Discoceras antiquissimum ( Eichwald, 1842 )

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 72-73

Discoceras antiquissimum ( Eichwald, 1842)

Fig. 31B

Clymenia antiquissima Eichwald, 1842: 33 , pl. 3, figs 16, 17.

Lituites trapezoidalis Lossen, 1860: 25 , pl. 1, fig. 2.

Clymenia antiquissima – Eichwald 1840: 15 (nomen nudum).

Lituites (Trocholites) antiquissimus – Schmidt 1861: 62, pl. 6, fig. 2a-g.

Lituites antiquissimus – Roemer 1861: 62, pl. 6, fig. 2f-g (non pl. 6, figs 2a-e = Discoceras roemeri Strand, 1934 , see below). ― Karsten 1869: 53, pl.19, fig. 7a-c. ― Roemer 1885: 68, pl. 4, fig. 12.

Discoceras antiquissimum – Foerste 1925: 17, 58, pl. 18, fig. 1. ― Strand 1934: 32, pl. 2, figs 4, 11, pl. 4, figs 2, 3, pl. 13, fig. 9. ― Thorslund 1936: pl. 2, fig. 11. ― Balashov 1953: 265, pl. 12, fig. 1a, b. ― Sweet 1958: 99, text-fig. 13q. ― Neben & Krueger 1973: pl. 77, figs 24, 25. ― Dzik 1984: 42, text-figs 9c-d, 12.39, pl.7, fig. 1.

Discoceras antiquissima – Hucke & Voigt 1967: 58, pl. 15, fig. 1.

Diagnosis

Discoceras with adult sizes more than 130 mm, WER of ca. 1.9 decreasing with increasing conch size; whorl cross section slightly depressed with WWI 1.2–1.5 becoming less depressed in maturity, broadly reniform in juvenile stages, rounded trapezoidal with venter wider than dorsum in later groth stages; free mature body chamber with constriction near aperture; ornamented with prominent ribs, one to two ribs per distance between two septa; siphuncle close to dorsal conch margin, ca. 0.15–0.17 of whorl height, septal necks orthochoanitic. (Adopted from Strand 1934.)

Type locality and horizon

Kärdla, Hiiumaa Island, Estonia, presumably Vormsi/Pirgu Regional Stage (“Lyckholm stage” of Teichert 1930).

Material

Two specimens, PMU 24739 and NRM-PZ Mo 8745, Kallholn, late Katian, Late Ordovician.

Description

This species was described in detail by Strand (1934). The specimen from the Boda Limestone is documented in order to add detail regarding the morphological variability of the species. The specimen with a maximum conch diameter of 92 mm is a fragment with three whorls, lacking the free part representing the mature stage. The maximum whorl height is ca. 22 mm and the maximum width 25 mm (WWI 1.14), the whorl expands with a WER of 1.89. The conch is prominently annulated, with ca. 30 sharply crested ribs per 360° revolution. The ribs slope slightly adapically toward the venter and form an angular v-shaped sinus on the venter. In addition to the conch annulation, the shell is ornamented with fine, irregularly spaced, transverse growth lines parallel to the annulations.

Remarks

It is very probable that the specimen NRM-PZ Mo8745is that referred to by Strand(1934:36) from the Boda Limestone.

Comparison

This species, with its prominent juvenile to adult annulation, is unique within Discoceras .

Stratigraphic and geographic range

Bønsnes Formation, Ringerike district, “Gastropod Limestone”, Oslo-Asker, and Skien-Langesund district, Norway ( Strand 1934); erratic limestone boulders from northern Germany ( Hucke & Voigt 1967), and erratic limestone boulders from Vormsi/Pirgu Regional Stage (“Lyckholm stage”), Zawidowice, Poland ( Dzik 1984); Vormsi/Pirgu Regional Stage (“Lyckholm stage”), Estonia ( Teichert 1930; Balasov 1953); 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.