Gorbormoceras vohilaidense, Kröger, 2025

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 : 22-23

Gorbormoceras vohilaidense gen. et. sp. nov.

urn:lsid:zoobank.org:act:820DB1BC-79F4-40A5-A0B0-19D08C785DD6

Figs 7B , 8F

Diagnosis

Same as for genus, by monotypy.

Etymology

The name refers to Vohilaid Island, the type locality.

Type material

Holotype

ESTONIA • Vohilaid Island , Vohilaid shore (E); Adila Formation, Pirgu Regional Stage; GIT 878-235 .

Description

The specimen is a slightly exogastrically curved mold of parts of a phragmocone and a body chamber. The conch cross section is apparently circular. At a conch diameter of 28–38 mm, the angle of expansion is 14°. At the base of the body chamber, the diameter is 31 mm. The preserved length of the body chamber is 60 mm. Although the outer shell is not preserved, traces of a relatively strong ornamentation with ca 20 longitudinal lirae around the circumference and a weak annulation are visible ( Fig. 7B ). Near the base of the body chamber, ca five annulations occur in a distance equal to the corresponding conch cross section.

The sutures form shallow lateral lobes and are slightly adorally shifted on the prosiphuncular side of the conch, the most adoral sutures are slightly crowded (chamber length 4.5 mm), at a conch height of 28 mm they are 6 mm distant (RCL = 0.21). The siphuncle is eccentrically positioned; near the apical end of the specimen, where the conch diameter is 31 mm, it is 10 mm distant from the conch margin and has a diameter of ca 3.3 mm (RSP = 0.36). The septal necks are short recumbent or achoanitic. The connecting ring is thin, and forms widely expanded segments with adnate areas at the adoral and apical septal surfaces (Fig. 8F). Where the septal foramen is 3.3 mm, and the chamber length is 4 mm, the segments expand toward a height of 8.6 mm (RSS = 2.15).

Remarks

This specimen is externally similar to Cycloceras fenestratum Eichwald, 1860 with regard to the general conch shape and its traces of the ornamentation (ca 20 longitudinal lirae and ca five annulations per distance equal to the corresponding conch cross section). However, it is larger (maximum diameter ca 25 mm in C. fenestratum ) and has a larger angle of expansion (angle of expansion <10° in C. fenestratum ) (see e.g., Kröger 2013). Internally, the specimen is similar to species of Ormoceras .

Comparison

The widely expanded siphuncle and the ornamentation of the new species are most similar to Troedssonoceras (?) obscuriliratum Flower, 1946, from which it differs in having a more centrally positioned siphuncle and a weakly annulated conch. Another orthoconic species with a nodular ornamentation, comparable to G. vohilaidense sp. nov., is Spyroceras (?) nodosum Sweet & Miller, 1957, from which the internal characters are not known.

Order Orthocerida Kuhn, 1940

Family Orthoceratidae M’Coy, 1844

Figures

Fig. 7. Actinocerida Teichert, 1933 of the Vormsi–Pirgu regional stages, Estonia. A. Nybyoceras bekkeri Troedsson, 1926, specimen GIT 426-1120 from Kasari, Vormsi Regional Stage, dorsal view. B. Gorbormoceras vohilaidense gen. et sp. nov., holotype GIT 878-235, from Vohilaid shore (E), Vohilaid Island, Pirgu Regional Stage, lateral view. C. Nybyoceras intermedium Teichert, 1930, specimen TUG 939-49, from Moe stratotype outcrop, Pirgu Regional Stage, lateral view. Scale bar = 10 mm for all figures, same scale in A–C.