Discoceras roemeri Strand, 1934

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 : 136-140

Discoceras roemeri Strand, 1934

Figs 48B , 49B , 51D, E , 52E , 53

Discoceras roemeri Strand, 1934: 38 , 42–43, pl. 2 figs 6a–b, 7, pl. 5 figs 1–2.

Schroederoceras balaschovi Stumbur, 1956: 181 , pl. 1 figs 5–6.

Lituites antiquissimus – Roemer 1861: 62, pl. 6 fig. 2a–e (non pl. 6 fig. 2f–g = Discoceras antiquissimum ( Eichwald, 1840)) . Schroederoceras angulatum – Teichert 1930: 276. ― Balashov 1953b: 263, pl. 13 figs 1a–b, 2a–b. Discoceras angulatum – Mutvei 1957: text-fig. 12, pl. 8.

Discoceras roemeri – Sweet 1958: 99, 102, text-fig. 13p. ― Dzik 1984: 41, 44, text-figs 10, 12.43, pl. 6 figs 5–6.

Schroederoceras roemeri – Stumbur 1962: 136, text-fig. 2.4.

Rectanguloceras balaschovi – Stumbur 1962: 142, text-fig. 2.9.

Rectanguloceras (Discoceras) cf. roemeri – Neben & Krueger 1973: pl. 65 figs 1–2.

Diagnosis

Discoceras with large adult conch diameters of> 140 mm, WER ca 1.9–2.5, decreasing with increasing conch size; whorl cross section slightly depressed with WWI ca 1.1–1.3, rounded, subquadratic with venter not wider than dorsum; body chamber nearly 180° long, free in mature growth stages and becoming markedly higher during mature stages; ornamented with imbricated lamellae, irregularly spaced costae and shallow ribs. (From Kröger 2013.)

Material examined

ESTONIA • 1 spec.; Aulepa quarry ; Kõrgessaare Formation , Vormsi Regional Stage; TUG 895-149 • 1 spec.; sine loco; Vormsi – Pirgu regional stages; GIT 426-379 • 1 spec.; same data as for preceding; TAM G432:597 • 1 spec.; Förby shore ; Pirgu Regional Stage; TUG 1745-175 • 1 spec.; Haapsalu holm ; Adila Formation , Pirgu Regional Stage; TUG 939-74 • 1 spec.; Vormsi Island, Hosholm shore ; Adila Formation , Pirgu Regional Stage; GIT 840-250 • 1 spec.; same data as for preceding; TUG 119-624 • 1 spec.; Vormsi Island, Hosholm shore (tower locality); Adila Formation , Pirgu Regional Stage; GIT 878-230 • 8 specs; Jootma ditch; Moe Formation , Pirgu Regional Stage; GIT 426-368 , GIT 426-369 , GIT 426-385 , GIT 426-403 to GIT 426-405 , GIT 426-441 , GIT 426-442 • 1 spec.; Kernu quarry ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-380 • 1 spec.; Kohila ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-424 • 1 spec.; Kolu quarry ; Pirgu Regional Stage; GIT 426-364 • 4 specs; Kõrgessaare quarry ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-396 , GIT 426- 400 to GIT 426-402 • 1 spec.; same data as for preceding; TUG 895-68 • 1 spec.; Küti, near ViruJaagupi ; Kõrgessaare Formation , Vormsi Regional Stage; TUG 1745-324 • 1 spec.; Lehtse ; Kõrgessaare Formation , Vormsi Regional Stage; TAM G432:593 • 1 spec.; same data as for preceding; TUG 42-368 • 1 spec.; Moe , Pirgu Regional Stage; GIT 426-366 • 1 spec.; Moe trench ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 878-106 • 2 specs; Niibi hillock; Moe Formation , Pirgu Regional Stage; TUG 39-711 , TUG 42-290 • 1 spec.; Odulemma old quarry ; Nabala Regional Stage; GIT 426-365 • 5 specs; Paluküla quarry ; Kõrgessaare Formation , Vormsi Regional; Stage; GIT 426-419 , GIT 426-420 , GIT 426-434 , GIT 426-435 , GIT 878-135 • 1 spec.; same data as for preceding; TUG 2-732 • 11 specs; Paope quarry ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-1076 to 426-1078 , GIT 426- 349 , GIT 426-350 , GIT 426-353 , GIT 426-406 , GIT 426-407 , GIT 426-410 , GIT 426-415 , GIT 426-417 • 1 spec.; same data as for preceding; TUG 1745-206 • 1 spec.; Röa-Jakobi quarry ; Ärina Formation , Porkuni Regional Stage; GIT 426-425 • 1 spec.; Saaremõisa (Lyckholm); Kõrgessaare Formation , Vormsi Regional Stage; GIT 878-122 • 1 spec.; same data as for preceding; TAM G432:680 • 1 spec.; same data as for preceding; TUG 1745-172 • 1 spec.; Saksi manor ; Pirgu Regional Stage; TUG 2-284 • 1 spec.; Salu ; Pirgu Regional Stage; GIT 426-362 • 1 spec.; same data as for preceding; TUG 939- 77 • 3 specs; Vormsi Island, Saxby shore ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426- 113 , GIT 426-343 , GIT 426-348 • 1 spec.; same data as for preceding; TUG 44-41 • 8 specs; Vormsi Island, Saxby shore (N); Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-1068 , GIT 426-1069 , GIT 878-24 , GIT 878-29 , GIT 878-38 , GIT 878-48 , GIT 878-52 , GIT 878-73 • 1 spec.; Sutlema old quarry ; Kõrgessaare Formation , Vormsi Regional Stage; TUG 1827-164 • 1 spec.; Sutlepa quarry ; Adila Formation , Pirgu Regional Stage; TUG 2-651 • 1 spec.; Tapa ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 878-98 • 2 specs; Vohilaid Island, Vohilaid shore (E); Adila Formation , Pirgu Regional Stage; GIT 878-202 , 2 GIT 878-203 • 1 spec.; same data as for preceding; TUG 1723-27 .

