Discoceras antiquissimum (Eichwald, 1842)

Figs 4–6; Table 3

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

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

Clymenia antiquissima – de Verneuil 1845: 361. — Eichwald 1860: 1301.

Lituites cornu arietites – Eichwald 1860: 1298.

? Discoceras antiquissimum – Rüdiger 1889: 55.

Lituites antiquissimus – Roemer 1861: 62, pl. 6 fig. 2f–g; 1885: 68(315), pl. 4(27) fig. 12.

Trocholites antiquissimus – Foord 1891: 51.

? Discoceras antiquissimum – Schröder 1891: (25)163.

Discoceras antiquissimum – Hyatt 1894: 500. — Foerste 1925: 17, 58, pl. 18 fig. 1. — Teichert 1930: 285. — Strand 1933: 33, pl. 2 figs 4, 11, pl. 4 figs 2–3, pl. 13 fig. 9. — Thorslund 1936: pl. 2 fig. 11 — Balashov 1953: 265, pl. 4 fig. 2, pl. 11 fig. 3, pl. 12 fig. 1; 1962: pl. 9 fig. 1; 1974: pl. 9 fig. 1. — Sweet 1958: 99, text-fig. 13Q. — Neben & Krueger 1973: pl. 77 figs 24–25. — Dzik 1984: 42, text-fig. 9c–d, 12.39, pl. 7 fig. 1. — Kröger 2013a: 72 fig. 31b; 2025: 129 figs 47c–d, 48a, 49a, 50a.

Discoceras cf. antiquissimum – Balashov 1964: 48, pl. 3 figs 3a–b.

non Lituites antiquissimus – Roemer 1861: pl. 6 fig. 2a–e.

non Lituites antiquissimus – Roemer 1885: 68, pl. 4 fig. 12.

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

Diagnosis (after Strand 1933; Kröger 2013a, 2025; emended)

Species of the genus Discoceras with adult sizes of up to about 150 mm, WER ah ~ 1.90 (decreasing with ontogeny), whorl profile slightly depressed and becoming less depressed to equidimensional in maturity (WWI decreases from 1.50 to ~ 1.00 during ontogeny), broadly reniform in juvenile stages, rounded trapezoidal with venter wider than dorsum in later growth stages; suture line with ventral and lateral lobe and ventrolateral saddle; free adult body chamber with constriction near aperture; shell ornamented with prominent annuli, one to two annuli per distance between two septa; siphuncle close to dorsal conch margin, diameter 0.15–0.17 of whorl height in late growth stages, septal necks orthochoanitic.

Type material

Neotype (designated here) ESTONIA • Piirsalu old quarry (Lääne County); Pirgu Regional Stage, Katian Stage; illustrated in Fig. 4; TAM G432:72.

Other material examined

ESTONIA • 1 spec.; Hosholm (Vormsi Island, Lääne County); Pirgu Regional Stage (late Katian, Late Ordovician); 2007; Krueger leg.; MB.C.32314 • 1 spec.; Härgla Quarry (Rapla County); Porkuni Regional Stage, Hirnantian Stage; illustrated in Fig. 5A; TUG 1862-1.

NORWAY • 1 spec.; Asker (Akershus County); Jerestadian Regional Stage (late Katian, Late Ordovician); von Buch Coll.; illustrated in Fig. 5B; MB.C.11569.

Description

Neotype TAM G432:72 (Fig. 4) is an internal mould of a nearly complete conch in the adult growth stage, 151 mm in maximum diameter; the inner volutions are obscured by the rock. The body chamber is preserved in the length of ~55 degrees and it is uncoiled adaperturally. At the diameter of 145 mm, the conch is extremely discoidal (CWI = 0.21) with a whorl expansion rate (WER wh) of ~ 1.62. The whorl profile is roughly equidimensional (WWI ~ 1.00), weakly round laterally and slightly flattened ventrally. The ornament consists from prominent, widely spaced annuli (~ 16 mm apart in the last half whorl); their crests are rather sharp and adapically inclined. The annuli are oblique laterally and produce a deep and sharp sinus ventrally. The annuli are locally covered by fine transverse and longitudinal lirae. The suture lines are directly transverse and extend with a ventrolateral saddle and a lateral and ventral lobe. The relative cameral length (RCL) is ~ 0.25 in the last half volution.

