Discoceras saemanni ( Hyatt, 1894 )

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 : 141-144

Discoceras saemanni ( Hyatt, 1894)

Figs 48C , 49C , 50B , 51A , 53

Lituites angulatus Saemann, 1853: 166 , pl. 21 fig. 1c–d, (not 1a–b).

Trocholites undosus Foord, 1891: 45 .

Schroederoceras saemanni Hyatt, 1894: 462 .

Discoceras hyatti Strand, 1934: 38–39 , pl. 2 fig. 2, pl. 5 fig. 3a–b.

Discoceras sp. – Foerste 1929: 176, pl. 20 fig. 2a.

Discoceras saemanni – Strand 1934: 40–42, pl. 2 fig. 1a–b, pl. 4 fig. 1. ― Sweet 1958: 102, fig. 13n.

Discoceras hyatti – Sweet 1958: 102, fig. 13g. ― Dzik 1984: 44, text-fig. 12.43.

Schroederoceras hyatti – Stumbur 1959: 26, fig. 1v.

Schroederoceras saemanni – Stumbur 1962: 137.

Emended diagnosis

Discoceras with adult conch diameters of up to ca 125 mm; mature WER of ca 2.0; mature whorl cross section with WWI ca 1.1–1.2, 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 mature body chamber with constriction near aperture; ornamented with crenulated costae and distinct growth lines that form a U-shaped hyponomic sinus, low ribs can occur.

Material examined

ESTONIA • 1 spec.; sine loco; Vormsi Regional Stage; TAM G432:598 • 2 specs; same data as for preceding; TUG 1745-59 , TUG 666-298 • 3 specs; Hosholm shore ; Adila Formation , Pirgu Regional Stage; GIT 426-354 to GIT 426-356 • 1 spec.; Imastu old quarry ; Kõrgessaare Formation , Vormsi Regional Stage; TUG 42-366 • 1 spec.; Jootma ditch ; Moe Formation , Pirgu Regional Stage; GIT 426-1137 • 1 spec.; Kersleti ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-377 • 3 specs; Kõrgessaare quarry ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-361 , GIT 426-395 , GIT 426-429 368 • 1 spec.; Moe stratotype outcrop ; Moe Formation , Pirgu Regional Stage; TUG 42-367 • 1 spec.; Moe trench ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 878-107 • 2 specs; Niibi hillock ; Moe Formation , Pirgu Regional Stage; GIT 426-357 , GIT 426-358 • 3 specs; same data as for preceding; TAM G432:423 , TAM G432:424 , TUG 939-66 • 1 spec.; Oru manor ; Kõrgessaare Formation , Vormsi Regional Stage; TUG 2-715 • 1 spec.; Paluküla quarry ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-422 • 2 specs; same data as for preceding; TUG 47-880 , TUG 66-294 • 8 specs; Paope quarry ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-159 , GIT 426-352 , GIT 426-408 , GIT 426- 413 to GIT 426-416 , GIT 426-1074 , GIT 426-1075 • 1 spec.; Rannaküla old quarry ; Adila Formation , Pirgu Regional Stage; GIT 426-360 • 1 spec.; Saaremõisa (Lyckholm); Kõrgessaare Formation , Vormsi Regional Stage; TUG 72-342 • 1 spec.; Salu ; Pirgu Regional Stage; GIT 426-36377 • 20 specs; Vormsi Island, Saxby shore ; Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-107 , GIT 878-155 , GIT 426-109 , GIT 426-112 , GIT 426-342 , GIT 426-344 , GIT 426-345 , GIT 426-347 , GIT 878-155 , GIT 426-1071 to GIT 426-1073 , GIT 878-17 , GIT 878-22 , GIT 878-56 , GIT 878-64 , GIT 878-66 to GIT 878-69 • 12 specs; same data as for preceding; TUG 39-710 , TUG 44-40 , TUG 66-290 to TUG 66-292 , TUG 119-625 , TUG 666-296 , TUG 939-65 , TUG 1666-16 , TUG 1743-36 , TUG 1743-5 , TUG 1745-214 • 1 spec.; Sutlepa quarry ; Adila Formation , Pirgu Regional Stage; GIT 426-359 • 2 specs; same data as for preceding; TAM G432:502 , TAM G432:503 • 1 spec.; same data as for preceding; TUG 39-706 • 1 spec.; Tapa ; Kõrgessaare Formation , Vormsi Regional Stage; TUG 1745-22 .

