Discoceras roemeri Strand, 1934

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 73-76

Discoceras roemeri Strand, 1934

Figs 31A , 32 , 33

Discoceras roemeri Strand, 1934: 38 , 42-43, pl. 2, figs 6a-b, 7, pl. 5, figs 1, 2.

Schroederoceras balaschovi Stumbur, 1956: 181 , pl. 1, figs 5, 6.

Lituites antiquissimus – Roemer 1861: 62, pl. 6, fig. 2a-e (non Discoceras antiquissimum ( Eichwald, 1840)) .

Discoceras angulatum – Mutvei 1957: text-fig. 12, pl. 8.

Discoceras roemeri – Sweet 1958: 99, 102, text-fig. 13p. ― Dzik 1984: 41, 44, text-figs 10, 12.43, pl. 6, figs 5, 6.

Schroederoceras roemeri – Stumbur 1962: 136, text-fig. 2.4.

Rectanguloceras balaschovi – Stumbur 1962: 142, text-fig. 2.9.

Rectanguloceras (Discoceras) cf. roemeri – Neben & Krueger 1973: pl. 65, figs 1, 2.

Diagnosis

Discoceras with large adult sizes of up to 150 mm, WER of 1.9–2.5, decreasing with increasing conch size; whorl cross section slightly depressed with WWI 1.1–1.3, rounded subquadratic with venter not wider than dorsum; body chamber nearly 180° long, free in mature growth stages and becoming

markedly higher during matures stages; ornamented with irregularly spaced costae and shallow ribs. (Adopted from Strand 1934.)

Type locality and horizon

Frognøya, Tyrifjorden, Norway, Sørbakken Formation (Stage 4css), late Katian, Ordovician.

Material

Four specimens ( PMU 26903–26906), Kallholn; one specimen ( PMU 26907), Skålberget, Dalarna, Boda Limestone; one specimen ( PMU 26908), Amtjärn, Dalarna, Johnstorp Formation; all late Katian, Late Ordovician.

Description

This species was described in detail by Strand (1934). The specimens from the Boda Limestone provide additional data regarding the ornamentation and the variability of the general conch shape. In specimen PMU 26903 the conch surface is well preserved ( Fig. 31A ). The maximum diameter of this specimen is 72 mm with a maximum whorl height of 24 mm and width of 29 mm (WWI 1.21); the WER of the specimen is 2.16. The whorl cross section is subquadratic with a ca. 12 mm wide, slightly rounded flattened venter, rounded umbilical shoulders and a shallow impression zone. The specimen is ornamented with ca. 17 shallow, slightly irregularly marginally spaced ribs on the outer whorl. The ribs occur only on the umbilical shoulders, not on the venter, which is almost flat. Narrowly spaced, pronounced growth lines or costae run parallel to the ribs and form a pronounced u-shaped hyponomic sinus.

The whorl expansion rate varies comparatively strongly amongst the five specimens from the Boda Limestone, with a maximum of 2.56 in specimen PMU 26904 ( Fig. 32B ), a specimen which agrees in all features with other specimens assigned to D. roemeri .

Remarks

Strand (1934: 46) described some variation in his specimen of D. roemeri . Within this material, the type, which is from the Sørbakken Formation, differs from younger specimens of the “Gastropod Limestone” in having a narrower venter. The Boda Limestone specimens are most similar to the specimen of the Gastropod Limestone and to the specimen figured by Roemer (1861: pl. 6, fig 2a). This is also apparent in the diagrammatic representation of the whorl expansion in Fig. 33 , where the variation of the specimens from the Boda Limestone is closer to the variation of the specimen figured by Roemer (1861).

Comparison

Several species of Discoceras , described from the Gastropod Limestone of the Oslo area ( Strand 1934), are very similar to the specimens assigned to D. roemeri herein. Generally, these species differ in having a lower expansion rate of ca. 1.8–1.9. Among them, D. angulatum is most similar to D. roemeri with respect to the conch cross section. D. angulatum also has similarly rounded umbilical shoulders, but in

contrast to D. roemeri the width of the ventral side of the whorl is always less than that of the dorsal side, and appears to be flatter.

The diagrammatic representation of the morphological variation of different specimens assigned to D. roemeri and the comparison of cross sections reveal that Schroederoceras balaschovi Stumbur, 1956 from the Pirgu Stage of Estonia and Discoceras vasegaardense Rasmussen & Surlyk, 2012 from the late Katian Lindegård Formation of Bornholm, Denmark are well within the range of variation of D. roemeri and must be synonymized with the latter ( Fig. 33 ). However, because details of the cross section of D. vasegaardense are not known and the ribbing in this species appears to be stronger than in specimens assigned to D. roemeri it cannot be synonymised with the latter. On the other hand, Schroederoceras balaschovi is interpreted as a junior synonym of D. roemeri , herein.

Stratigraphic and geographic range

Venstøp and Sørbakken formations, Ringerike district, and Venstøp and Grimsøy formations, Oslo-Asker district, upper Herøya Formation (“Gastropod Limestone”), Skien-Langesund district, Norway ( Strand 1934); erratic limestone boulders from Vormsi/Pirgu Regional Stage (“Lyckholm stage”), Zawidowice, Poland ( Dzik 1984); Vormsi/Pirgu Regional Stage (“Lyckholm stage”), Estonia ( Teichert 1930; Stumbur 1956, 1962); Boda Limestone, Dalarna, Sweden; all late Katian.

Figures

Fig. 31. Discoceratids of the Boda Limestone of Kallholn. A. Discoceras roemeri Strand, 1934, PMU 26903. B. Discoceras antiquissimum (Eichwald, 1842), NRM-PZ Mo 8745. Scale bar = 10 mm for both figures.

Fig. 32. Median cross sections of Discoceras roemeri Strand, 1934, Boda Limestone, Kallholn. A. PMU 26903. B. PMU 26904. Scale bar = 10 mm for both figures.

Fig. 33. Morphological variation in Discoceras roemeri Strand, 1934. Black dots = specimens of the Boda Limestone. Black dot with grey margin = Rectanguloceras balaschovi Stumbur, 1962. Grey dots = Lituites antiquissimus of Roemer (1861: pl. 6, figs 2a-e). Grey dot with black margin = Discoceras roemeri of Dzik (1984: pl. 6, figs 5,6). White dot with black circle = Discoceras roemeri of Strand (1934: pl. 5, fig. 1). WER = whorl expansion rate.