Svalbardoceras sterna Kröger & Pohle 2021, gen. et sp. nov.

Svalbardoceras sterna gen. et sp. nov.

urn:lsid:zoobank.org:act: F4069C31-5F45-44C0-A05D-03223749C9DE

Figs 12B, 16C–D, 19D, 20– 21, 22E–G, 23A, C, E, G

Diagnosis

Slightly curved longicones with compressed conch cross section; with shallow irregularly spaced undulations that are more pronounced in later growth stages and with fine, irregularly spaced, directly transverse imbricate growth lines which form a shallow hyponomic sinus at prosiphuncular side of the conch; thin marginal siphuncle with rSD ≈ 0.13, siphuncle positioned at concave side of conch curvature; septal necks orthochoantic to loxochoantic, connecting ring nearly tubular, weakly concave.

Etymology

Referring to the laridid genus Sterna Linneaus, 1758, birds which were common companions during our field work at Profilstranda, Ny Friesland.

Type material

Holotype Specimen FMNH-P30368.

Paratypes Forty-four specimens (P30368 to P30382, P30437 to P30449, P30451 hasta P30462, P30465, P30466, P30468 to P30470; see Suppl. file 1 for list of specimens) from type locality, two from bed PO 07, fortytwo from bed PO 7.5, 4 m and 4.5 m above the base base of the Olenidsletta Member, Valhallfonna Formation, V1a trilobite zone, Blackhillsian, Floian. Two additional specimens (FMNH-P30169, FMNH-P30172) from bed PO 7.5 are apical fragments.

Type locality and horizon

From Profilstranda section, adjacent to Hinlopenstretet, Spitsbergen, from bed PO 7.5, 4.5 m above the base of the Olenidsletta Member, Valhallfonna Formation, V 1a trilobite zone, Blackhillsian, Floian.

Description

The conchs are slightly curved with an angle of expansion of conch width <ca 9° (mean angle of expansion = 6.7°, 1 st –3 rd quantile: 5.7–7.8; n = 12), and of conch height <12° (mean angle of expansion = 7°, 1 st –3 rd quantile: 6.6–9.3; n = 14). The conchs reach their largest angle of expansion in growth stages with conch heights of 10–17 mm (Fig. 20). The conch cross section is compressed with a mean rW = 0.89 (1 st –3 rd quantile: 0.86–0.92; n = 29) (Fig. 21).

The conch surface is ornamented with growth lines or growth bands and weak irregularly spaced annulations, which are more pronounced in specimens with conch height> 10 mm. Annulation and ornamentation are slightly obliquely transverse, shifted toward aperture at antisiphuncular side and form a shallow hyponomic sinus at the prosiphuncular side. In some specimens, e.g., FMNH-P30379, the conch surface appears to be finely imbricated (Fig. 22E).

The siphuncle is marginal, positioned at the concave side of the conch curvature with a mean rSD = 0.13 (1 st –3 rd quantile: 0.11–0.15; n = 4). The sutures are directly transverse with an rCL between 0.27 and 0.36 (Fig. 21).

The septal necks are loxochoanitic to orthochoanitic. The connecting ring is relatively thin and forms slightly concave segments, which can vary within one specimen from nearly tubular to slightly s-shaped (Fig. 23E–G). The largest known specimen is a fragment of a phragmocone, FMNH-P30446, with a maximum diameter of 22 mm.

Two specimens preserve the apical parts of the conch. In the apical 0.5 mm FMNH-P30169 reaches 1.7 mm in diameter; increasing to 2.2 mm at a distance 2 mm from the apex, and then respectively 3 mm at 5 mm and 6 mm at 17 mm. The conch has a slightly compressed cross section. The extreme ca 0.5 mm of the apex of specimen FMNH-P30169 was broken during preparation, is poorly preserved and the presence or absence of a cicatrix is not possible to determine.

Comparison

Svalbardoceras sterna gen. et sp. nov. differs from S. skua gen. et sp. nov. in having a compressed conch cross section. Svalbardoceras sterna gen. et sp. nov. is similar to Bactroceras morguesi Kröger & Evans,

2011 with respect to the angle of expansion and the slight conch curvature, but the latter differs in having a circular conch cross section and in not being annulated.