Type locality and horizon

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

Description

The species has been described by Strand (1934) based on Saemannʼs (1853) specimen of Lituus angulatus and three additional specimens of D. saemanni , all of them have conch diameters of more than 100 mm, the largest has a mature diameter of 125 mm. The two specimens, described by Strand (1934) under D. hyatti , are smaller, with mature conch diameters of 84 mm and 95 mm, respectively. In three specimens of the Estonian material, the mature divergence of the body chamber is preserved; it occurs at a conch diameter of 74 mm in TUG 1745-214, and at 110 mm in GIT 426-425.

GIT 426-425 is a fragment of a mature specimen with the complete adult body chamber preserved ( Fig. 50B ). The diameter at the base of the body chamber is ca 100 mm. The body chamber is ca 100 mm long and has a trapezoidal cross section at its base. There, it is widest at the venter with a width of 34 mm and a height of 30 mm (WWI = 1.13). The adult aperture has a subquadratic, rounded cross section and a width of 32 mm. The specimen has a subquadratic conch cross section at a diameter of 33 mm with a conch width of 15 mm and corresponding conch height of 11 mm (WWI = 1.36). At a conch diameter of 51 mm, the conch cross section is rounded subquadratic with a flat venter and a maximum width at mid-flank of 18 mm, and with a corresponding height at 17 mm (WWI = 1.06).

A change from a subquadratic whorl cross section with maximum width at mid-flank to trapezoidal whorl cross section with its maximum width at the venter occurs at conch diameters of ca 40–50 mm and corresponding conch widths of 16–17 mm (e.g., TUG 1745-59, GIT 426-345, GIT 426-354, GIT 426-425, GIT 426-1072).

The ornamentation is well-preserved in specimens GIT 426-425 and 426-391, where it forms well-developed, irregularly spaced, crenulated costae or frills which are imbricated toward the aperture, and which form a wide U-shaped hyponomic sinus. The costae are more regularly spaced in juvenile portions (first one–two volutions), with a distance of ca 1 mm at the venter; they are less distinct in later growth stages and on the mature body chamber with a distance of ca 1–1.5 mm at the venter. In specimen GIT 426-425, a distinct band is apparent at mid-flank in the first two whorls, where the frills appear to be more elevated and broken off the remainder of the shell. On the initial ca two volutions of specimen GIT 878-67, the costae are developed more strongly at distances of 3–4 mm, forming low ribs. In specimen GIT 878-67, the mold preserves the ornamentation and the septa and shows that ca three costae occur in the distance between two successive septa on the second volution. Identical features of the ornamentation are best preserved and figured, in specimens assigned herein to Discoceras sp. ( Fig. 52A–C ).

The features of the siphuncle and septal neck are well-preserved in specimen GIT 426-107. The siphuncle is located at the dorsal margin of the whorl throughout its preserved length (conch diameter 41–92 mm). At a conch diameter of 92 mm, the whorl height is 24 mm, and the diameter of the septal foramen is 4 mm (RSH = 0.17), and the septal distance is 4.5 mm. At a position with whorl height 19 mm, the septal foramen is 2.5 mm in diameter (RSH = 0.13); the septal distance is 4 mm. The septal neck is straight, suborthochoanitic and 1.5 mm long. The connecting ring is 0.7 mm thick, reaching its greatest thickness near the adapical part of the segment.

Remarks

The emendation herein regards the variability of the adult size (given as ca 120 mm in Strand 1934), and the WWI (given as 1.2–1.5 in Strand 1934). Because of the high ontogenetic variability of this character, the range of the mature WWI rather than its total range throughout the growth of the conch (as used by Strand 1934) is included in the diagnosis. The maximum adult size is taken from the largest of the types described by Strand (1934: 41) under this species.

