Rizosceras teres Kröger, 2025, sp. nov.
Creators: name: Kröger, Björn
Publisher: name: Zenodo
Year: 2025
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Cephalopoda
Oncoceratidae
Rizosceras
Rizosceras teres

description: Rizosceras teres sp. nov.
urn:lsid:zoobank.org:act: 8A906D01-DEF8-402F-9CF3-7CCE72E51F9B
Figs 27F, 28E, 29B
Diagnosis
Rizosceras with nearly smooth conch surface, ornamented only with fine, irregularly spaced growth lines or lirae; mature conch height ca 45 mm; phragmocone expands with an angle of ca 20°; conch cross section slightly elliptically compressed; siphuncle close to conch margin, with expanded segments, which are wider at their adoral ends.
Etymology
Refers to the Latin ‘ teres ’ (‘fine’, ‘well-rounded’) because of its relatively smooth conch surface.
Type material
Holotype
ESTONIA • Saaremõisa (Lyckholm); Kõrgessaare Formation, Vormsi Regional Stage; TUG 1745-188.
Paratypes
ESTONIA • 2 specs; Kõrgessaare quarry; Kõrgessaare Formation, Vormsi Regional Stage; GIT 426- 550, GIT 426-1141 • 1 spec.; Vormsi Island, Saxby shore; Kõrgessaare Formation, Vormsi Regional Stage; TUG 80-508.
Description
The holotype, TUG 1745-188 (Figs 27F, 28E), is a slightly deformed (compressed) fragment of a phragmocone and a body chamber. The conch surface was apparently smooth. The conch height increases at a length of 25 mm from of 22 mm to 30 mm (angle of expansion = 18°). The sutures are straight and directly transverse, 5 mm apart where the conch height is 25 mm (RCL = 0.2). The siphuncle is close to the conch margin at a conch height of ca 22 mm. There, the septal foramen is almost marginally positioned and has a width of 2 mm. The siphuncular segments are fusiform, expanded into the chambers, wider near the adoral septal surface.
Specimen GIT 426-550 (Fig. 30B) is a fragment of a body chamber and one chamber of the phragmocone, it has a length of 28 mm and an apical angle of 20°. The conch surface is nearly smooth, ornamented with fine, irregularly spaced growth lines or lirae. At its apical end, the conch height and width are 32 mm and 27 mm, respectively (CHI = 1.18). Its maximum conch height is ca 43 mm at the peristome. The peristome is simple, directly transverse; no traces of a hyponomic sinus are visible at the venter, which is relatively poorly preserved. In lateral view, the body chamber is nearly conical, straight, with a very slight convexity at the dorsal and ventral sides. The relatively narrow spacing of the two preserved septa indicates that this is a body chamber of a mature specimen. The sutures are 4 mm apart where the conch height is 32 mm (RCL = 0.13) and are straight and directly transverse. The septal foramen is located near the conch margin but too poorly preserved to be measured.
In specimen TUG 80-508, the septal foramen is ca 3 mm in diameter and the siphuncle expands toward 6 mm into the chambers where the septa are 7 mm apart and the conch height is ca 30 mm (RSH = 0.1, SCR = 0.9).
Remarks
This new species is placed in Rizosceras based on similarity of the general conch shape, the shape of the siphuncular segments, and the position of the siphuncle in the type-species of this genus. Its relatively smooth surface could be used as an argument to erect a new genus, similar to Rizosceras, but with a smooth conch surface. Rizosceras is a very species rich and long ranging genus (see e.g., Barskov 1972) probably containing unrelated species with similar conch shapes and ornamentation. A revision, therefore, would be desirable but is out of scope of this work. Moreover, the erection of a new genus, based on relatively poorly preserved fragments of Rizosceras teres sp. nov. seems unwise until better material is available and a detailed comparison with the type species of the genus is possible.
Comparison
This is a Rizosceras with a relatively smooth surface, low angle of expansion and a relatively strongly curved apical conch part. The shape of the mature body chamber almost resembles the Silurian Metarizoceras Foerste, 1930b.
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 92-93, DOI: 10.5852/ejt.2025.978.2801, http://zenodo.org/record/14988376