Pernostrea, Munier-Chalmas, 1864

Kosenko, Igor N., 2017, Late Jurassic-Early Cretaceous oysters from Siberia: A systematic review, Acta Palaeontologica Polonica 62 (4), pp. 759-778 : 771-773

Pernostrea ? robusta sp. nov.

Fig. 11 .

2016 Deltoideum (Deltoideum) delta ( Smith, 1817) ; Kosenko 2016a: pl. 3: 3.

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

Etymology: From Latin robustus, powerful, massive; in reference to gross shell morphology.

Type material: Holotype, adult right valve with exposed ligament area: TsSGM 2068/35 ( Fig. 11C ). Paratypes: TsSGM 2068/36, TsSGM 2068/38 from the type locality ( Fig. 11B, D ) and TsSGM 2068/37 from the Yatriya River ( Fig. 11A ) .

Type locality: Tolya River , the Northern Urals (north-west of Western Siberia), Russia .

Type horizon: Middle Volgian silt ( Pavlovia iatriensis Zone , Strajevskya strajevskyi Subzone ) of the Tolya River.

Material.— Type material only.

Diagnosis.— Inequivalve; LV convex with deep internal cavity, RV flat; ligament area extremely large; PAM length-oval, strongly depressed.

Description.— Shell inequivalve, outlines subrectangular or trapezoidal. LV convex, very thick (maximum thickness 10 mm), relatively capacious (deepest part of LV located near anteroventral margin with maximum depth of 15 mm). RV slightly convex, relatively capacious, maximum depth near anteroventral margin as in LV. Attachment area large, almost entire LV. Observed maximum size (measurements were made on complete shells, hypothetic size of specimen 2068/37 is bigger than the measured specimens), H= 110 mm, L = 10 mm. Shell height to length ratio (H/L) varies 0.94–1.21 (average 1.11, based on measurements of 5 shells). LV and RV covered with thick growth lamellae. Radial stria questionable because preservation of shells does not allow to observe them. Ligament area ostreoid, trapezoidal or rectangular, extremely elongated in length. Resilifer very shallow and extremely wide. Ligament area length to shell length ratio (Lla/L) varies 0.47–0.79 with average of 0.65 (based on the results of measuring 5 shells). Resilifer length to ligament area length ratio (Lr/Lla) varies 0.42– 0.54 with average of 0.49 (based on the results of measuring 5 shells). PAM length-oval, very depressed, posterocentral. Quenstedt muscle located under posterior border of resilifer. Chomata absent. Microstructure not observed.

Remarks.— This species is characterized by length-oval shape of PAM whereas other oysters from the genus Pernostrea have circular shape of PAM. Due to these differences attribution of this species to the genus Pernostrea is questionable. This species has been found in the Middle Volgian silt of Tolya River and the Middle Volgian silt of Yatriya River. Only LV presented in studied collection is attached to valve of other oyster.

Helvetostrea expansa (J. Sowerby, 1819) is the most morphologically similar species to Pernostrea ? robusta sp. nov. Helvetostrea expansa differs by having a much less broad ligament area. Koppka (2015) noted, that shells of H. expansa have a small umbonal cavity and large chambers below the ligament area, while such structural chambers are absent in P.? robusta sp. nov. Pernostrea gibberosa Zakharov, 1974 (Zakharov and Mesezhnikov 1974: 147, pl. 31: 1, pl. 32: 1; SOM: fig. 2B, C) differentiates by the presence of an U-shaped fold on the LV and a corresponding depression on the RV. Pernostrea? planoconvexa Zakharov, 1974 (Zakharov and Mesezhnikov 1974: 148. pl. 33: 1) differs by a less broad triangular ligament area, more convex elongated in length LV and the orbicular shape of PAM which is situated in a subcentral position. Pernostrea uralensis Zakharov, 1972 (SOM: fig. 1A) differs by having an orbicular and centrally positioned PAM. Other species of Pernostrea can be distinguished by the presence of an orbicular PAM, while the PAM of P.? robusta sp. nov. is length-oval and strongly depressed.

Stratigraphic and geographic range. —Middle Volgian of the Northern and the Subpolar Urals (Yatriya and Tolya rivers), the northwest of Western Siberia, Russia.

Systematic review of Late Jurassic–Early Cretaceous Ostreoidea from Siberia

Family Flemingostreidae Stenzel, 1971

Subfamily Crassostreinae Scarlato and Starobogatov, 1979

Genus Helvetostrea Koppka, 2015

Helvetostrea exotica ( Kosenko, 2014) ; Upper Kimmeridgian

(Fig. 12L, M)

Subfamily Liostreinae Vialov, 1983

Genus Praeexogyra Charles and Maubeuge, 1953

Praeexogyra siberica (Zakharov, 1974) ; Middle Volgian–

Ryazanian (Fig. 12A, B, E, F, G)

Family Gryphaeidae Vialov, 1936

Subfamily Gryphaeinae Vialov, 1936

Tribe Pernostreini nov.