Strand (1934) distinguished D. saemanni from D. hyatti based on the whorl cross section (wider in the former) and by the expansion rate (lower in the latter). However, a comparison of the WWI and WER of the types of Strand (1934) reveals that these differences are well within the variability of the Estonian sample (see Fig. 48C ). The combined samples of D. saemanni and D. hyatti from Norway (i.e., the Strand 1934 types) and from Estonia differ in WWI but not in WER (see Table 2 ). However, the variation between specimens appears to form a continuum between the extremes. Therefore, a distinction between two species based on expansion rate and whorl shape is practically impossible with the available material.

Additionally, the ontogenetic trends between the types of D. saemanni and D. hyatti differ, their WWI decreases with conch size in the former, and it remains relatively constant in the latter (see, e.g., Strand 1934: pl. 2 figs 1–2; Fig. 49C ). However, the Estonian material contains specimens with decreasing, increasing, and nearly constant WWI trends, and specimens in which WWI initially increases and later decreases. In total, these trends result in a decreasing WWI–conch size relation ( Fig. 49C ) and it is impossible to distinguish morphologically well-delimited groups among them.

Strand (1934) also mentioned the ornamentation as a feature that distinguishes D. saemanni from D. hyatti , but it remains unclear what exactly these differences are. In both species, he described similar distinct costae and growth lines and the occurrence of low ribs during juvenile growth stages. In the Estonian material, it was not possible to distinguish two or more groups based on differences in ornamentation. Consequently, D. hyatti is interpreted as a junior synonym of D. saemanni herein.

Comparison

Discoceras saemanni differs from D. roemeri and D. angulatum in having a whorl cross section with greatest width at the venter. In D. roemeri and D. angulatum , the greatest width occurs at a position of the umbilical shoulders but not at the venter. In D. roemeri and D. angulatum , the umbilical shoulders are generally more rounded throughout their conch lengths.

In several specimens of D. saemanni , the whorl cross section changes during ontogeny from rounded, subquadratic or depressed reniform in early growth stages to trapezoidal with greatest width at the venter during later ontogeny ( Fig. 53 ). This change occurs at diameters of smaller than ca 50 mm, and at whorl widths smaller than 17 mm. It is therefore impossible to distinguish between D. saemanni , D. roemeri , and D. angulatum in specimens with diameters below ca 50 mm, and with whorl widths below 17 mm when the whorl cross section is not unambiguously trapezoidal. However, in several specimens assigned to Discoceras saemanni (including a specimen figured in Strand 1934: pl. 2 fig. 1a), a trapezoidal whorl cross section with its greatest width at venter appears already from the first volution onward, making such a distinction possible. As a general ontogenetic trend, a change from a more rounded toward a more flattened venter occurs in all specimens ( Fig. 53 ). In mature specimens, however, the aperture, again, is rounded and has no flattened venter. A considerable intraspecific and ontogenetic variation occurs also in ornamentation of D. saemanni , with at least one specimen (GIT 426-1072), showing an annulation on the inner two whorls.

The embryonic shells of specimens assigned to D. hyatti and D. antiquissimum have been described by Stumbur (1959). In both species, the first whorl is tightly coiled, leaving no or only a very narrow umbilical window and comprising five ( D. hyatti ) to seven ( D. antiquissimum ) chambers. The diameters of the embryonal shell are 10.1 mm for D. hyatti , and 14.7 mm for D. antiquissimum ( Stumbur 1959) . In both species the siphuncle starts near the venter in the first chamber, perforates the first septum slightly ventral from the whorl center and reaches a nearly dorsal position at the 3 rd to 4 th septum ( Stumbur 1959).

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. 50. Early growth stages of species of Discoceras Barrande, 1867 of the Pirgu–Porkuni regional stages, Estonia. A. D. antiquissimum (Eichwald, 1842), specimen GIT 425-375, from Haapsalu holm, Pirgu Regional Stage. B. D. saemanni (Hyatt, 1894), specimen GIT 426-425, from Röa-Jakobi quarry, Porkuni Regional Stage. Scale bars = 10 mm.

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. 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).

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.