Pernostrea mesezhnikovi, Kosenko, 2017

Kosenko, Igor N., 2017, Late Jurassic-Early Cretaceous oysters from Siberia: A systematic review, Acta Palaeontologica Polonica 62 (4), pp. 759-778 : 765-771

publication ID	https://doi.org/ 10.4202/app.00387.2017
persistent identifier	https://treatment.plazi.org/id/40598788-F46C-FFA2-FC8E-FCBB50AAFAAA
treatment provided by	Felipe
scientific name	Pernostrea mesezhnikovi
status	sp. nov.

Treatment

Pernostrea mesezhnikovi sp. nov.

Figs. 7–9 View Fig View Fig View Fig , 10B, D View Fig .

2014 Deltoideum delta ( Smith, 1817) ; Kosenko 2014: pl. 6: 4, 5, 7.

2014 Deltoideum View in CoL ex gr. delta ( Smith, 1817) ; Kosenko 2014: text-fig. 4, pl. 6: 6, 8.

2016 Deltoideum (Deltoideum) delta ( Smith, 1817) ; Kosenko 2016a: 342, pl. 3: 4a–c, 5, 7, 8, 11.

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

Etymology: After Mikhail S. Mesezhnikov (1931–1989), specialist on the boreal Jurassic stratigraphy.

Type material: Holotype, a complete adult specimen, both valves with exposed ligament areas, TsSGM 2048/17 ( Fig. 7A View Fig ). Paratypes: TsSGM 2048/17-1, 2048/17-2, 2048/23, 2048/24, 2068/28, 2068/29, 2068/30, 2068/32, 2068/33, 2068/34, 2068/84-1, 2068/84-2, 2068/85, 2068/86, 2068/87); from the type locality.

Type locality: Lopsiya River , outcrop 41, the Northern Urals (NorthWest of Western Siberia), Russia .

Type horizon: Eosphinctoceras magnum Zone , base of the Lower Volgian, horizon with large “ Ostrea ” (sensu Mesezhnikov 1959) .

Material.— Only type material.

Diagnosis.— Ligament area broad with resilifer subparallel to anterior and posterior bourrelets, PAM circular in a posterocentral position. Outer surface of valves covered by concentric growth squamae. RV with radial striae.

Description.— Large (maximum H = 170 mm, maximum L = 116 mm), usually equivalve ( Table 1). Outlines variable, including oval, deltoid, trapezoidal and subtriangular shapes, usually slightly elongated in height. Shell shape sometimes irregular in clusters. Attachment area commonly very large, sometimes cemented with entire LV. Rarely attachment areas are very small and in such a case the shells have convex LVs and flat RVs (see Fig. 9 View Fig ). Shell height to length ratio (H/L) varies 0.95–1.46 with average of 1.23 (based on measurements of 15 shells from one layer). LV and RV covered by concentric growth squamae and by weak radial striae. Ligament area ostreoid, well developed, very wide, trapezoidal, rare rectangular, with a straight or slightly undulating resilifer. LV resilifer deep, flanked by equally broad, elevated bourrelets, angle between resilifer and commissural plane around 10–30°. RV resilifer less deep than LV, cavity under RV resilifer absent. Ligament area length to shell length ratio (Lla/L) varies from 0.26 to 0.55 with average of 0.41 (based on measurements of 11 shells). PAM orbicular to oval; position usually subcentral or posterocentraland in LV usually buttressed. Quenstedt muscle scar displaced to posterior side of valve, its position corresponds to projection of posterior side of resilifer. No chomata. No chambers in shell microstructure. LV and RV with simply foliated structure.

Remarks.— The species has been found in the uppermost Kimmeridgian–lowermost Volgian clay of the Lopsiya River section in association with ammonites, belemnites and bivalves. According to Zakharov in Zakharov and Mesezhnikov (1974), this assemblage is autochtonous or paraautochtonous. The species is found usually in aggregates consisting of two shells. Individually grown specimens are characterized by inequivalve shells with a convex LV and a concave RV (morphotype “ Gryphaea -like recliner”, see Kosenko 2014: fig. 5).

Some species of Pernostrea and morphologically similar forms are discussed below in alphabetic order. Pernostrea luciensis ( Orbigny, 1850) ( Stenzel 1971: N1104, fig. J78: 1–3) is type species of the genus Pernostrea distributed in Bathonian–Callovian of France and England. It can be distinguished from P. mesezhnikovi sp. nov. by rectangular and circular outlines and broader ligament area (occasionally multivincular). Deltoideum delta ( Smith, 1817) ( Smith 1817: 18, pl. 5: 6; Arkell 1932: 149–155, text-figs. 22–26, pl. 16: 2–4, pl. 15: 6, pl. 14: 6; Fig. 10A, C View Fig ) is widely distributed in Kimmeridgian–Tithonian species. Furthermore, Bogdanova (1988: 147, pl. 23: 2, pl. 24: 1, pl. 25: 4, pl. 26: 12) described D. delta from the Berriassian of Mangyshlak (Central Asia). This species first time has been described from the Upper Oxfordian–Lower Kimmeridgian of England. It can be distinguished from P. mesezhnikovi sp. nov. by extremely flat both LV and RV, narrowing of shell near ligament area and narrow and usually strongly dorsally elongated shape of ligament area. P. cucurbita (Zakharov, 1966) (Zakharov 1966: 111–112, pl. 41: 1, 2, pl. 42: 1; SOM: fig. 4) from the Valanginian of north of Eastern Siberia (Khatanga depression). This species differs by a dorsoposterior position of PAM, a less wide ligament area and the absence of radial striae on RV. “ Ostrea ” expansa Sowerby, 1819 ( Sowerby 1819: 65, pl. 238: 1) was first described from the Tithonian of England. This species is also common in the Tithonian of northern France and was recently assigned to Helvetostrea by Koppka (2015). The species differentiates from P. mesezhnikovi sp. nov. by having a reniform shapeed PAM and a less developed ligament area. P. gibberosa (Zakharov, 1974) (Zakharov 1974: 147–148, pl. 31: 1, pl. 32: 1; SOM: fig. 2B, C) from the Lower–Middle Volgian of the Northern and the Subpolar Urals. This species can be distinguished by having a well-developed radial fold on the LV and a corresponding radial depression on its RV. Pernostrea? planoconvexa (Zakharov, 1974) (Zakharov 1974: 148, pl. 33: 1) from the Middle Volgian of the Subpolar Ural differs by an inequivalve shaped shell with a convex LV and a concave or flat RV. Pernostrea ? robusta sp. nov. ( Fig. 10 View Fig ) differs by a broader ligament area with a less deep resilifer and the strongly depressed, oval elongated, reniform PAM. P. uralensis (Zakharov, 1972) (Zakharov in Saks et al. 1972: 225–226, pl. 41: 1, pl. 42: 9, pl. 43: 5; SOM: fig. 1A) from the Upper Volgian–Ryazanian stage of the Northern and the Subpolar Urals. This species differs from P. mesezhnikovi sp. nov. by its isometric orbicular shell outlines, a central position of PAM and a low and broad ligament area. Some inequivalved specimens of P. mesezhnikovi sp. nov. with a prolonged height are similar to Helvetostrea and Crassostrea but differ from these genera by the absence of an umbonal cavity and structural chambers.

Stratigraphic and geographic range. —Upper Kimmeridgian–Lower Volgian of the Northern Urals (Lopsiya and Tolya rivers), the northwest of Western Siberia, Russia.