Isorthoceras sp. D

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 : 52

Isorthoceras sp. D

Fig. 11I

Material examined

ESTONIA • 2 specs; Moe trench; Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-114 , GIT 878-125 • 1 spec.; Paluküla quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 426-433 • 2 specs; Vormsi Island , Saxby shore; Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-44 , GIT 878-83 • 2 specs; same data as for preceding; TUG 939-3 , TUG 939-4 .

Description

Specimen GIT 878-114 (Fig. 11I) is a fragment of a body chamber and one chamber of the phragmocone. The body chamber is slightly deformed and does not permit measurement of the angle of expansion. The conch cross section at the base of the body chamber is circular with a diameter of 10 mm. The preserved two septa are 1.8 mm apart (RCL = 0.18) and the septal foramen is nearly centrally positioned (RSP = 0.46) with a diameter of ca 1 mm. The outer shell is not preserved. Specimen TUG 939-4 is a fragment of a body chamber and seven chambers of the phragmocone with a circular conch cross section. The septal distance varies between 2.5–3.5 mm (RCL = 0.23–0.39) and is most distant at smaller diameters (9 mm) and decreases toward the base of the body chamber at the base of the body chamber 11 mm. The angle of expansion of the specimen is ca 3°. The siphuncle is nearly central (RSP = 0.45), its shape is nearly tubular, and the septal necks are very short suborthochoanitic to orthochoanitic. The largest specimen is a fragment of a 28 mm long body chamber with a maximum diameter of ca 13 mm (GIT 878-83).

Remarks

The adult size of this species is indicated by mature septal crowding occurring in specimens TUG 939- 4, and GIT 878-114 at conch diameters of ca 10–11 mm. The maximum adult diameter at the aperture was probably not much more than 13 mm. These specimens are assigned to Isorthoceras based on their similarity with other small orthocerids known from the Late Ordovician of Baltoscandia, and because of their short suborthochoanitic septal necks. They differ from I. sp. E in having a central siphuncle. Better preserved material is needed for a species level determination.