Discoceras Barrande, 1867

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 71

Genus Discoceras Barrande, 1867

Type species

Clymenia antiquissima Eichwald, 1842 , Kärdla, Hiiumaa Island, Estonia, presumably Vormsi/Pirgu Regional Stage (“Lyckholm stage” of Teichert 1930); by secondary designation ( Schröder 1891).

Diagnosis

Gradually expanded tarphycerids with smooth to strongly annulated conch; impression zone slightly developed; subquadrate conch cross section; siphuncle central in initial half-volution, gradually shifting toward dorsal conch margin during later growth stages; thick connecting rings. (Adopted from Furnish & Glenister 1964b.)

Remarks

The genus Schroederoceras Hyatt, 1894 was regarded as synonymous with Discoceras Barrande, 1867 by Sweet (1958) and Furnish & Glenister (1964b). In the opinion of these authors, Discoceras included strongly annulated forms such as the type of Discoceras , Clymenia antiquissima Eichwald, 1842 , as well as smooth conchs such as Lituites angulatus Saemann, 1852 , the type of Schroederoceras . Because several species show transitional morphologies from weakly ornamented to strongly annulate, Schroederoceras and Discoceras are often difficult to distinguish and the position advocated by Sweet (1958), who discussed this issue in detail, is followed herein. Stumbur (1962) distinguished groups of discoceratids by their conch cross section and erected new generic names. However, Dzik (1984) already pointed out that the variation between the different shapes is continuous and sometimes changes within single specimens. Based on the known variability of the material from the Boda Limestone, it is impossible to distinguish different genera by conch cross section alone.