Type locality and horizon

Herøya, near Porsgrunn, Norway; Herøya Formation (“Gastropod limestone”), latest Katian Stage ( Strand 1934).

Description

Specimen GIT 878-230 is an exceptionally well-preserved, complete mature specimen ( Figs 51E , 52E ). This specimen has a conch diameter of 123 mm, the base of the body chamber is at a conch diameter of 105 mm. The body chamber is 107 mm long and extends around ca ¼ of a volution. The body chamber diverges from the rest of the conch at ca 112 mm. The sutures form wide lobes over the umbilical margin and are nearly straight at the venter. They are ventrally 10 mm apart where the corresponding conch diameter is 105 mm. At the aperture, the whorl is slightly constricted and forms wide U-shaped hyponomic sinus. At the aperture, the whorl cross section is subquadratic with a rounded venter, the whorl cross section has a distinctively flattened venter at the base of the body chamber. There, the whorl width, and height are 33 mm, and the umbilical margins are slightly rounded with their greatest width near the mid-flank. At a conch diameter of 69 mm, the whorl cross section has a width of 29 mm and a height of 27 mm, and the venter is distinctively flattened. There, the umbilical margins are rounded with greatest width at mid-flanks. The path of the ornament follows a broad U-shaped hyponomic sinus on the venter ( Fig. 52E ). The costae are irregularly spaced: at a conch diameter of 80 mm, ca six to seven occur at a length of 10 mm at the venter. On the first three whorls at ca mid-flank position, a 2–3 mm wide band is present in which the lamellae are slightly more pronounced and thicker. Ventrally of that band, the lamellae generally appear to be slightly longer, creating a stronger relief. On the external cast of this specimen, parts of the shell are attached at the position of the longitudinal band and ventrally, giving the false impression of a longitudinal color banding (see also Fig. 52C ).

Four specimens are preserved with the mature divergence of the body chamber; in one of them the divergence occurs at a conch diameter 93 mm (TUG 1745-175); in the others it occurs at diameters 118 mm (GIT 878-230), 124 mm (TUG 1723-27) ( Fig. 51D ), and 125 mm (TUG 2-651). The only complete adult specimens in the collection are GIT 878-230, and TUG 1723-27 with adult diameters of 124 mm and 128 mm, respectively.

Remarks

The specimens described by Balashov (1953b) under Schroederoceras roemeri , represent a different species. Based on similarities in ornamentation and siphuncular position, they are probably related to D. boreale Sweet, 1958 .

The finely transversely lamellate or costate conch surface ( Fig. 52 ) is not preserved in most specimens because it is often firmly attached to the external sediment. During natural weathering or artificial preparation, the shell splits from the smooth, inner cast of the specimen and the surface of its external side remains invisible. In few, exceptionally preserved specimens, a peculiar ornamentation is visible: in superficial view it is longitudinal banded, giving the impression of a color band at approximately mid-flank position (compare, e.g., Kröger & Aubrechtová 2019: fig. 5). The apparent banding, however, is produced by subtle differences in ornamentation between the dorsal part of the flank and the ventral part of the flank and the venter: the frills or crenulate lamellae are deeper and more pronounced at the ventral section of the flank.

Comparison

The holotype of D. roemeri is a large, mature specimen (conch diameter ca 140 mm, Strand 1934: 45). In comparison, the holotype of D. angulatum , which also is a complete mature specimen and the only type specimen of this species (see Strand 1934), is small (diameter ca 90 mm, Strand 1934: 37). The shape of the whorl cross section of D. roemeri is described as “rounded quadrangular” ( Strand 1934: 43), whilst that of D. angulatum was described as “subquadrangular […] with the venter narrower than the dorsum” ( Strand 1934: 37). Additionally, the length of the mature body chamber differs between the types of the two species: in D. roemeri it is ca ½ of a volution, whilst it is only ¼ of a volution in D. angulatum . The types of D. angulatum and D. roemeri differ also in WWI, with D. angulatum having a smaller WWI at a comparable conch size ( Fig. 49B ; supplementary data 5). Despite these differences in the types, it is impossible to distinguish between the two species in the Estonian material.

