Discoceras Barrande, 1867

Kröger, Björn, 2025, The Lyckholm acme of cephalopods - Review of the late Katian (Vormsi-Pirgu regional stages) Ordovician cephalopods of Estonia, European Journal of Taxonomy 978, pp. 1-169 : 128-129

Genus Discoceras Barrande, 1867

Type species

Clymenia antiquissima Eichwald, 1842 , Kärdla, Hiiumaa Island, Estonia; Vormsi–Pirgu regional stages (“Lyckholm Stufe” of Teichert 1930), late Katian; by secondary designation ( Schröder 1891).

Diagnosis

Tarphycerid with slightly to moderately depressed evolute shell with variously spaced frills; caecum subcentral, shifted toward ventral side, the siphuncle shifts in second and third chambers toward dorsal side, its final dorsal or subdorsal position is attained in third septum. (From Manda & Turek 2018: 414.)

Remarks

The genus Discoceras was originally erected to encompass species with a subquadratic cross section and a dorsal siphuncle, which are either strongly annulated, such as in the type species or smooth such as in Lituites angulatus Saemann, 1853 ( Barrande 1867). Subsequently, Discoceras has been restricted to the annulated D. antiquissimum , and smooth or slightly ribbed forms have been assigned to Schroederoceras Hyatt, 1894 .

The high ontogenetic and intraspecific variability in ornamentation and whorl cross section led Strand (1934) and Sweet (1958) to the conclusion that it is not possible to distinguish between a smooth or slightly ribbed Schroederoceras and a strongly ribbed Discoceras . Following Strand (1934: 32–33), Discoceras comprises closely coiled forms, with subquadratic to trapezoidal conch cross sections and with a sculpture “formed by imbricate lamellae which overlap in apical direction (…) and ribs, which are either strongly elevated or weaker and more closed set.” This concept of Discoceras was adopted in the Treatise ( Furnish & Glenister 1964) and is followed herein. It was not accepted by, e.g., Balashov (1953b), Stumbur (1962), and Dzik (1984).

Furthermore, to tackle the high intrageneric and intraspecific variability of Discoceras, Stumbur (1962) erected two new genera ( Rectanguloceras Stumbur 1962 ; and Sweetoceras Stumbur, 1962 ), rejected Sweet’s (1958) synonymisation of Discoceras and Schroederoceras , and developed a phylogenetic hypothesis based on these genera (or vice versa). However, Stumbur’s (1962) genus diagnoses do not permit effective differentiation between these two novel genera and between Discoceras and Schroederoceras , respectively. His diagnoses of the two genera are so vague, that they are inapplicable in practice.

In several specimens assigned to D. roemeri , and D. saemanni the ornamentation is preserved: The external shell is ornamented with finely crenulated, imbricate lamellae, costae or frills (see, e.g., Strand 1934: 37, 44). Additionally, a faint lateral band is visible in a specimen assigned to D. roemeri by Kröger (2013: fig. 31a, see below). Several external casts and fragments of the outer shell of species of Discoceras are preserved in the Estonian collections, which also show a longitudinal banding, which in a glance resembles a preserved color stripe. Closer examination, however, reveals that this band results from different strengths of the imbricate lamellae: at approximately mid-flank a narrow band is present with exceptionally prominent costae. Ventrally, the frills are more distinct and long: overlapping more strong than dorsal of the band.

Without doubt, the peculiar ornamentation, with a lateral band of elevated costae is a character of taxonomic value. However, it is preserved only under exceptional circumstance and, therefore, probably has never been explicitly described or included within the diagnoses of species of Discoceras , it occurs in specimens assigned to D. saemanni and D. roemeri . The feature probably is related to the “lateral furrow” described in two Silurian species of Discoceras by Manda & Turek (2018).

Strand (1934) distinguished two pairs of species of Discoceras in the latest Katian strata of Norway based on the shape of the whorl cross section ( D. angulatum – D. hyatti ; D. roemeri – D. saemanni ). Each pair supposedly contains a species with a relatively narrow venter ( D. angulatum , D. roemeri ) and a species with a broadly trapezoidal cross section and a broad venter ( D. hyatti , D. saemanni ). The two pairs, in turn were distinguished by their expansion rate and relative whorl cross section width. However, the new data available from the Estonian material show that the high variability of the shape of the whorl cross section during ontogeny, within and between individual specimens does not permit distinctions between D. saemanni and D. hyatti , respectively (see below). Herein, D. hyatti is interpreted as a junior synonym of D. saemanni , leaving three late Katian species to be distinguished: one with a whorl cross section with broadly trapezoidal venter ( D. saemanni ) and two with a relatively narrow venters ( D. angulatum , D. roemeri ). Discoceras angulatum remains restricted to its genotype until revision. Hence, it is possible that these species represent sexual dimorphs because in many specimens, the relatively broad venter only occurs during late growth stages. A similar distinction between a species of Discoceras with a narrow venter and a species with a wide venter can be seen in D. amtjaernense Kröger & Aubrechtová, 2019 and D. nilssoni Kröger & Aubrechtová, 2019 from the early Katian Kullsberg Limestone, Sweden.