Isorthoceras saaremense Kröger 2025
Creators: name: Kröger, Björn
Publisher: name: Zenodo
Year: 2025
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Cephalopoda
Orthocerida
Proteoceratidae
Isorthoceras
Isorthoceras saaremense

description: Isorthoceras saaremense (Balashov, 1959) comb. nov.
Figs 15D, 19D, G
Hedstroemoceras saaremense Balashov, 1959: 42, pl. 5 fig. 11.
Hedstroemoceras saaremense – Balashov & Zhuravlyeva 1962: pl. 13 fig. 2; 1974: pl. 13 fig. 2.
Emended diagnosis
Isorthoceras with circular conch cross section; smooth shell surface; weakly curved growth axis with siphuncle slightly eccentric (RSP = 0.29) on concave side of curvature; angle of expansion of up to ca 12°; mature body chamber with maximum diameter more than 40 mm; siphuncular segments pyriform to fusiform; with weakly developed endosiphuncular deposits; septal necks short, suborthochoanitic to orthochoanitic.
Material examined
ESTONIA • 1 spec.; Kersleti quarry; Kõrgessaare Formation, Vormsi Regional Stage; TUG 1745-305 • 1 spec.; Paluküla quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 426-570 • 1 spec.; Paope quarry; Kõrgessaare Formation, Vormsi Regional Stage; TUG 939-2 • 1 spec.; Salutaguse quarry; Moe Formation, Pirgu Regional Stage; GIT 878-138 • 4 specs; Vormsi Island, Saxby shore; Kõrgessaare Formation, Vormsi Regional Stage; TUG 1743-4, TUG 1745-223, TUG 1745-246, TUG 1745-259 • 4 specs; Vormsi Island, Saxby shore (N); Kõrgessaare Formation, Vormsi Regional Stage; GIT 426- 1133, GIT 878-40, GIT 878-41, GIT 878-20 • 1 spec.; Sutlema quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-128.
Type locality and horizon
Tapa, Estonia; Vormsi–Pirgu regional stages.
Description
Specimen GIT 878-41 is a 155 mm long fragment of part of a body chamber and phragmocone with a diameter of 19–39 mm (angle of expansion = 7.4°). The preserved part of the body chamber is 61 mm long with a conch diameter of 33 mm at its base. The conch cross section is circular or slightly compressed. The conch is slightly curved. Where preserved, the conch surface is smooth. The sutures are directly transverse and 5.5 mm distant at a corresponding conch diameter of 32 mm and 4.5 mm at a conch diameter of 24 mm. The septal foramen is located 10 mm from the margin of the conch at the convex side of the conch curvature, and has a diameter of 2.3 mm where the conch height is 31 mm.
The complete set of measurements available reveals a decreasing angle of expansion from 12° at corresponding conch diameters of ca 15 mm toward 9°–4° in more mature specimens (Fig. 20A). The relative cameral length decreases.from ca 0.25 to less than 0.15 (mean RCL = 0.21, n = 10).
The details of the internal characters are preserved in specimens GIT 878-40, GIT 878-20, TUG 939-2, TUG 1745-246, 1745-259, and TUG 1745-305, These represent fragments of the phragmocone with diameters between 19–37 mm (Fig. 19D, G). In all specimens the shape of the siphuncle is slightly expanded, barrel-shaped with greatest widths at ca mid-length of the siphuncular segments, and the septal necks are short orthochoanitic to suborthochoanitic. The position of the siphuncle varies between RSP 0.2–0.3 with no apparent ontogenetic trend (mean RSP = 0.3, n= 7).
Remarks
This species was originally classified within the Middle Ordovician genus Hedstroemoceras Foerste, 1930a, without explicit justification, probably based on its eccentric siphuncle. It has been originally described from a single poorly preserved small fragment of a phragmocone. The outer shell in the holotype is not preserved. The specimen can be identified, based on its siphuncular position and septal neck features, as an Isorthoceras. It has an eccentric siphuncle position (RSP = 0.27) at a conch diameter of 44 mm, and an apparently a very low angle of expansion. In the context of the additional material, the holotype can be interpreted as an adoral fragment of a phragmocone of a near mature specimen with a low angle of expansion, and, in contrast to I. luhai, an eccentric siphuncle during late ontogeny.
Comparison
This species is in almost all aspects like I. wahlenbergi from the Boda Limestone, Late Katian, Sweden. It differs from the latter in its larger size. Isorthoceras wahlenbergi reaches an adult size of ca 30 mm, which is documented in numerous specimen (Niko 2008; Kröger 2013: fig. 26). Immature specimens of I. saaremense can be distinguished from I. wahlenbergi by their more central siphuncular position: the mean RSP is 0.22 in I. wahlenbergi (Kröger 2013: 66), it is 0.29 in I. saaremense.
description: Published as part of Kröger, Björn, 2025, The Lyckholm acme of cephalopods - Review of the late Katian (Vormsi-Pirgu regional stages) Ordovician cephalopods of Estonia, pp. 1-169 in European Journal of Taxonomy 978 on pages 49-50, DOI: 10.5852/ejt.2025.978.2801, http://zenodo.org/record/14988376