Discoceras paopense, Kröger, 2025

Kröger, Björn, 2025, The Lyckholm acme of cephalopods - Review of the late Katian (Vormsi-Pirgu regional stages) Ordovician cephalopods of Estonia, European Journal of Taxonomy 978, pp. 1-169 : 132-136

Discoceras paopense sp. nov.

urn:lsid:zoobank.org:act:22516E44-23DC-42BD-9D93-444068FBC3F0

Figs 47A–B , 48A , 49A , 51C

Diagnosis

Discoceras with large adult conch diameters of more than 120 mm, adult WER ca 1.7–2, decreasing with increasing conch size; whorl cross section slightly compressed with WWI 0.8–0.9 in mature stages, with flattened venter not wider than dorsum; free whorl in mature growth stages; ornamented with irregularly spaced costae and distinct shallow ribs, ca three to four costae occur between two ribs, the ribs and the costae run obliquely across the umbilical margin and form a deep U-shaped hyponomic sinus.

Etymology

Refers to the type locality.

Type material

Holotype

ESTONIA • Paope quarry; Kõrgessaare Formation, Vormsi Regional Stage; TUG 2-719 .

Paratypes

ESTONIA • 3 specs; same data as for holotype; GIT 426-114 , GIT 426-35 , GIT 426-373 • 1 spec.; sine loco; Kõrgessaare Formation , Vormsi Regional Stage; TUG 2-730 • 1 spec.; Vormsi Island , Saxby shore; Kõrgessaare Formation Vormsi Regional Stage; GIT 426-160 .

Description

The holotype, TUG 2-719, consists of ⅔ of a mold of a whorl with a conch diameter of 88 mm of which only the apical 33 mm of the phragmocone are preserved in 3D ( Fig. 47B ). The other parts are strongly flattened, without inner septa and siphuncle preserved. The whorl height at the maximum diameter is 28 mm. At a whorl width of 19 mm, the corresponding whorl height is 19 mm. There, the conch cross section is rounded, subquadratic. At a whorl width of 19 mm, the septal distance is 2–3 mm. The conch surface is ornamented with shallow annulations in a distance of 3–4 mm at the venter combined with distinct striae, which run parallel to the annuli with ca 4–5 striae per cycle of annulations. The annuli and striae form a deep and broadly rounded hyponomic sinus (in the inner whorls of the specimen ca 5 mm deep) and exhibit an apically deflected, curved path over the flanks.

The largest and most complete specimen is TU 2-730 ( Fig. 47A ). This specimen is a mold of a nearly mature individual with a maximum diameter of 119 mm. The preserved part of the body chamber is 105 mm long and spans nearly 45°, it is detached from the rest of the conch at a diameter of ca 114 mm. The whorl cross section of the adoralmost part of the body chamber has a width of ca 28 mm and a height of 32 mm, at the base of the body chamber the width is 25 mm and the height 28 mm, its umbilical flanks are rounded with the greatest width near the dorsum, the venter is flattened with the flattened part narrower than the umbilical margin. The cross section in a juvenile part of the conch is rounded with a width and height of 14 mm.

The septal distance at the base of the body chamber is 4–5 mm. The sutures form shallow, wide lobes on the umbilical flanks and on the venter and a pointed saddle at the ventrolateral shoulders.

The ornamentation consists of shallow, irregularly spaced, but distinct annuli (at the base of the body chamber ca three occur at a length of 10 mm). Between the annuli, ca two to four costae are present. The ornamentation runs obliquely across the umbilical margins of the whorl and forms a deep broad hyponomic sinus.

Specimen GIT 426-114 is a 75 mm long fragment of a mold of a body chamber with a rounded subquadratic cross section with a whorl width of 30 mm and a whorl height of 26 mm. The specimen is ornamented with annuli (ca 3 in a distance of 10 mm) which run obliquely, curved at the flanks adapically and form a deep and U-shaped hyponomic sinus (> 10 mm deep).

Specimen GIT 426-351 is a well-preserved mold of a phragmocone with a rounded subquadratic to trapezoidal cross section with greatest whorl width of 23 mm near the flattened venter and with a whorl height of 20 mm. The ornamentation is identical to that of specimen GIT 426-114. The siphuncular foramen is located at the dorsal margin of the septum and has a diameter of 4.5 mm (RSH = 0.23).

Specimen GIT 426-373 is a fragment of a phragmocone with a rounded subquadratical conch cross section with whorl width 13 mm and corresponding whorl height of 11 mm. The septal distance is 2.5 mm and the siphuncular foramen is located at the dorsal margin with a diameter of 2 mm (RSH = 0.18). The specimen is ornamented with distinct annuli, which run obliquely over the flanks and form a deep U-shaped hyponomic sinus, ca 4 annuli occur per 10 mm.

