Hosholmoceras ovalis Kröger, 2025, gen. et sp. nov.
Creators: name: Kröger, Björn
Publisher: name: Zenodo
Year: 2025
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Cephalopoda
Discosorida
Cyrtogomphoceratidae
Hosholmoceras
Hosholmoceras ovalis

description: Hosholmoceras ovalis gen. et sp. nov.
urn:lsid:zoobank.org:act: 221233FB-02D1-4F7C-A010-B574D890913D
Figs 37C, 39E–G
Diagnosis
Elliptically to oval compressed Hosholmoceras gen. nov., with CHI of ca 1.3–1.5, with angle of expansion of up to ca 26°, maximum mature conch size of ca 50 mm, mature body chamber length of ca 30 mm; ornamented with fine, narrowly spaced transverse lirae.
Etymology
The name refers to Hosholm, Vormsi Island, the type locality.
Type material
Holotype
ESTONIA • Vormsi Island, Hosholm shore (tower locality); Adila Formation, Pirgu Regional Stage; GIT 840-252.
Paratypes
ESTONIA • 3 specs; same data as for holotype; GIT 878-171 to GIT 878-173 • 11 specs; Vohilaid Island, Vohilaid shore (E); Adila Formation, Pirgu Regional Stage; GIT 878-170, GIT 878-174 to GIT 878-177, GIT 878-179 to GIT 878-184.
Description
The holotype is an endogastrically curved, compressed fragment of a phragmocone and body chamber (Fig. 39G). The conch is slightly deformed (compressed). Parts of the outer shell are preserved on the antisiphuncular margin near the adoral end of the specimen. There, it is ornamented with fine transverse lirae of which ca 8 occur per 5 mm where the conch height is 53 mm. In lateral view, the pro-, and antisiphuncular conch margins, respectively, are convex and concave throughout their entire length. The concave side is nearly straight adorally, so that in lateral view the entire specimen has a bulbous shape. The greatest width is near the adoral end of the phragmocone. The greatest rate of expansion occurs in adapical parts of the phragmocone, where it increases in height from 28 mm to 42 mm over a distance of ca 30 mm (angle of expansion = 26°). The length of the preserved part of the body chamber is 20 mm. The conch cross section is rounded elliptical to oval with a slightly narrower margin at the antisphuncular side. At the base of the body chamber the height and width are 52 mm and 34 mm, respectively (CHI = 1.53). At the apical end of the specimen, the height and width are 28 mm and 21 mm, respectively (CHI = 1.33). The sutures are narrowly spaced, ca 2–3 mm apart throughout the entire length of the phragmocone (RCL ca 0.05–0.07). The sutures form wide lateral lobes. Details of the siphuncle and septal necks are preserved at the apical end of the specimen (Fig. 37C). The septal foramen is nearly marginal (ca 1 mm from the conch margin), very small (ca 1 mm in diameter, RSH = 0.04) and the septal necks are cyrtochoanitic. The siphuncular segments are rounded, expanded within the chambers with a width of 4 mm where the chamber length is ca 1.8 mm (SCR = 2.2). Thin bullettes are present.
Three other specimens in the collection (GIT 878-170, GIT 878-181, GIT 878-183) have a maximum conch height of ca 50–52 mm, suggesting that this is the mature size of this species. A nearly complete body chamber is preserved in specimen GIT 878-170 (Fig. 39E–F), it is simple conical, 23 mm long, and has a straight peristome at the antisiphuncular side of the conch.
Remarks
See Remarks on Hosholmoceras triangulatum gen. et sp. nov. (below).
description: Published as part of Kröger, Björn, 2025, The Lyckholm acme of cephalopods - Review of the late Katian (Vormsi-Pirgu regional stages) Ordovician cephalopods of Estonia, pp. 1-169 in European Journal of Taxonomy 978 on pages 103-105, DOI: 10.5852/ejt.2025.978.2801, http://zenodo.org/record/14988376