Discoceras saemanni

Discoceras saemanni (Hyatt, 1894) Figs 6–8; Table 4 Schroederoceras Saemanni Hyatt, 1894: 462. Lituites angulatus Saemann, 1853: 166, pl. 21 fig. 1c–d. Trocholites undosus Foord, 1891: 45. Discoceras hyatti Strand, 1933: 38, pl. 2 fig. 2, pl. 5 fig. 3a–b. Discoceras sp. – Foerste 1930: 176, pl. 20 fig. 2a. Discoceras saemanni – Strand 1933: 40, pl. 2 fig. 1a–b, pl. 4 fig. 1. — Sweet 1958: 102, text-fig. 13n. — Dzik 1984: 44, text-fig. 12.43. — Kröger 2025: 141, text-figs 48c, 49c, 50b, 51a, 53. Discoceras hyatti – Sweet 1958: 102, text-fig. 13g. Schroederoceras hyatti – Stumbur 1959: 26, text-fig. 1c; 1962: 137. — Dzik 1984: 44, text-fig. 12.44. Schroederoceras saemanni – Stumbur 1962: 137. Rectanguloceras sp. – Neben & Krueger 1973: pl. 76 figs 16–17. non Lituites angulatus Saemann, 1853: pl. 21 fig. 1a–b. Diagnosis (after Kröger 2025) Species of the genus Discoceras with adult conch diameters of up to ca 125 mm; adult WER ~2.00; adult whorl profile with WWI ~1.10–1.20, subtrapezoidal with flattened venter, venter considerably wider than dorsum except at adult aperture and in juvenile growth stages; WER and WWI decrease with conch size; free adult body chamber with constriction near aperture; ornamented with crenulated costae and distinct growth lines that form U-shaped hyponomic sinus, low ribs can occur. Material examined GERMANY – Brandenburg • 1 spec.; Hohensaaten; Vormsi or Pirgu regional stages (late Katian Stage, Late Ordovician); Stober leg.; illustrated by Neben & Krueger (1973: pl. 76 figs 16–17), re-illustrated here in Fig. 7; MB.C.32315. POLAND – Pomerania • 1 spec.; Gdańsk; probably Late Ordovician; Rühle von Lilienstern Coll.; illustrated in Fig. 8; MB.C.32316. Description Specimen MB.C.32315 is a conch of an adult individual, where a little less than one half is missing (Fig. 7). The maximum diameter is 102 mm; 3.25 whorls are preserved. The whorl width and profile had to be reconstructed due to the conch being embedded in rock on one side and the imprecise preparation of the specimen by a previous researcher. In the last half whorl, the conch is discoidal (CWI ~ 0.34) and increasingly evolute (UWI increases from 0.46 to 0.49); the coiling rate drops (WER decreases from 2.09 to 1.91), and the whorls are weakly embracing (IZR close to zero). The whorl profile is almost equidimensional (WWI = 0.96) and weakly trapezoidal in shape, with the widest point at the subangular ventrolateral shoulder. The sutures line has lateral and external lobes and a ventrolateral saddle. The body chamber is preserved over nearly 90 degrees of the whorl and is uncoiled from the preceding whorl. The end of the body chamber flares laterally, causing the whorl profile to change from trapezoidal to rectangular, with flattened dorsal and ventral sides. The relativa cameral length decreases during ontogeny, from RCL = 0.60 in the earliest whorl to 0.19 in the latest part of the phragmocone (dm = 72 mm). The initial chamber measures approximately 1.7 mm in length and 2.3 mm in width at the position of the first septum. The siphuncle is located close to the dorsal shell wall in the inner whorls and is nearly in contact with it in the outer whorl; the RSH is 0.20 at the largest preserved whorl height. The shell ornament in the earliest whorl is only very faintly preserved and appears to consist of widely and regularly spaced raised lirae or thin ribs. The second whorl has low, lamellar lirae and at the end of the body chamber, the raised lirae with symmetric crests are present. Specimen MB.C.32316 is a nearly complete adult conch, broken off on one side (Fig. 8). The conch measures 96 mm in diameter, with a reconstructed maximum size of 120 mm, and consists of 3.25 whorls. In the last half-whorl, the conch becomes increasingly discoidal (CWI decreases from 0.34 to 0.29) and more evolute (UWI increases from 0.49 to 0.53), while the coiling rate decreases (WER drops from 1.99 to 1.82). The imprint zone rate (IZR) is close to zero. The whorl profile is weakly depressed (WWI decreases from 1.16 to 1.13) and weakly trapezoidal in shape, with the widest point at the subangular ventrolateral shoulder and a convex umbilical wall. The suture line extends with external and ventral lobes as well as a ventrolateral saddle. The last whorl begins to uncoil at approximately the rear part of the terminal body chamber or slightly earlier. The body chamber is 110 degrees in length, with a laterally flared aperture forming a ventral sinus. The relatice cameral length (RCL) is 0.25 in the last whorl. In the latest part of the phragmocone, the siphuncle is subdorsal, with an RSH of approximately 0.19. The shell ornament in the first whorl transitions from sharp, widely and regularly spaced raised lirae or thin ribs to low, lamellar lirae. The outer whorl bears fine lirae, either with symmetrical crests or imbricated, while very flat ribs appear at the end of the body chamber. A pair of very low, narrow ridges is present on the ventral shell surface of the last whorl. Remarks According to the original museum label, the specimen MB.C.32316 comes from the “Orthoceratite Limestone”, which would correspond to the Darriwilian Stage (Middle Ordovician). However, the lithology of the specimen indicates that the actual age is the Katian Stage (Late Ordovician). Additionally, no Darriwilian representatives of Discoceras exhibit a conch morphology similar to that of specimen MB.C.32316, suggesting that the stratigraphic information on the label is erroneous. Instead, specimen MB.C.32316 is almost identical to specimen MB.C.32315 and other late Katian–early Hirnantian representatives of Discoceras with a trapezoidal, ventrally flattened whorl profile. Its coiling parameters, ontogenetic trends and shell ornamentation perfectly align with the morphological variation recognised in D. saemanni by Kröger (2025); the two specimens are therefore assigned to that species. Hyatt (1894) based D. saemanni on two specimens originally illustrated by Saemann (1853: pl. 21 fig. 1c–d). Both specimens come from the latest Katian of Herøya (near Porsgrund, Norway) and are now kept at the Museum of Comparative Zoology (Massachusetts, USA) under coll. no. MCZ:IP:109000 and MCZ:IP:109001. Neither of the syntypes were available for study in this work. Discoceras saemanni is most similar to D. antiquissimum in terms of conch proportions, ratios and whorl profile, but its shell surface lacks annuli; instead, crenulated costae and, in some cases, low ribs are present. A weakly trapezoidal whorl profile is also present in specimens of the stratigraphically older (Sandbian) D. spongistratum, but the latter differs in shell ornamentation (narrow ribs or annuli), a more compressed whorl profile (WWI ~ 0.70) and a greater coiling rate (WER = 2.12) in the terminal growth stage. Geographic and stratigraphic occurrence Norway, Estonia, northern Germany and northern Poland; late Katian–Hirnantian Stage, Late Ordovician.