Isorthoceras sp. A

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 : 50-51

Isorthoceras sp. A

Fig. 12A

Material examined

ESTONIA • 1 spec.; Sutlema quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 878-128 .

Description

The specimen is a ca 45 mm long fragment of parts of a mature body chamber and three phragmocone chambers with a circular conch cross section. The maximum diameter of the specimen is 16 mm at ca mid-length of the body chamber. The body chamber is at least 34 mm long and at its base 15 mm in diameter. At ca 15 mm from its base, the body chamber has a wide, shallow constriction occurs. The three preserved chambers of the phragmocone decrease in length from 4 mm to 3.2 mm, to 2 mm in adoral direction. The septal foramen is well-preserved at 3.7 mm from the conch margin, where the conch cross section is 15 mm in diameter (RSP = 0.27), and it has a diameter of 1.2 mm. The conch surface is apparently smooth, although fine details of the ornamentation are not preserved.

Remarks

This specimen can be related to I. heroyense ( Strand, 1934) , and I. angelini Kröger, 2013 , with respect to the mature conch size, position of the siphuncle and general conch shape. However, because the nature of the conch surface is uncertain, a species level determination is impossible.