Discoceras polonicum sp. nov.

urn:lsid:zoobank.org:act: ADC2FF6A-2F7C-48AD-9147-99C6A6D76B02

Figs 23, 27; Table 16

Diagnosis

Species of the genus Discoceras with adult conch diameter of ~ 100 mm where the last 50 degrees are uncoiled (diameter of coiled part ~ 70 mm); conch extremely discoidal (CWI ~ 0.30) and evolute (UWI up to 0.45); whorls not imprinted dorsally (IZR close to zero); whorl profile in the last whorl elliptic and weakly compressed (WWI ~0.90); whorl expansion rate decreases to WER ~ 2.20); shell ornament of imbricated lirae and evenly distributed thin ribs.

Etymology

From the Latin ‘polonicum’ (adj., neut.) = ‘polish’, referring to the origin of the holotype.

Type material

Holotype POLAND – West Pomerania • Ustronie Morskie; Moldåan Regional Stage (early Katian Stage, Late Ordovician); Müldner Coll.; illustrated in Fig. 27; MB.C.11992.

Description

Holotype MB.C.11992 is a plaster cast of a nearly complete conch in the adult growth stage. The conch has a maximum diameter of 100 mm and consists of 3.25 volutions (Fig. 27). The last 50 degrees of the external volution are widely uncoiled (diameter of the coiled part is 73 mm). In the last 0.75 whorl, the conch is extremely discoidal (CWI decreases from 0.32 to 0.29), more evolute (UWI increases from 0.41 to 0.45), and less expanded (WER decreases from 2.42 to 2.15). The whorl profile is weakly compressed (WWI around 0.90), elliptic, and barely imprinted dorsally (IZR close to zero). The shell ornament consists of imbricated lirae and evenly distributed thin ribs.

Remarks

The discoidal, compressed and adaperturally uncoiled conch of the holotype of Discoceras polonicum sp. nov. is similar to the holotypes of D. sovaki sp. nov. and D. alienum sp. nov. However, both D. sovaki and D. alienum are smaller in the adult stage (100 mm vs 70 mm and 55 mm, respectively), less compressed (WWI = 0.92 vs 0.86 and 0.77, respectively, at maximum conch diameter), and more expanded (WER wh = 2.15 vs 2.42 and 2.61, respectively, at maximum conch diameter). Additionally, D. alienum has a finer shell ornamentation than D. polonicum, and the body chamber does not uncoil as strongly in the former.

The holotype of D. polonicum sp. nov. (MB.C.11992) is also somewhat similar to the holotype of D. brandenburgense sp. nov. (MB.C.32324). The similarities include umbilical width, conch width, expansion rate, and the wide uncoiling of the last volution. However, the whorl profile in MB.C.32324 is not compressed but equidimensional or weakly depressed (WWI = 1.03–1.09), and the last 90 degrees of the conch expand much less rapidly (WER = 2.04–1.97 vs 2.27– 2.15 in D. polonicum).

Geographic and stratigraphic occurrence

Northern Poland; early Katian Stage, Late Ordovician.