The acrotretacean brachiopod Ceratreta tanneri (Metzger) from the Upper Cambrian of Baltoscandia

Ceratreta tanneri is a widespread and biostratigraphically significant species within Baltoscandia. In northern Estonia and the Leningrad district (USSR) it is restricted to the Upper Cambrian quartzose Ülgase and Ladoga formations (lower Westergaardodina Biozone). In Sweden it has been found in the Upper Cambrian alum shales in Östergötland, where it is associated with trilobites of the Leptoplastus to lower Peltura biozones; in the Siljan district (Dalarna) the species has been isolated from the so-called ‘Obolus’ beds, situated directly above the Precambrian basement. Previously, Cambrian rocks were thought to be missing from this area, and the occurrence of C. tanneri (associated with Ungula inornata) is of palaeogeographical importance as it implies a late Cambrian, rather than an early Ordovician dating for the transgression of the early Palaeozoic sea into the Siljan district. C. tanneri is redescribed mainly on the basis of new material from Sweden and USSR.