Eosomichelinoceras borealis, 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 : 58-60

Eosomichelinoceras borealis sp. nov.

urn:lsid:zoobank.org:act:C53AC527-FC9D-4433-A853-5BF9AE4C2B6C

Figs 12C, F–G , 13B , 28C , 33C–E , 39–40

Diagnosis

Eosomichelinoceras with circular to slightly compressed conch cross section; conch in early growth stages slightly curved; shell surface ornamented with narrowly spaced transverse striae which form shallow hyponomic sinus at concave side of conch curvature; siphuncle eccentric, at convex side of conch curvature with rSP of 0.36, relative siphuncle diameter (rSD) ca 0.16 of corresponding conch cross section; septal necks loxochoanitic during earliest growth stages, orthochoanitic during later ontogeny; ca three chambers per length similar to corresponding conch cross section; connecting ring thin, tubular or slightly expanded.

Etymology

From ancient Greek ‘Βορέας’, ‘north wind’.

Type material

Holotype Specimen FMNH-P30279 .

Paratypes Fourty-one specimens ( FMNH-P30171 , P30182 , P30193 , P30278 , P30280 to P30318 , P30434 ; see Supp. file 1 for list of specimens) from type horizon and one specimen from bed PO 131, 128 m above base of Olenidsletta Member, all from type locality. Additionally, two microscopic fragments ( FMNH-P30170 , FMNH-P30184 ) from type horizon and one ( FMNH-P30182 ) from bed PO 131 .

Type locality and horizon

From Profilstranda, 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

Conch with circular to slightly compressed cross section, exogastrically curved in juvenile growth stages, becoming nearly straight in growth stages with diameter> 10 mm. The holotype (specimen FMNH-P30279) is a fragment of a phragmocone which growths within a length of 53 mm from 3.2 to 10 mm (angle of expansion ca 7°). The largest conch cross section diameter known is 11.5 mm (specimen FMNH-P30297), which is a completely preserved, nearly straight body chamber with a length of 28 mm and an angle of expansion of ca 5°. The low angle of expansion of this fragment indicates that a diameter of ca 12 mm is probably the adult size of this species (see also Fig. 39 ).

The ornamentation consists of fine striae, which run slightly obliquely transverse, which are deflected adorally on the convex side of the conch and form a shallow, but distinct hyponomic sinus at the concave side of the conch curvature ( Fig. 13B ). The holotype is ornamented with 7–10 transverse striae in a length of 1 mm where the conch is 9 mm in diameter. Six random measurements from the holotype and other specimens result in a mean distance of 0.11 mm between striae.

The sutures are directly transverse with a mean relative chamber length of 0.28 of the corresponding cross section. The chamber length is 2.0– 2.5 mm at 8–9 mm conch diameter in the holotype. The septal

perforation has a mean relative diameter of 0.16 in all measured specimens (rSD 1 st –3 rd quantile: 0.13– 0.16; n = 26).

The siphuncle is eccentrically positioned, with rSP ≈ 0.36 on the convex side of the conch curvature (1 st –3 rd quantile: 0.32–0.38; n = 5) ( Fig. 40 ). The septal necks are short loxochoanitic in specimen FMNH-P30193 at 7 mm diameter conch diameter and orthocoanitic in larger fragments with a larger conch diameter ( Figs 28C , 33C–E ). The connecting ring is thin with tubular or slightly expanded segments. No endosiphuncular or cameral deposits known.

The extreme apical part is preserved in specimens FMNH- P30171, FMNH-P30182, and FMNH-P30184 ( Fig. 12C, F–G ). The initial ca 1.5 mm in both specimens are set apart from the subsequent shell in being distinctly bent in growth direction toward the convex side of the curvature of the subsequent shell, which is the prosiphuncular side. The initial ca 1.5 mm differs also in lacking the fine transverse striation of the subsequent conch parts. The protoconch has a very distinctive stump-like form with a diameter of 1 mm (specimen FMNH-P30182) and 1.4 mm (specimen FMNH-P30184), and a length of 0.9 mm (specimen FMNH-P30182) and 0.7 mm (specimen FMNH-P30184). The shaft is endogastrically curved and starts at a diameter of 1.2 mm (specimen FMNH-P30184) and 0.9 mm (specimen FMNH-P30182), respectively. The conch grows toward 2.8 mm within ca 17 mm in FMNH-P30184, and reaches 2.3 mm within ca 15 mm in FMNH-P30182. A second distinct change in growth pattern occurs at a diameter of ca 1.5–1.7 mm and is best seen in specimen FMNH-P30171. There, the conch curvature abruptly decreases and the angle of expansion increases ( Fig. 12C ).

