Svalbardocreas skua, Kröger & Pohle, 2021

Kröger, Björn & Pohle, Alexander, 2021, Early-Middle Ordovician cephalopods from Ny Friesland, Spitsbergen - a pelagic fauna with Laurentian affinities, European Journal of Taxonomy 783 (1), pp. 1-102 : 32-33

Svalbardocreas skua gen. et sp. nov.

urn:lsid:zoobank.org:act:9385D872-8656-4B53-9CA8-05F7575BEB6F

Figs 16B , 19C , 20–21 , 22A–D , 23B, D, F

Diagnosis

Slightly curved longicones with circular conch cross section; shell ornamented with shallow, irregularly spaced undulations that are more pronounced in later growth stages and with fine, irregularly spaced, directly transverse growth lines or bands; thin, nearly marginal siphuncle with relative siphuncle diameter of ca 0.16 of corresponding conch cross section, siphuncle positioned at concave side of conch curvature; septal necks orthochoanitic; connecting ring relatively thick with slightly concave segments, weak hyposeptal and episeptal deposits occur.

Etymology

Referring to Stercorarius skua (Brünnich, 1764) , a bird which was a common companion during our field work at Profilstranda, Ny Friesland.

Type material

Holotype Specimen FMNH-P30272 .

Paratypes Nine specimens from type locality, one from bed PO 123 ( FMNH-P30271 ), eight from bed PO 123.3 ( FMNH-P30263 , P30264 , P30273 , P30275 to P30277 , P30408 , P30471 ), 120–120.3 m above the base of the Olenidsletta Member, V2 a trilobite zone , Blackhillsian , Floian .

Type locality and horizon

From Profilstranda section, adjacent to Hinlopenstretet, Spitsbergen, bed PO 123.3, 120.3 m above the base of the Olenidsletta Member, V 2a trilobite zone, Blackhillsian, Floian.

Description

The conchs are slightly curved with angle of expansion of <ca 7° (mean angle of expansion = 4.8°, 1 st –3 rd quantile: 3.3–4.8; n = 7), which decreases with conch size from ca 11° to 5° ( Figs 20 , 22A–D ). The conch cross section is invariantly circular.

The conch surface is ornamented with fine growth lines or growth bands ( Fig. 29C ) and annulations, which are slightly obliquely transverse, and shifted toward the aperture at the antisiphuncular side.

The sutures are slightly obliquely transverse and shifted toward the aperture at the antisiphuncular side. The relative chamber length (rCL) decreases with increasing conch cross section ( Fig. 21 ), ranging from ca 0.25 to more than 0.3.

The siphuncle is marginally positioned, very slightly removed from the conch margin ( Fig. 23B ), at the concave side of the conch curvature with a mean rSD of 0.16 (1 st –3 rd quantile: 0.13–0.19; n = 4). The septal necks are orthochoanitic. The connecting ring is moderately thick and forms slightly concave siphuncular segments ( Fig. 23F ). Weak epi-, and hyposeptal deposits occur in the extreme apical part of specimen FMNH-P30274 at a conch height of ca <2.8 mm. In one specimen part of the body chamber is preserved (specimen FMNH-P30276); the body chamber is> 26 mm long and the conch diameter at the position of the last septum is 5.5 mm.

Comparison

See above, under Svalbardoceras sterna gen. et sp. nov.

Figures

Fig. 16. Reconstruction and interpretation of details of connecting ring and septal necks in rioceratid cephalopods from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A. Ethanoceras solitudines gen. et sp. nov., FMNH-P30406, see Fig. 15B.B. Svalbardoceras skua gen. et sp. nov., FMNH-P30274, see Fig. 23F. C–D. Svalbardoceras sterna gen. et. sp. nov., note variability of septal neck shape, which is interpreted as an effect of poor preservation. C. Specimen, FMNH-P30368, see Fig. 23G. D. Specimen, FMNH-P30371, see Fig. 23E. Black: septal neck and septum. Dark grey: connecting ring. Without scale.

Fig. 19. Details of the conch surface of rioceratid cephalopods from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A. Hinlopoceras venti gen. et sp. nov., FMNH- P30262, from bed PO 07, lateral view with ventral, prosiphuncular side toward left. B. Hinlopoceras tempestatis gen. et sp. nov., FMNH-P30359, from bed PO 07, lateral view with ventral, prosiphuncular side toward right. C. Svalbardoceras skua gen. et sp. nov., FMNH-P30408, from bed PO 123.3. D. Svalbardoceras sterna gen. et sp. nov., FMNH-P30373, from bed PO 7.5. Scale bar = 5 mm for all figures.

Fig. 20.Diagrams of relative conch width and angles of expansion of species of Svalbardoceras gen. nov. from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. See Supp. file 1 for details of measurements.

Fig. 21. Diagrams of relative chamber length and relative siphuncular diameter of species of Svalbardoceras gen. nov. from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. See Supp. file 1 for details of measurements.

Fig. 22. Svalbardoceras gen. nov. from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A–D. Svalbardoceras skua gen. et sp. nov. from bed PO 123.3. A. Specimen FMNH-P30277, lateral view, venter, prosiphuncular side toward the right. B. Specimen FMNH-P30272, holotype, lateral view, venter, prosiphuncular side toward the left. C. Specimen FMNH-P30276, lateral view of adult (?), body chamber, with venter, prosiphuncular side toward the left. D. Specimen FMNH-P30263, lateral view of a fragmentary body chamber, with venter, prosiphuncular side toward the left. E–G. Svalbardoceras sterna gen. et sp. nov., from bed PO 7.5. E. Specimen FMNH-P30379. F. Specimen FMNH-P30373, lateral view with venter, prosiphuncular side toward the right. G. Specimen FMNH-P30368, holotype, lateral view with venter, prosiphuncular side toward the right. Scale bar = 10 mm for all figures.

Fig. 23. Median sections of phragmocones of Svalbardoceras gen. nov., from Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A, C, E, G. Svalbardoceras sterna gen. et sp. nov., from bed PO 7.5. A, E. Specimen FMNH-P30371. B, D, F. Svalbardoceras skua gen. et sp. nov., from bed PO 123.3. B. Specimen FMNH-P30275. C, G. Specimen FMNH-P30368, holotype. D, F. Specimen FMNH-P30471, oblique sagittal section through siphuncle, distance from conch margin is cutting effect. Scale bars: A–D = 5 mm; E–G = 1 mm.

Fig. 29. Median sections of phragmocones of Bactroceras Holm, 1898. A, C–D. Bactroceras fluvii sp. nov., from Profilbekken river basin, locality PR-phosphatic. A. Specimen FMNH-P30159, with adapical siphuncular segment, taphonomically slightly distorted.B. Bactroceras boliviensisAubrechtová, 2015, FMNH-P30176 from bed PO 131, extreme apical part. C–D. Specimen FMNH-P30177, holotype. C. Details of the protoconch. D. Apical part with transition toward juvenile growth stages; note the slightly expanded siphuncular segments. Scale bars: A = 5 mm; B–C = 1 mm; D = 2 mm.