Cyrtorizoceras 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 : 85

Cyrtorizoceras sp. A

Fig. 32D

Material examined

ESTONIA • 1 spec.; Vohilaid Island , Vohilaid shore (E); Adila Formation, Pirgu Regional Stage; GIT 878-260 .

Description

The specimen is a fragment of an exogastrically curved mature body chamber and five chambers of the phragmocone. At the base of the body chamber, the conch height and width are 58 mm and ca 45 mm, respectively (CHI = 1.29), and the conch cross section is ovate, oval compressed with the prosiphuncular margin slightly narrower than the antisiphuncular margin. The preserved part of the body chamber is 22 mm in height. In lateral view, only the antisiphuncular side of the body chamber is sufficiently preserved to describe its shape; it is slightly concave throughout the preserved length. The prosiphuncular side is convex and more strongly curved than the antisiphuncular side. The phragmocone height expands with an angle of 26° through an increase in conch height of 48–58 mm. The sutures form a shallow lateral lobe and are slightly more adaperturally located on the antisiphuncular side of the conch. The adoral-most sutures are crowded and 4 mm distant. At a conch height of 53 mm the sutures are 5 mm apart (RCL = 0.09). The siphuncle is almost marginally positioned with segments that are expanded within the chambers.

Remarks

Like C. hariense sp. nov., C. sp. A is a large Cyrtorizoceras . It differs from C. hariense in being more compressed, in having a larger angle of expansion (angle of expansion = ca 20° in C. hariense ) and a shorter mature body chamber. No similar species is known. Species of Diestoceras , which have similar high angles of expansion and short body chambers, may be distinguished by the contraction of the mature body chambers.

Figures

Fig. 32. Oncoceratidae Hyatt, 1884 of the Vormsi–Pirgu regional stages, Estonia. A–B. Beloitoceras sinuososeptatum (Roemer, 1861). A. Specimen TUG 939-56, from Haapsalu holm, Pirgu Regional Stage, lateral view. B. Specimen TUG 1723-26, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage, lateral view. C. Beloitoceras cf. sinuososeptatum (Roemer, 1861), specimen GIT 878-231, from Hosholm shore (tower), Vormsi Island, Pirgu Regional Stage. D. Cyrtorizoceras sp. A., specimen GIT 878-260, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage, lateral view.E–F. Cyrtorizoceras hariense sp. nov., holotype GIT 878-71, from Saxby shore (N), Vormsi Island, Estonia, Vormsi Regional Stage. E. View of prosiphuncular side. F. Lateral view. Scale bar = 20 mm, same scale in all figures.