The WWIs of the Estonian specimens assigned to D. roemeri , herein, are generally lower than of the Norwegian types of this species at comparable growth stages, and are more similar to that of the type of D. angulatum ( Fig. 49B ). However, several of the measured specimens in the Estonian collections are also larger than 90 mm in conch diameter, and therefore could only be placed within D. angulatum after emending the diagnosis of this species and accepting larger adult conch diameters. To distinguish different species based on WWI is not possible, as no sharp boundaries exist between the two groups.

Distinction between the two species based on the whorl cross sections is also not possible in the Estonian material. Many interspecific and ontogenetic variations of D. angulatum -like and D. roemeri -like whorl cross section shapes occur in the Estonian material, with a flattened, slightly rounded venter, and subquadratic to broadly reniform shapes ( Fig. 53 ; also compare Strand 1934: pl. 2 figs 3, 5–7). This is also the case in a specimen figured by Kröger (2013: fig. 32a) assigned to D. roemeri , where the premature whorl cross section is almost identical to that of the adult D. angulatum . It is therefore impossible to distinguish between D. angulatum and D. roemeri in premature specimens with conch diameters <90 mm.

Furthermore, the available material does not permit distinctions between two or more size classes of maturity. Among the seven type specimens of D. roemeri described by Strand (1934), four have a conch diameter of more than 130 mm. The Estonian material contains four mature specimens belonging to the D. angulatum – D. roemeri group, three of them have mature conch diameters of more than 120 mm (see above). An additional specimen with a mature conch diameter of ca 100 mm is known from Balashov 1953b: pl. 13 fig. 1a–b, described under Schroederoceras angulatum . With these data it is not possible to distinguish between clearly constrained size-groups of D. roemeri and D. angulatum .

Too little is known about the intraspecific and ontogenetic variability of the length of the body chamber. Significant differences exist between specimens described, herein (compare Fig. 51D–E ). However, the low number of specimens with complete body chambers available does not permit the detection of sets of features covarying with the body chamber length. Future work (that includes the Norwegian material) may show whether D. roemeri is a subjective junior synonym of D. angulatum or if both species represent distinct groups. Therefore, for practicality, herein, we assign specimens with a D. angulatum – D. roemeri aspect to the latter and reserve D. angulatum to specimens with small adult size and an adult conch cross section identical to the type of this species.

Discoceras roemeri differs from D. saemanni , which also has a distinctively flattened venter, in having a whorl cross section with greatest width between the venter and the dorsum, but not strictly at the venter. Throughout the entire length of the conch, the umbilical margins are generally more rounded in D. roemeri compared to D. saemanni . The average WER of mature D. roemeri is ca 1.8–1.9; slightly lower than that of mature D. saemanni (WER ca 2.0) (compare Fig. 48B–C for ontogenetic trends; supplementary data 5).

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.

Fig. 52. Details of the ornamentation of species of Discoceras Barrande, 1867 of the Vormsi–Pirgu regional stages, Estonia. A–D. Discoceras sp. A. Specimen TUG 42-371, from Kohila, Vormsi Regional Stage. B. Specimen TUG 1745-212, from Saxby shore, Vormsi Island, Vormsi Regional Stage. C. Specimen TUG 1745-203, from Paope quarry, Vormsi Regional Stage. D. Specimen TUG 2-726, from Saaremõisa (Lyckholm), Vormsi Regional Stage. E. D. roemeri Strand, 1934, specimen GIT 878- 230, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage. Arrows mark areas in which the partly imbricated strong crenulation is well-preserved and visible. Scale bars = 5 mm in all figures.

Fig. 53. Camera lucida drawings of whorl cross-sections of specimens assigned to Discoceras roemeri Strand, 1934 or Discoceras saemanni (Hyatt, 1894) of the Vormsi–Pirgu regional stages, Estonia. Note the high ontogenetic and interspecific variability, and the taphonomic distortion among the specimens. Numbers refer to specimens: 1. GIT 426-348. 2. GIT 840-250. 3. GIT 878-68. 4. TUG 1745-175. 5. GIT 426-425. 6. GIT 426-109. 7. GIT 426-357. 8. GIT 426-365. 9. GIT 426-364. 10. GIT 426-1074. 11. GIT 426-343. 12. TUG 42-290. 13. TUG 939-65. 14. TUG 72-342. 15. TUG 1745-59. 16. GIT 878-69. 17. GIT 426-358. 18. GIT 878-56. 19. TAM G432:598. 20. GIT 426-361. 21. GIT 426-355. 22. TUG 42-367. 23. TAM G432:503. 24. GIT 426-354. 25. TUG 939-66. 26. GIT 426-107. 27. GIT 426-363 (see file 1 for specimen details).