiplandi being a common species in the Middle Devonian of the Orcadian Basin, Scotland. A number of other bones from Tamme Cliffs are also placed in this genus. It is already known that the coccosteids Coccosteus cuspidatus and probably Millerosteus minor are present in both areas. A final coccosteid genus Watsonosteus has been shown previously to also be present in both areas. We can confirm that the sequence of coccosteid genera is the same in both the Orcadian Basin and Estonia and is, from the lowest horizon, Coccosteus–Dickosteus–Millerosteus–Watsonosteus. The coccosteids in Estonia are also associated with other Scottish faunal elements (including co-specific species), such as other placoderms, acanthodians and sarcopterygians. This implies a very close connection between the two areas in the Middle Devonian. Some of the acanthodian species of the Orcadian Basin are also present in Belarus and Severnaya Zemlya, suggesting that other Orcadian Basin forms, including coccosteids may be present at these localities and perhaps elsewhere on the Old Red Sandstone continent. Further investigation may allow a continent-wide correlation at species level across the whole continent.