Diestoceras 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 : 68-70

Diestoceras sp. A

Fig. 27K

Material examined

ESTONIA • Vormsi Island, Hosholm shore (tower locality); Adila Formation, Pirgu Regional Stage; GIT 878-252.

Fig. 27 (next page). Breviconic cephalopods from the Vormsi–Pirgu regional stages, Estonia. A. Dowlingoceras kallholnense Frye, 1987 , specimen GIT 878-247, from Hosholm shore (tower),Vormsi Island, Pirgu Regional Stage, lateral view. B–C, E. Dowlingoceras sp. , specimen GIT 878-249, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage. B. Lateral view. C. View of antisiphuncular side. D. Danoceras sp. A ., specimen GIT 878-269, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage, lateral view. E. Adoral view. F. Rizosceras teres sp. nov., holotype TUG 1745-188, from Saaremõisa (Lyckholm), Vormsi Regional Stage, lateral view. G–H. Kallholnoceras sp. G . GIT 878- 323, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage, lateral view. H. Specimen GIT 878-233, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage, lateral view. I. Dowlingoceras tornense sp. nov., holotype GIT 840-254, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage, lateral view. J. Dowlingoceras sp. B , specimen GIT 47-866, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage, lateral view. K. Diestoceras sp. , specimen GIT 878-252, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage, lateral view. L. Diestoceras stensioei ( Troedsson, 1926) , specimen GIT 426-987, from Piirsalu quarry, Pirgu Regional Stage. M. Diestoceratidae gen. et sp. indet., specimen TUG 1745-192, from Salu, Pirgu Regional Stage, view from antisiphuncular (?) side. N. Diestoceras stensioei ( Troedsson, 1926) , specimen GIT 840-264, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage, lateral view. Scale bar = 10 mm, same scale in all figures.

Description

The specimen preserves parts of the mature body chamber and the phragmocone. Two thirds of the shell (in longitudinal direction) are preserved, so that the conch cross section cannot be determined with certainty; but was apparently circular. The body chamber is 45 mm long. The conch height at the base of the body chamber is 64 mm, ca 10 mm adorally the conch height reaches its maximum at 67 mm and at the peristome the height is ca 57 mm. In longitudinal direction, the outline of the body chamber is convex, and the peristome is simple and directly transverse. The hyponomic sinus is not preserved. At the base of the body chamber, a ca 0.4 mm wide band of oncomyarian muscle scars is preserved. The phragmocone is slightly curved, its height increases from 30 mm to 62 mm at a length of 38 mm (angle of expansion = 46°). At a conch height of 62 mm, the sutures are 7 mm apart (RCL = 0.11). The sutures are straight and directly transverse. Traces of the septal foramen and siphuncle are not preserved.

Remarks

The short, rounded conch, simple mature peristome, and apparently circular conch cross section identify this specimen as a species of Diestoceras . The size of the mature body chamber differs from known species of the genus described from Baltoscandia. However, because of the fragmentary preservation of this single specimen a species level determination is not possible.