Genus Pernostrea Munier-Chalmas, 1864

Pernostrea cucurbita (Zakharov, 1966) ; Lower Valanginian (SOM: fig. 4)

Pernostrea gibberosa (Zakharov, 1974) ; Lower Volgian (SOM: fig. 2B, C)

Pernostrea mesezhnikovi sp. nov.; Upper Kimmeridgian–Lower Volgian ( Figs. 7 , 9 , 10B, D )

Pernostrea ? robusta sp. nov.; Middle–Upper Volgian ( Fig. 11 )

Pernostrea cf. quadrangularis ( Arkell, 1927) ; Lower Kimmeridgian (SOM: fig. 5)

Pernostrea uralensis (Zakharov, 1972) ; Upper Volgian–Ryazanian (SOM: fig. 1A)

Genus Deltoideum Rollier, 1917

Subgenus Boreiodeltoideum Kosenko, 2016a

Deltoideum (Boreiodeltoideum) borealis sp. nov.; Lower Kimmeridgian ( Fig. 5A, E )

Deltoideum (Boreiodeltoideum) praeanabarensis (Zakharov, 1963) ; Lower Volgian ( Fig. 5B–D, F )

Deltoideum (Boreiodeltoideum) anabarensis ( Bodylevsky, 1949) ; Ryazanian–Lower Hauterivian (SOM: fig. 3)

Tribe Gryphaeini Vialov, 1936

Genus Gryphaea Lamarck, 1801

Subgenus Gryphaea (Gryphaea) Lamarck, 1801

Gryphaea (Gryphaea) curva ( Gerasimov, 1955) ; Lower Kimmeridgian (Fig. 12C, D)

Gryphaea? borealis Zakharov, 1966 ; Lower Valanginian (Fig. 12J)

Gryphaea sp. ; Lower Volgian (Fig. 12I)

Subfamily Exogyrinae Vialov, 1936

Tribe Amphidontini Vialov, 1983

Genus Nanogyra Beurlen, 1958

Nanogyra ? cf. thurmanni (Étallon, 1862); Middle Volgian (Fig. 12O)

Nanogyra ? sp.; Lower Kimmeridgian (Fig. 12H)

Subfamily? Pycnodonteinae Stenzel, 1959

Genus Argutostrea Kosenko in Kosenko and Seltser, 2016

Argutostrea roemeri ( Quenstedt, 1843) ; Kimmeridgian–Middle Volgian (Fig. 12K, N)

Subfamily indet.

Genus indet.

“ Ostrea ” cf. moreana ( Buvignier, 1852); Middle Volgian (SOM: fig. 2A)

Figures

Fig. 11. Gryphaeid oyster Pernostrea? robusta sp. nov., Middle Volgian, Yatriya River, Subpolar Urals,. A. TsSGM 2068/37, interior of right valve. B. TsSGM 2068/36, interior of right valve. C. TsSGM 2068/35, interior of right valve. D. TsSGM 2068/38, interior of left valve attached to other oyster shell.

Fig. 7. Gryphaeid oyster Pernostrea mesezhnikovi sp. nov., Lower Volgian, Lopsiya River, Northern Urals. A. TsSGM 2048/17, holotype, interior of right valve (A1), exterior of left valve with attached oyster (A2). B. TsSGM 2048/24, paratype, interior of right valve. C. TsSGM 2068/33, paratype, interior of left valve.

Fig. 9. Gryphaeid oyster Pernostrea mesezhnikovi sp. nov., Lower Volgian, Lopsiya River, Northern Urals. A. TsSGM 2048/23, left (A1) and right (A2) valve views, view from posterior side (A3). B. TsSGM 2068/34, left (B1) and right (B2) valve views. C. TsSGM 2068/32, exterior (C1) and interior (C2) of left valve, view from anterior side (C3).

Fig. 10. Comparison of morphology Deltoideum vs. Pernostrea. A, C. Deltoideum delta (Smith, 1817) (labelled as “Ostrea deltoidea”), Kimmeridgian, La Hève, France. A. NHM 204314, interior of right valve. C. NHM 204314, interior of left valve. B, D. Pernostrea mesezhnikovi sp. nov., Lower Volgian, Lopsiya River, Northern Urals. B. TsSGM 2068/30, interior of right valve. D. TsSGM 2068/84, interior of left valve.

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.