Specimen TUG 1862-1 (Fig. 5A) is an internal mould of a conch in the subadult or adult growth stage, 109 mm in diameter, with locally preserved shell wall. One side of the conch is slightly worn resulting in the need to reconstruct whorl width, but otherwise, the specimen is not significantly deformed. A little over three, tightly coiled whorls are preserved, which are fully chambered; only the last ~ 24 degrees belong to the body chamber. At the maximum diameter, the conch is evolute (UWI = 0.50) and extremely discoidal (CWI ~ 0.29) with a whorl expansion rate (WER wh) of ~ 1.90. The whorl profile is slightly depressed (WWI ~ 1.07) and very weakly imprinted dorsally, rounded laterally and slightly flattened ventrally. The shell is ornamented with sharp and widely spaced annuli (~ 13 mm apart at dm = 109 mm); their crests are rather sharp and inclined towards the adapical part. The annuli run obliquely across the flanks and produce a sharp ventral sinus. Finer ornament elements are locally preserved, especially on the ventral side of the outer whorl. These fine elements are transverse lirae and very fine longitudinal lirae. The suture lines are directly transverse; they produce a ventrolateral saddle and a lateral and ventral lobe. The relative cameral length (RCL) is 0.18 at the dm = 104 mm).

Specimen MB.C.11569 is a conch in the subadult growth stage with a diameter of 81 mm and three preserved whorls (Fig. 5B). The conch is evolute (UWI = 0.53) and discoidal (CWI = 0.36) with moderate whorl expansion (WER ah = 1.82). The last whorl is weakly depressed (WWI = 1.36) and very weakly embracing (IZR = 0.02); the whorl profile is rounded trapezoidal in shape, being narrowest dorsolaterally and widest ventrolaterally. A short part of the body chamber is preserved; it is very slightly uncoiled from the preceding whorl. The shell is ornamented with distinct, widely spaced annuli (~ 11 mm apart at dm = 81 mm) with rounded, symmetrical crests, along with coarse lirae (up to 1 mm apart); the annuli have low nodes mid-ventrally and at the ventrolateral shoulder. Extremely fine longitudinal ornament is discernible at the ventral side at the end of the last whorl. The septa are relatively crowded (RCL = 0.15 at dm = 60 mm).

Specimen MB.C.32314 is a large, nearly adult conch with a diameter of 119 mm; only the outer whorl is preserved. The conch is extremely discoidal (CWI = 0.26) and evolute (UWI = 0.56), expanding with a WER ah of 1.71. The whorl profile at the maximum diameter of the conch is weakly depressed (WWI = 1.10) and trapezoidal in shape (widest ventrally); weakly imprinted dorsally or not imprinted (IZR = 0.02–0.00). The relative cameral length (RCL) is 0.22 (dm = 86 mm). The incomplete body chamber is preserved in the length of 100 degrees and uncoils from the preceding whorl at near its beginning. The siphuncle has a position close to the dorsal shell wall but its distance and the diameter cannot be precisely measured. The ornament consists of widely spaced annuli (~ 10 mm apart at dm = 87 mm) with rounded and symmetric crests.

Remarks

The holotype of Discoceras antiquissimum comes from the late Katian (Vormsi–Pirgu regional stages) of Kärdla (Island of Hiiumaa, Estonia). It was described by Eichwald (1840: 115), illustrated by Eichwald (1842: pl. 3 figs 16–17) and then re-illustrated by Foerste (1925: pl. 18 fig. 1) and Balashov (1953: pl. 12 fig. 1). The latter author stated the holotype was held at the “Prussian Geological Institute?” in Berlin. However, any attempts of the present authors to trace the current location and status of the specimen were unsuccessful and it is probable that the specimen was lost. For this reason, we designate TAM G432:72 (Fig. 4) from the Pirgu Regional Stage (Katian Stage) of west Estonia (Piirsalu old quarry, Lääne County) as the neotype. The specimen is roughly corresponds stratigraphically to the holotype and bears all the diagnostic features of the species, namely the coarse annulation on the shell surface. The specimen was previously described by Strand (1933: 34, pl. 2 fig. 11) and Kröger (2025: 132; under the obsolete collection number TAM G1:170), and is re-described and illustrated here for the purpose of neotype designation.

Specimen MB.C.11569 described above was originally used by Lossen (1860) to establish the species Lituites trapezoidalis. As already noted by Strand (1933: 36), the specimen corresponds in all aspects to the diagnosis of D. antiquissimum and is thus considered to represent an earlier growth stage attributable to the latter taxon.

Discoceras antiquissimum is similar to D. saemanni due to its weakly depressed, trapezoidal whorl profile, which is widest at the ventrolateral shoulder. However, it can be readily distinguished by the distinctly annulated shell surface; the annuli are clearly visible also on the internal mould. The thickness of the annuli varies between specimens, and the annuli are present in all growth stages except the earliest and latest growth stage (Kröger 2025).

Geographic and stratigraphic occurrence

Baltoscandia, north-eastern Russia, northern Germany and northern Poland; late Katian–early Hirnantian, Late Ordovician.