Genus Discoceras Barrande, 1867

Type species

Clymenia antiquissima Eichwald, 1842; by subsequent designation (Schröder 1891).

Diagnosis (after Kröger & Aubrechtová 2018; Manda & Turek 2018)

Genus of the family Trocholitidae with adult conch diameters between 40 and 150 mm or more; body chamber usually divergent from preceding whorl, aperture may be slightly constricted or ventrally flared; whorl profile weakly depressed, equidimensional or weakly compressed; shape of whorl profile circular, elliptic, subquadrate, trapezoidal or heart-shaped; whorls weakly embracing or not embracing; caecum subcentral, shifted towards ventral side, siphuncle shifts dorsally in second and third chamber, final dorsal or subdorsal position is attained in third septum; thick, layered connecting rings; shell surface smooth or ornamented with frilled and imbricated to lamellar lirae, narrow to strong annuli or ribs present in some taxa; ornament forms ventral sinus.

Remarks

Among the Ordovician Baltoscandian trocholitids, the genera most similar to Discoceras are Curtoceras Ulrich, Foerste, Miller & Furnish 1942 and Trocholites Conrad, 1838 (see Aubrechtová & Korn 2025 for detailed discussions). The species of both genera are smaller in adult diameters (usually between 20 and 60 mm), and their shells are more finely ornamented; the ornamentation elements are neither frilled nor strongly imbricated. In Curtoceras, the whorl profile is equidimensional or nearly so, with convex or only slightly flattened ventral and lateral sides. In Trocholites, the conch is stouter, more tightly coiled, the whorls embrace each other and the whorl profile is much more depressed.

In Discoceras, the adult conch size usually exceeds 100 mm, and the shell is ornamented with frilled and imbricated elements. The terminal body chamber is typically uncoiled to varying degrees. The adult growth stages of Discoceras are extremely discoidal (CWI <0.35), and the whorls usually have only a weak imprint zone or no imprint zone at all. The whorl profile in Discoceras is typically weakly depressed in earlier growth stages (WWI = 1.00–1.50), becoming equidimensional or compressed in later growth (WWI = 0.50–1.00). The shape of the whorl profile ranges from circular or oval to subrectangular, subquadrate, trapezoidal or heart-shaped. Both the whorl profile WWI and its shape are highly variable within species and during ontogeny (Kröger 2025).

Geographic and stratigraphic occurrence

North America, Baltoscandia and St Petersburg Region of Russia, Kazakhstan, India, China, Middle–Late Ordovician. Baltoscandia, Czech Republic, North America, China and Australia, Llandovery–Wenlock series, Silurian.

Species included

North America (Meek & Worthen 1870, 1873; Whiteaves 1897; Flower 1968): Lituites Graftonensis Meek & Worthen, 1870; Lituites? Ortoni Meek & Worthen, 1873; Discoceras canadense Whiteaves, 1897; Discoceras perornatus Flower, 1968.

Northern Europe and St Petersburg Region of Russia (Eichwald 1840; Saemann 1853; Lossen 1860; Remelé 1880, 1890; Schröder 1891; Hyatt 1894; Strand 1933; Balashov 1953; Sweet 1958; Kröger et al. 2011; Rasmussen & Surlyk 2012; Manda & Turek 2018; Kröger & Aubrechtová 2019; Kröger 2025): Clymenia antiquissima Eichwald, 1840; Clymenia rarospira Eichwald, 1840; Lituites angulatus Saemann, 1853; Lituites arcuatus Lossen, 1860; Lituites danckelmanni Remelé, 1880; Discoceras bandonis Remelé, 1890; Trocholites Damesii Schröder, 1891; Schroederoceras saemanni Hyatt, 1894; Schroederoceras tubulatum Hyatt, 1894; Discoceras roemeri Strand, 1933; Discoceras spongistratum Balashov, 1953; Schroederoceras chrevitzaense Balashov, 1953; Schroederoceras gubkovense Balashov, 1953; Schroederoceras ievense Balashov, 1953; Schroederoceras tammikuense Balashov, 1953; Schroederoceras vasalemmense Balashov, 1953; Schroederoceras vesenbergense Balashov, 1953 Discoceras amplicameratum Sweet, 1958; Discoceras boreale Sweet, 1958; Discoceras depressum Sweet, 1958; Discoceras fleischeri Sweet, 1958; Discoceras siljanense Kröger et al. 2011; Discoceras costatum Rasmussen & Surlyk, 2012; Discoceras vasegaardense Rasmussen & Surlyk, 2012; Discoceras lindstroemi Manda & Turek, 2018; Discoceras stridsbergi Manda & Turek, 2018; Discoceras amtjaernense Kröger & Aubrechtová, 2019; Discoceras nilssoni Kröger & Aubrechtová, 2019; Discoceras paopense Kröger, 2025.

Bohemia (Barrande 1865): Lituites (Ophioceras) amissus Barrande, 1865.

Kazakhstan (Tarbagatay, East Kazakhstan) (Barskov 1972): Discoceras kazakhstanense Barskov, 1972; Discoceras tshingizense Barskov, 1972.

China (Xainza, Tibet) (Lai 1982): Discoceras xainzaense Lai, 1982.

China (Xinjiang) (Lai & Wang 1981): Discoceras aqalense Lai & Wang, 1981; Discoceras rareplicatum Lai & Wang, 1981; Discoceras robustum Lai & Wang, 1981; Discoceras semicirculare Lai & Wang, 1981; Discoceras ventrosellatum Lai & Wang, 1981; Discoceras yinganense Lai & Wang, 1981.

China (Shaanxi, Gansu, Ningxia) (Gao et al. 1982): Discoceras huanxianense Gao, Lai & Wen, 1982.

China (Altun area, Xinjiang) (Lai & Wang 1986): Discoceras altunense Lai & Wang, 1986.

China (Mt Charchag, Xinjiang) (Lai 1987): Discoceras xinjiangense Lai, 1987.

India (Spiti, Himachal Pradesh) (Salter in Reed 1912): Trocholites juliformis Salter, 1912.