Diestoceras stensioei ( Troedsson, 1926 )

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 : 67-68

Diestoceras stensioei ( Troedsson, 1926)

Fig. 27L, N

Cyrtogomphoceras stensioei Troedsson, 1926: 108–109 , pl. 63 fig. 5, pl. 64 figs 1–2.

Diestoceras stoermeri Strand, 1934: 85 (partim).

Cyrtogomphoceras stensioei – Foerste 1929: 231.

Diestoceras stensioei – Teichert 1930: 294, 296. ― Miller & Carrier 1942: 538. ― Flower 1946: 398. ― Balashov 1953a: 208. ― Stumbur 1956: 176. ― Flower in Flower & Teichert 1957: 65. ― Zhuravlyeva 1962: pl. 26 fig. 6. ― Bolton 1977: 36.

Diagnosis

Diestoceras with slightly curved shell and compressed conch cross section; mature body chamber highest near its base with height of ca 80 mm, width of ca 65 mm and length of ca 30–50 mm; sutures straight, more narrowly spaced at venter. (Compiled from Troedsson 1926: 108–109.)

Material examined

ESTONIA • 5 specs; Vormsi Island , Hosholm shore (tower locality); Adila Formation, Pirgu Regional Stage; GIT 840-264 , GIT 840-249 - 1 , GIT 878-254 , GIT 878-258 , GIT 878-165 • 1 spec.; Piirsalu quarry; Moe Formation, Pirgu Regional Stage; GIT 426-987 • 4 specs; Vohilaid Island , Vohilaid shore (E); Adila Formation, Pirgu Regional Stage; GIT 878-253 , GIT 878-255 , GIT 878-259 , GIT 878-262 .

Type locality and horizon

Rannaküla old quarry, near Haapsalu, western Estonia; Adila Formation, Pirgu Regional Stage.

Description

The most complete specimen is GIT 840-264 ( Fig. 15N ), which preserves parts of the mature body chamber and of the phragmocone. The complete cross section is not preserved but the conch height can be measured throughout the preserved parts of the conch. The maximum conch height of 74 mm is located at the base of the body chamber. Toward the peristome the conch height decreases to 58 mm at the peristome. The peristome is simple with a very shallow (ca 5 mm) hyponomic sinus. The outline of the mature body chamber in lateral view is convex. The cross section apparently circular or slightly compressed. At the base of the body chamber traces of a thin band of oncomyarian muscle scars are preserved; the band is ca 5 mm wide. The phragmocone has a convex outline in lateral view and decreases in diameter from 74 mm to 56 mm in 31 mm (angle of expansion = 32°). The septa are shallowly curved, 6 mm apart where the conch height is 68 mm (RCL = 0.09). No traces of the septal foramen or siphuncle are preserved.

The phragmocone is also well-preserved in GIT 878-259. It has a circular cross section and increases in diameter from 36 mm to 48 mm in a distance of 11 mm (angle of expansion = 43°). GIT 878-253, GIT 878-259, and GIT 878-262 preserve the circular conch cross section.

Remarks

This is a large Diestoceras . Other species known from Norway and Sweden reach mature diameters of ca 55 mm or less ( Strand 1934; Frye 1987). Diestoceras breviconum ( Portlock, 1843) reaches a smaller adult size (<35 mm), and has a wider angle of expansion (38°).

In his description of Diestoceras stoermeri Strand, 1934 , the author describes a specimen with a mature body chamber diameter of 75 mm as paratype ( Strand 1934: 85). The holotype of this species has a conch diameter of only 50 mm at the base of the mature body chamber. The complete original diagnosis given for D. stoermeri is very brief and without explicitly mentioning the adult size: “Short rounded Diestoceras with circular cross section and low camerae” ( Strand 1934: 85). Hence, based on the size of the holotype D. stoermeri has an adult diameter of ca 50 mm. Consequently, the paratype must be assigned to another, larger, species, which is most probably D. stensioei . Therefore, it can be assumed that this species occurs in the uppermost Katian strata (“Gastropod Limestone”) of Norway.

The holotype of C. stensioei differs slightly from the specimens described herein. The height of the mature body chamber of the holotype is 79 mm (compared with 74 mm in specimen GIT 840-264) and it has a compressed conch cross section (CHI = 1.22). However, Troedsson (1926) describes only the mature body chamber and not the conch cross section of the phragmocone and its intraspecific variability. Moreover, depending on the orientation of the specimens in the sediment and on the specific sediment stratum, various degrees of taphonomic compaction occur among the specimens. The deviations of the material described herein from the holotype are therefore interpreted as within the range of the intraspecific variability or as a result of taphonomic processes.

Figures

Fig. 15. Pseudorthoceratidae Flower & Caster, 1935 of the Vormsi–Pirgu regional stages, Estonia. A–C. Ephippiorthoceras vormsiense sp. nov. from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage. A. Specimen GIT 878-192, lateral view. B–C. Specimen GIT 878-228. B. Lateral view. C. Ventral view. D. Isorthoceras saaremense (Balashov, 1959) comb. nov., specimen GIT 878-41, from Saxby shore (N), Vormsi Island, Vormsi Regional Stage. Scale bar = 10 mm in all figures, same scale in A–C.