Remarks

The most complete specimen (TUG 2-230) of this new species cannot serve as a holotype, because it is from an unknown locality (although its stratigraphy, Vormsi Regional Stage, is stated on the label, which is coherent with the distinct Kõrgessaare Formation lithology of the specimen).

Comparison

The new species differs from D. antiquissimum in ornamentation. In contrast to the latter, the path of the ornament follows a broad curve obliquely across the umbilical margin of the whorl, forming a deep U-shaped hyponomic sinus. It consists of a succession of alternating strong annuli and weaker costae (ca two to three) in between two ribs. The ornamentation is more similar to that of D. boreale Sweet, 1958 , and D. arcuatum ( Lossen, 1860) . Discoceras paopense sp. nov. differs from both these species, in having a whorl cross section of the aspect of D. roemeri with a flattened venter. The relatively low WWI of D. paopense sp. nov. is similar to that of D. rarospira ( Eichwald, 1860) , but the latter species differs in having a more rounded whorl cross section and a siphuncle that is further from the dorsal conch margin.

Probably, the specimen assigned to D. antiquissimum , and figured in Dzik (1984: pl. 7.1.) from erratic boulders of Poland (collection of Roemer 1861) belongs to D. paopense sp. nov. because it has a similar ornamentation. This specimen, however, differs in having a (taphonomically?) depressed whorl cross section.

Figures

Fig. 47. Annulated species of Discoceras Barrande, 1867 of the Vormsi–Pirgu regional stages, Estonia. A–B. Discoceras paopense sp. nov. from Vormsi Regional Stage. A. Paratype TUG 2-730, from unknown locality, Estonia. B. Holotype TUG 2-719, Paope limestone quarry, Vormsi Regional Stage. C–D. Discoceras antiquissimum (Eichwald, 1842) from Pirgu Regional Stage. C. Specimen GIT 878- 208, from Hosholm shore (tower), Vormsi Island. D. GIT 878-148, from Vohilaid (E), Vohilaid Island. Scale bar = 20 mm, same scale in all figures.

Fig. 48. Whorl expansion rate of species of Discoceras Barrande, 1867 of the Vormsi–Pirgu regional stages, Estonia. A. WER of D. antiquissimum (Eichwald, 1842) (= circles) and D. paopense sp. nov. (= black triangles). White circles = Estonian specimens described herein; black circles = specimens described in Strand (1934). B. WER of D roemeri Strand, 1934. White circles = Estonian specimens described herein; black circles = specimens described in Strand (1934), and in Balashov (1953b); black squares = specimens described as D. angulatum (Saemann, 1853) in Strand (1934); black triangles = specimens described as Schroederoceras balaschovi Stumbur, 1956 in Stumbur (1956). C. WER of D. saemanni (Hyatt, 1894). White circles = Estonian specimens described herein; black circles = specimens described in Strand (1934); black squares = specimens described as D. hyatti Strand, 1934 in Strand (1934).

Fig. 49. Whorl width index species of Discoceras Barrande, 1867 of the Vormsi–Pirgu regional stages, Estonia. A. WWI of D. antiquissimum (Eichwald, 1842) (= circles) and D. paopense sp. nov. (= black triangles). White circles = Estonian specimens described herein; black circles = specimens described in Strand (1934). B. WWI of D roemeri Strand, 1934. White circles = Estonian specimens described herein; black circles = specimens described in Strand (1934), and in Balashov (1953b); black squares = specimens described as D. angulatum (Saemann, 1853) in Strand (1934); black triangles = specimens described as Schroederoceras balaschovi Stumbur, 1956 in Stumbur (1956). C. WWI of D. saemanni (Hyatt, 1894). White circles = Estonian specimens described herein; black circles = specimens described in Strand (1934); black squares = specimens described as D. hyatti Strand, 1934 in Strand (1934).

Fig. 51. Species of Discoceras Barrande, 1867 of the Vormsi–Pirgu regional stages, Estonia. A. D. saemanni (Hyatt, 1894), 1934, specimen GIT 426-345, from Saxby shore, Vormsi Island, Vormsi Regional Stage. B. D. roemeri Strand, 1934, specimen TUG 39-711, from Niibi hillock, Pirgu Regional Stage. C. D. paopense sp. nov., paratype GIT 426-160, from Saxby shore, Vormsi Island, Vormsi Regional Stage. D. D. roemeri, specimen TUG 1723-27, from Vohilaid (E), Vohilaid Island, Pirgu Regional Stage. E. D. roemeri, specimen GIT 878-230, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage. Scale bar = 20 mm, same scale in all figures.