Stratigraphic and geographic range

120.3 m (PO 123.3) and 128 m (PO 131) above base of Olenidsletta Member, V2 trilobite zone, Blackhillsian, Floian.

Comparison

Eosomichelinoceras borealis sp. nov. is unique among Eosomichelinoceras in having a slightly curved juvenile conch and a relatively wide angle of expansion of ca 7°. The type species differs additionally in having a wider chamber spacing. In addition, in Eosomichelinoceras guizhouense Yang, 1978 the siphuncle is more central in position; in Eosomichelinoceras ordosoense Chen & Zou, 1984 additionally the siphuncle is more eccentrically positioned. Eosomichelinoceras baldisii Kröger et al., 2007 differs from the new species in having a compressed conch cross section. Apical parts of species assigned to Eosomichelinoceras are only known so far from specimens from the Olenidsletta Member, making a comparison of this growth stage with other species impossible at present.

Figures

Fig. 12 (previous page). Apical parts of orthoconic cephalopods from the Olenidsletta Member, Floian, Ordovician, near Hinlopenstretet, Ny Friesland, Spitsbergen. A. Cyptendoceras sp. B., FMNH-P30472 from Profilbekken river basin, locality PR-phosphatic. Lateral view with siphuncle toward the left. B. Svalbardoceras sterna gen. et sp. nov, FMNH-P30169, from PO 7.5. Lateral view with siphuncle toward left. C, F–G. Eosomichelinoceras borealis sp. nov. C. Specimen FMNH-P30171, from bed PO 123.3. Lateral view with prosiphuncular side toward the right. D. Bactroceras fluvii sp. nov., FMNH-P30168, from Profilbekken river basin, locality PR-phosphatic, showing the ventral side with ventral sutural lobe. E, H, J. Bactroceras boliviensis Aubrechtová, 2015 from bed PO 131. E. Specimen FMNH-P30165, nearly dorsal view, showing the transition of the extreme apical part toward the juvenile part of the conch. F. Specimen FMNH-P30182, lateral view with siphuncle toward the right. G. Specimen FMNH-P30184, lateral view with siphuncle toward the left. H. Specimen FMNH-P30183, apical view, showing the smooth apical surface of the protoconch. I. Ethanoceras solitudines gen. et sp. nov., FMNH-P30173, from bed PO 123.3, note the fine transverse striation. J. FMNH-P30160, showing details of fine transverse ornamentation. Scale bar = 2 mm for all figures.

Fig. 13. Details of the conch surface of transversally ornamented orthocones from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A. Ethanoceras solitudines gen. et sp. nov., FMNH-P30383, holotype, from bed PO 131. B. Eosomichelinoceras borealis sp. nov., FMNH-P30279, holotype, from bed PO 123.3. C. Bactroceras boliviensis Aubrechtová, 2015, FMNH-P30232, from bed PO 131, conch section with nearly cyclic spacing of transverse striae. Scale bar = 10 mm for all figures.

Fig. 28.Reconstructionand interpretation of detailsof connecting ring and septal necks.A–B. Bactroceras boliviensisAubrechtová, 2015.A. Specimen FMNH-P30243, see Fig. 27D.B. Specimen FMNH-P30215, see Fig.29A.C. Eosomichelinoceras borealis sp. nov., from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen, specimen FMNH-P30305, see Fig. 33C. Black: septal neck and septum. Dark grey: connecting ring. Light grey: endosiphuncular deposits. Without scale.

Fig. 33. Median sections of phragmocones. A–B. Hemichoanella occulta sp. nov., from bed PO 131. C–E. Eosomichelinoceras borealis sp. nov., from bed PO 123.3, Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A–B. Specimen FMNH-P30420, holotype. C. Specimen FMNH-P30305. D. Specimen FMNH-30288. E. Specimen FMNH-30284. Scale bars: A–B, E = 5 mm; C–D = 1 mm.

Fig. 39. Diagrams of spacing of ornamentation, angle of expansion, and relative chamber length of Eosomichelinoceras borealis gen. et sp. nov., from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. See Supp. file 1 for details of measurements.

Fig. 40. Diagrams of relative siphuncular diameter and relative siphuncular distance of Eosomichelinoceras borealis gen. et sp. nov. from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. See Supp. file 1 for details of measurements.