Diestoceratidae, Foerste, 1926

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 : 73

Diestoceratidae gen. et sp. indet.

Fig. 27M

Material examined

ESTONIA • Salu; Pirgu Regional Stage ; TUG 1745-192 .

Description

The specimen is a fragment of a nearly straight mold of a mature body chamber and nine chambers of the phragmocone which is partially deformed. Parts of the shell are eroded. The conch surface is not preserved. The fragmentary preservation does not permit measurement of the original conch cross section. The maximum conch diameter (28 mm) is located at the base of the body chamber. The body chamber is ca 28 mm long with slightly convex conch margins, it contracts adorally to a diameter of ca 27 mm where a shallow, inconspicuous constriction is located ca 5 mm distant from the peristome. The phragmocone diameter increases from 23 mm to 27 mm at a length of 16 mm (angle of expansion = 18°). The sutures are directly transverse and 3.5 mm apart where the conch diameter is 27 mm (RCL = 0.13). On the surface of the phragmocone, faint longitudinal lirae may be seen.

Remarks

The conch shape, shape of the mature body chamber as well as spacing and pattern of sutures of this specimen are suggestive of a small diestoceratid. The small mature size and the relatively long body chamber are not known from any species of Danoceras , Dowlingoceras or Diestoceras described in the literature. However, as crucial features for determination, including the siphuncle position and shape, are not preserved, while the conch cross section cannot be fully be reconstructed, a genus and species level determination of this specimen is impossible.

Figures

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.