Striatocycloceras, Kroger & Isakar, 2006

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 : 30-31

Striatocycloceras ? sp.

Fig. 13E

Material examined

ESTONIA • 1 spec.; Saaremõisa (Lyckholm); Kõrgessaare Formation , Vormsi Regional Stage; TUG 42-414 .

Description

The specimen is a nearly complete body chamber of an adult specimen, ca 60 mm long, with a conch diameter at the position of the last septum of 16 mm and at the aperture of 13.5 mm. The conch diameter decreases rapidly in the adapical half of the body chamber and in its adoral half remains nearly constant at ca 14 mm. The conch cross section is circular. In longitudinal direction the conch is (probably taphonomically) slightly curved. Annulations are regular, directly transverse and with ca four annulations in a distance equal to the corresponding conch diameter. The troughs of the annulations are widely rounded, and the crests narrow and sharp. Very fine growth lines or striae, 20 per annulation are present parallel to the annulation.

The single preserved septum is three millimeter deep and positioned in the trough of an annulation. The suture line is parallel to the annulation. The septal foramen is central with a diameter of ca 2 mm.

Remarks

The specimen was collected by F. Schmidt (1832–1908) and originally determined as “ Orthoceras anellus ” [sic]. Orthoceras anellum Conrad, 1843 , is the type species of Anaspyroceras , which is ornamented with fine longitudinal lirae. Fine lirae are not visible in specimen TUG 42-414 despite a well-preserved conch surface. Details of the septal neck and siphuncular shape are not preserved in this specimen, making a genus determination questionable. Herein, the specimen is questionably assigned to Striatocycloceras because of the apparent absence of longitudinal elements in the ornamentation. The spacing of the annulation of this specimen is similar to S. undulatostriatum , from which it differs in having a smaller adult size (the conch diameter of the latter reaches 30 mm, see Kröger & Isakar 2006: 150). The specimen differs from specimens assigned to S. foerstei ( Teichert, 1930) in having a wider annulation (in S. foerstei six to seven annulations occur in a distance equal to the conch cross section). More material is needed for a species-level determination.

Figures

Fig. 13. Annulated Orthocerida of the Vormsi–Pirgu regional stages, Estonia. A. Palaeodawsonocerina senckenbergi (Teichert, 1930), specimen TUG 939-81, from Saxby shore (N), Vormsi Island, Vormsi Regional Stage. B, D. Striatocycloceras hosholmense sp. nov., holotype GIT 878-297, from Hosholm shore, Vormsi Island, Pirgu Regional Stage. C. Striatocycloceras cf. hosholmense, specimen TUG 939- 20, from Hosholm shore, Vormsi Island, Pirgu Regional Stage. E. Striatocycloceras sp., specimen TUG 42-414, from Saaremõisa (Lyckholm), Vormsi Regional Stage. Scale bars = 10 mm for all figures, same scale in A, C–E.