Hosholmoceras triangulatum, 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 : 105

Hosholmoceras triangulatum gen. et sp. nov.

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Fig. 39A–B, D

Diagnosis

Slender Hosholmoceras gen. nov. (angle of expansion ca 15°), with rounded triangular conch cross section, with CHI ca 1, maximum mature conch size of ca 50 mm, and mature body chamber length of ca 30 mm; ornamented with fine, narrowly spaced transverse lirae.

Type material

Holotype

ESTONIA • Vormsi Island , Hosholm shore; Adila Formation, Pirgu Regional Stage; TUG 119-2 .

Paratypes

ESTONIA • 2 specs; same data as for holotype; GIT 878-168 , GIT 878-169 • 1 spec.; Vohilaid Island , Vohilaid shore (E); Adila Formation, Pirgu Regional Stage; GIT 878-178 .

Description

The holotype is a fragment of the phragmocone and the mature body chamber. The conch surface is not preserved ( Fig. 39A–B, D ). The conch is endogastrically curved (siphuncle at the concave side of the conch curvature). The complete conch cross section is preserved only in the apical portion of the specimen. There, it is rounded triangular with three sides of equal width and a narrowly rounded venter. The siphuncle is not visible or not preserved but apparently lies in the mid-position on the flat dorsum. At the apical end, the conch height and width are ca 27 mm and 26 mm, respectively. The greatest width of the specimen is reached ca 80 mm in the adoral direction from apical end of the specimen with 45 mm (angle of expansion = 14°). Beyond this point, the conch width decreases and at the base of the body chamber is ca 42 mm. The greatest conch width is located ca 20 mm adorally from the base of the body chamber. The preserved part of the body chamber is ca 25 mm long. The sutures forms shallow lateral lobes; they are narrowly spaced, ca 15–20 occur in a distance equal to the corresponding conch height (RCL ca 0.05–0.07).

Specimen GIT 878-169 is a small fragment of a phragmocone with ca four chambers preserved, it shows the rounded triangular conch cross section where the conch height and width are 32 mm and 37 mm, respectively (CHI = 0.86). The third specimen is a poorly preserved fragment of a phragmocone with a triangular conch cross section with a conch width of 27 mm.

Remarks

The siphuncle is only poorly preserved in the four specimens available. The poor preservation of the siphuncle is also a feature that can be seen in H. ovalis sp. nov. Amongst the 14 specimens of H. ovalis available for study only two preserved traces of the thin marginal siphuncle. Hosholmoceras triangulatum sp. nov. is similar to the type species of Hosholmoceras in having extremely narrowly spaced chambers and a bulbous phragmocone but differs in having a triangular conch cross section.

Figures

Fig. 39. Hosholmoceras gen. nov., and an indeterminate fragment of a multiceratid of the Pirgu Regional Stage, Estonia. A–B, D. Hosholmoceras triangulatum sp. nov., holotype TUG 119-2, from Hosholm shore (tower), Vormsi Island. A. View of the antisiphuncular side. B. Lateral view. C. Multiceratoidea gen. et sp. indet. B, specimen TUG 1743-37, from Saxby shore, Vormsi Island, Vormsi Regional Stage, lateral view. D. Adapical view. E–G. Hosholmoceras ovalis gen et sp. nov., specimen GIT 840-252 (holotype), from Hosholm shore (tower), Vormsi Island. E–F. Specimen GIT 878-170. E. View of the prosiphuncular side. F. Lateral view. G. Specimen GIT 840-252 (holotype), lateral view.