Deltoideum (Boreiodeltoideum) borealis, Kosenko, 2017

Kosenko, Igor N., 2017, Late Jurassic-Early Cretaceous oysters from Siberia: A systematic review, Acta Palaeontologica Polonica 62 (4), pp. 759-778 : 764-765

Deltoideum (Boreiodeltoideum) borealis sp. nov.

Fig. 5A, E .

1963 Liostrea ex gr. delta ( Smith, 1817) ; Zakharov 1963: 44, fig. 1a–c. 1966 Liostrea delta ( Smith, 1817) ; Zakharov 1966: 101, pl. 37: 2a,b,

pl. 38: 1.

ZooBank LSID: urn:lsid:zoobank.org:pub:26FBF7FF-F554-48DCB02D-069507B4202B

Etymology: From Greek boreas, north; in reference to type area.

Holotype: TsSGM 150/3887 ( Fig. 5E ) left and right valves from the same shell with exposed ligament area.

Type locality: Outcrop along the Levaya Boyarka River (N70°33, E97°20), north of Eastern Siberia, Russia .

Type horizon: Kimmeridgian sand of the Sigovskaya Formation (the Sigovskaya Formation consists mainly of sandstone; for the detail information see Shurygin et al. 2000), the Levaya Boyarka River, north of Eastern Siberia.

Material.— Holotype and five left and right valves from the Lower Kimmerigian deposits exposed at the Lopsiya River (TsSGM, collection 2068, holotype TsSGM 150/3887, paratypes TsSGM 2068/31, 2068/83, 2068/84, 2068/85, 2068/86).

Diagnosis.— Shells medium to large, inequivalve, LV convex, RV flat or slightly concave, umbo opisthogyrate, ligament area triangular, with triangular resilifer, height of posterior bourrelet smaller than height of anterior bourrelet, PAM circular andin a posterodorsal position.

Description.— Shell inequivalve, outlines deltoid, subtriangular with height greater than length. LV convex, RV flat. Margins of valves taper dorsally. Attachment area small to relatively large, restricted to umbonal region. Maximum convexity close to anteroventral side. Sculpture of LV with growth lamellae, covered by weak radial stria. Growth lamellae of RV less thick than of LV. Ligament area ostreoid, small, triangular. Resilifer triangular, not deep, poorly marked, posteriorly directed. Height of anterior bourrelet greater than height of posterior bourrelet. Posterior adductor scar circular; position posterodorsal. Quenstedt muscle scar not observed. Chomata absent. Microstructure unknown.

Remarks.— At the type locality, shells of D. (B.) borealis sp. nov. have been found in sand and glauconitic sand of Kimmeridgian age, as separated valves randomly distributed in the layers or rarely in lens-shaped shell accumulations (Zakharov 1966). According to Zakharov (1966) this species inhabited shallow, turbulent waters. However, in the Lopsiya River section, this species was also found in Lower Kimmeridgian clay. In this place the oysters inhabited a low-energy soft bottom environment. LV’s of oysters from the clay are more convex compared to specimens found in sand.

Deltoideum (Boreiodeltoideum) praeanabarensis (Zakharov, 1963) (Zakharov 1963: 50, pl. 7: 1–4; 1966: 103, pl. 38: 3–6, pl. 39: 1–2, pl. 40: 1–2; Kosenko 2016a: pl. 3: 12, 13, 15; Fig. 6B–D, F ) is morphologically close to D. (B.) borealis sp. nov. It differs by a more elongated shell with the length greater than height, and the presence of an alate expansion posterior of the umbo.

D. (B.) anabarensis ( Bodylevsky, 1949) (Zakharov 1966: 108, pl. 39: 3, pl. 40: 4, 5, pl. 42: 4–5; Kosenko 2016a: pl. 3: 9, 14, 16; SOM: fig. 3, Supplementary Online Material available at http://app.pan.pl/SOM/app62-Kosenko_SOM.pdf) differs by an extremely elongated and crescentic shaped shell.

“ Ostrea ” eduliformis (Zieten, 1832) (Zieten 1832: 60, pl. 45: 1; Madsen 1909: 200, pl. 7: 1–5) (this species belongs most likely to Pernostrea and it was usually assigned to Liostrea ) from the Bajocian of Germany and Greenland differs by a less convex LV, a larger ligament area and the position of the PAM is less posteriorly shifted.

Stratigraphicandgeographicrange.— LowerKimmeridgian of the Northern Urals, north of Eastern Siberia.

Figures

Fig. 5. Microstructure of studied gryphaeid oysters. A. Deltoideum delta (Smith, 1817), TsSGM 2068/87; Kimmeridgian, Dorset, England. B. Pernostrea uralensis (Zakharov, 1972), TsSGM 2068/45; Upper Volgian, Maurynya River, eastern slopes of the Northern Urals. Microstructure (A1, B1), cross section (A2, B2). RF, regularly foliated structure; HCF,?herringbone cross-foliated structure.

Fig. 6. Gryphaeid oysters from the subgenus Boreiodeltoideum. A, E. Deltoideum (Boreiodeltoideum) borealis sp. nov. A. TsSGM 2068/31, paratype, Lower Kimmeridgian, Lopsiya River, Northern Urals; exterior (A1) and interior (A2) of left valve. E. TsSGM 150/3887, holotype, Lower Kimmeridgian, Khatanga depression, north of Eastern Siberia; interior of RV (E1) and exterior of LV (E2). B–D, F. Deltoideum (Boreiodeltoideum) praeanabarensis Zakharov, 1966), Lower Volgian, Dyabaka-Tari River, north of Eastern Siberia. B. TsSGM 2068/10, view on left (B1) and right (B2) valves. C. TsSGM 2068/57, interior of right valve. D. TsSGM 2068/8, exterior (D1) and interior (D2) of left valve. F. TsSGM 150/2031, interior of left valve.