Discoceras danckelmanni

Discoceras danckelmanni (Remelé, 1880) Figs 14, 18–19; Table 10 Lituites Danckelmanni Remelé, 1880: 241, pl. 1 figs 7–8. ? Lituites (Trocholites) cornuarietis – Schmidt 1861: 199. ? Discoceras danckelmanni – Rüdiger 1889: 56. — Sweet 1958: 102, text-fig. 13f. Discoceras (Lituites) Danckelmanni – Remelé 1890: 32, pl. 1 figs 7–8. Discoceras Danckelmanni – Schröder 1891: 25 (163). — Hyatt 1894: 469. Schroederoceras danckelmanni – Balashov 1953: 261, pl. 9 fig. 2. Discoceras danckelmanni – Sweet 1958: 102, text-fig. 13f. Rectanguloceras danckelmanni – Stumbur 1962: 137, 142. ? Schroederoceras danckelmani (sic) – Saladžius 1966: 36, pl.1 fig. 2. Rectanguloceras (Discoceras) – Neben & Krueger 1973: pl. 76 fig. 18. non Discoceras danckelmanni – Hucke 1967: pl. 15 fig. 2. non Rectanguloceras (Discoceras) danckelmanni – Neben & Krueger 1973: pl. 65 figs 3–4. Diagnosis Species of the genus Discoceras with adult conch diameters of ~ 100 mm; adult conch evolute (UWI = 0.50–0.60), moderately expanding (WER ~2.00); whorl profile elliptic and compressed (WWI ~ 0.70 in late growth stage); body chamber uncoiled from preceding whorl for most of its length, nearly tubular in shape, only weakly curved and slightly flared ventrally in its apertural part; shell ornament with regularly spaced, weakly frilled lirae with symmetric crests, and irregularly spaced, low ribs on the body chamber. Type material Lectotype (designated herein) GERMANY – Brandenburg • Eberswalde; Moldåan Regional Stage (early Katian Stage, Late Ordovician); Remelé Coll.; illustrated by Remelé (1880: pl. 1 fig. 7; 1890: pl. 1 fig. 7), re-illustrated here in Fig. 18; MB.C.11603. Paralectotype (designated herein) GERMANY – Brandenburg • 1 spec.; same data as for lectotype; illustrated by Remelé (1880: pl. 1 fig. 8) and Remelé (1890: pl. 1 fig. 8); MB.C.11570. Other material examined GERMANY – Brandenburg • 1 spec.; Hohensaaten; Moldåan Regional Stage (early Katian Stage, Late Ordovician); Neben and Krueger Coll.; illustrated by Neben & Krueger (1973: pl. 65, figs 3–4), re-illustrated here in Fig. 19; MB.C.32321. – Mecklenburg-Vorpommern • 1 spec.; Salem (Malchin); Moldåan Regional Stage (early Katian Stage, Late Ordovician); MB.C.32320. Description Lectotype MB.C.11603 is a conch in the adult growth stage with an uncoiled body chamber (Fig. 18). The specimen is strongly secondarily compressed, but some measurements can still be obtained. The diameter of the conch is around 100 mm in total, and it is evolute (UWI ~0.50). The whorls were probably weakly compressed, elliptic, and weakly imprinted dorsally or not imprinted. The coiling rate decreases in the last whorl from 2.10 to 1.94. The body chamber is tubular, about 104 degrees in length, and its departure from the previous whorl starts shortly after the last septum; the uncoiled part is weakly curved. The shell ornament is not preserved; only indistinct traces of some irregularly thick, flat transverse ornament elements are visible on the internal mould of the body chamber. The ornament is much better preserved in paralectotype MB.C.11570 (Fig. 19). It consists of regularly spaced lirae, which are weakly frilled and have symmetric crests; in addition, low, rounded ribs are indicated ventrolaterally. The whorl profile in the paralectotype is not as strongly distorted as in the lectotype; it appears to be compressed (WWI = 0.70) and elliptic in shape. Specimen MB.C.32320 is a compacted, fragmentary conch in the adult growth stage. The specimen shows the uncoiled body chamber, about 85 degrees in length, with a slightly flared ventral side. Specimen MB.C.32321 is a nearly complete conch, consisting of over three whorls and with a maximum diameter of 93 mm. At this diameter, the conch is evolute (UWI = 0.61) and moderately expanding (WER = 1.98). The conch is embedded in rock from one lateral side, hence the whorl width cannot be measured. The shell wall is preserved only locally and bears fine, relatively regularly spaced transverse lirae. The relative cameral length (RCL) is about 0.20 in the last whorl; suture lines appear to be nearly straight or only with a shallow lateral and ventral sinus. The last 100 degrees of the conch are occupied by the body chamber, which is uncoiled from about the last septum and is nearly tubular in shape; the uncoiled part is weakly curved and almost straight. Remarks Discoceras danckelmanni was described by Remelé (1880) based on two distorted (compacted) specimens. Additionally, the type specimens are not accurately illustrated by Remelé (1880, 1890); the ornamentation is only faintly preserved, and the specimens do not show the siphuncle or septa. Since the specimens are compacted, the whorl profile is not as strongly compressed as indicated by Remelé (1880, 1890) and Sweet (1958: text-fig. 13f). Despite the poor preservation of the type material, some measurements and descriptions could still be made, and two other specimens, MB.C.32320 and MB.C.32321 from the herein studied collection, were assigned to the species. Discoceras danckelmanni can be distinguished from other species of the genus by its large, discoidal conch. The body chamber is probably rather short, with a low expansion rate, and remains nearly tubular except for a slight ventral flaring in its apertural region. The whorls are also relatively slowly expanding and have an elliptic and weakly compressed profile. Most similar to D. danckelmanni is D. bandonis, in which the whorl profile is less compressed (WWI ~0.90 vs 0.70) and which has a higher coiling rate (WER of 2.00 in late growth stage); its body chamber uncoils from the previous whorl at about midlength or later. The holotypes of three species from the Haljala Regional Stage (Sandbian) of Estonia and the St Petersburg Region of Russia (D. tammikuense, D. gubkovense, and D. spongistratum) are also similar to the specimens of D. danckelmanni. All three of these species have rather large (95–145 mm), discoidal and slowly expanding conchs (WER ~ 2.00) with weakly subrectangular, subtrapezoidal, or elliptic, compressed whorl profiles (WWI = 0.70–0.80), and their shells are ornamented with lirae and/or low ribs. In contrast to the specimens of D. danckelmanni, only the adapertural end of the body chamber is uncoiled in D. tammikuense, while in D. gubkovense and D. spongistratum, the body chamber is not uncoiled at all. The specimens of D. paopense are larger in adult size (120 mm vs 100 mm) than D. danckelmanni, and the whorl profile appears less compressed (WWI 0.80–0.90 vs ~ 0.70) with a flattened venter; the body chamber uncoils later in ontogeny. Geographic and stratigraphic occurrence Northern Germany and northern Estonia; early Katian Stage, Late Ordovician.