Isorthoceras sp. E

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 : 52-53

Isorthoceras sp. E

Fig. 19E

Material examined

ESTONIA • 1 spec.; Vormsi Island, Saxby shore , Kõrgessaare Formation , Vormsi Regional Stage; GIT 426-1132 .

Description

The specimen is a 110 mm long, 27–32 mm high fragment of a phragmocone (angle of expansion ca 4°) with 16 chambers preserved (RCL 0.21–0.31), and with a circular cross section. The outer shell is not preserved. The siphuncle is eccentric (RSP = 0.31) and, where preserved, with a straight, tubular connecting ring. The septal necks are short (ca 0.8 mm long) orthochoanitic-suborthochoanitic and shallow endosiphuncular annuli occur.

Remarks

This specimen is unique among the larger Isorthoceras fragments in having a combination of relatively low angle of expansion (comparable to I. luhai at similar conch heights) and a relatively eccentric siphuncle position (comparable to I. saaremense at similar conch heights). Additionally, the siphuncle is more tubular than compared to the other species of Isorthoceras known from Estonia, and shallow endosiphuncular deposits occur. More material is needed with external characters preserved to allow for a species level determination.

Figures

Fig. 19 (next page). Median sections of phragmocones of Isorthoceras Flower, 1962 of the Vormsi– Pirgu regional stages, Estonia. A–C. Isorthoceras luhai (Stumbur, 1956) comb. nov. A. Specimen GIT 426-1108 from Paluküla quarry, Vormsi Regional Stage. B. Specimen GIT 838-30 from Saxby shore, Vormsi Island, Vormsi Regional Stage. C. Holotype TUG 939-1, from Saxby shore, Vormsi Island, Vormsi Regional Stage. D. Isorthoceras saaremense (Balashov, 1959) comb. nov., specimen GIT 878- 20, from Saxby shore, Vormsi Island, Vormsi Regional Stage. E. Isorthoceras sp. E, GIT 426-1132, from Saxby shore, Vormsi Island, Vormsi Regional Stage. F. Isorthoceras luhai, specimen GIT 426- 1108, from Paluküla quarry, Vormsi Regional Stage. G. Isorthoceras saaremense, TUG 1745-305, from Kersleti old quarry, Vormsi Regional Stage. Scale bars = 5 mm in all figures.