The genus Discoceras (Tarphyceratida, Cephalopoda; Ordovician) from Estonia and glacial erratics in north-central Europe

The cephalopod genus Discoceras Barrande, 1867 (family Trocholitidae Chapman, 1857; order Tarphyceratida Flower, 1950) is a characteristic component of the late Middle and Late Ordovician carbonates of Baltoscandia and its glacial erratics in north-central Europe. The genus consists of tens of species, most of which are, however, poorly constrained, partly due to the type material being inaccessible or lost. Consequently, a comprehensive revision of the genus is needed. Here, we describe 39 specimens from in-situ rocks in northern Estonia and Norway, and from boulders in Pleistocene glaciofluvial sediments in Germany and Poland. Six previously described species were identified: Discoceras antiquissimum (Eichwald, 1842), D. saemanni (Hyatt, 1894), D. ievense (Balashov, 1953), D. damesii (Schröder, 1891), D. bandonis Remelé, 1890 and D. danckelmanni (Remelé, 1880). Two species were left in open nomenclature: D. cf. paopense Kröger, 2025 and D. cf. gubkovense Balashov, 1953; seven species are newly described: D. aseriense sp. nov., D. brandenburgense sp. nov., D. angustum sp. nov., D. circulare. sp. nov., D. sovaki sp. nov., D. alienum sp. nov. and D. polonicum sp. nov. The single specimen and holotype of Discoceras aseriense (Aseri Regional Stage, Darriwilian, Middle Ordovician) is the stratigraphically earliest known representative of the genus from Baltica and possibly globally. Lituites trapezoidalis is herein synonymised under the type species, Discoceras antiquissimum. A neotype is proposed for the latter, stabilising the concept of the type species and the genus Discoceras. The study showed that the separation of Discoceras at the family level is not justified and the monogeneric family Discoceratidae Hyatt, 1894 is herein synonymised under the family Trocholitidae Chapman, 1857.