Pycnogryphaea weberae

Kosenko, Igor N., 2018, The origin of the Pycnodonteinae and relationship between gryphaeas and true pycnodontes, Acta Palaeontologica Polonica 63 (4), pp. 769-778 : 772-774

publication ID	https://doi.org/ 10.4202/app.00494.2018
publication LSID	lsid:zoobank.org:pub:1B95E3C0-3368-4C88-B80B-62110A6A0940
persistent identifier	https://treatment.plazi.org/id/03D19639-FFC1-FFD3-FC94-FDA5FA58F821
treatment provided by	Felipe
scientific name	Pycnogryphaea weberae
status

Treatment

Pycnogryphaea weberae View in CoL (Yanin in Tschelzova, 1969)

Fig. 4.

1969 Gryphaea weberae Yanin View in CoL sp. nov.; Tschelzova 1969: 56, pl. 4: 6, 7–8 (microstructure).

1997 Pycnodonte weberae (Yanin in Tschelzova, 1969); Arkadiev et al. 1997: 78, pl. 18: 8.

2012 Pycnodonte weberae (Yanin in Tschelzova, 1969); Arkadiev et al. 2012: 251, pl. 43: 16.

Type material: Holotype CP MSU 331 View Materials /16 ( Tschelzova 1969: pl. 4: 6). Type locality: Central Crimea , sections Sarysu river and Balki village. Type horizon: Middle Berriasian.

Material.—11 left valves and 4 right valves, section Sarysu river, Pseudosubplanites grandis Zone ; 15 left valves, section Solnechnoselye village, beds with Malbosiceras chaperi ; 53 left valves and 22 right valves, section Balki village, Dalmasiceras tauricum Zone ; 1 juvenile left valve and 2 right valves, section Chernokamenka village, lower part of the Riasanites crassicostatum Subzone ; 1 left valve and 5 right valves, section Balki village, beds with Symphythyris arguinensis ; 1 left valve and 3 right valves, section Kuchki village, beds with Symphythyris arguinensis ; 1 left valve and 3 right valves, section Mezhgorye village, beds with Zeillerina baksanensis (base of the Berriasella callipso Subzone to the Berriasian–Valanginian boundary). All material is housed in the Central Siberian Geological Museum, and referenced as collection TsSGM 2088 (TsSGM 2088/1–110).

Emended diagnosis.—Shell medium-sized, strongly inequivalve and thick. Left valve gryphaeoid, strongly convex with well-defined posterior sulcus and posterior flange, right valve flat or slightly concave. Outlines triangular or subtriangular. Attachment area very small. Left valve umbo prominent, rising well above hinge line. Both valves with concentric growth squamae, right valve with weak antimarginal grooves. Left valve lacks umbonal cavity. Ligament area triangular with triangular resilifer. Posterior adductor scar located posterodorsally, rounded (gryphaeate). Quenstedt muscle scar well distinguishable on right valve,

Fig. 4. Pycnodontein oyster Pycnogryphaea weberae (Yanin in Tschelzova, 1969) from Berriasian of Crimea, section Balki village, Dalmasiceras tauri- → cum Subzone (A, C, D, F–L) and Mezhgorye village, beds with Zeillerina baksanensis (B, E). A. TsSGM 2088/28, interior of right valve (A 3); elongated chomata near the hinge (A 1, A 2), simple lath chomata near the ventral margin of right valve (A 4). B. TsSGM 2088/29, interior of right valve. C. TsSGM 2088/30, interior of right valve. D. TsSGM 2088/31, exterior of right valve. E. TsSGM 2088/34, interior of right valve. F. TsSGM 2088/32, interior of right valve. G. TsSGM 2088/3, exterior of left valve. H. TsSGM 2088/33, interior of left valve. I. TsSGM 2088/14, interior of left valve. J. TsSGM 2088/15, exterior of left valve (J 1) and view from the posterior side (J 2). K. TsSGM 2088/5, exterior of left valve. L. TsSGM 2088/2, exterior of left valve (L 1) and view from the anterior side (L 2). Scale bars 10 mm.

located ventrally from resilifer. Commissural shelf narrow, well-defined, delimited by circumferential curb. Chomata developed all along the circumferential curb. Elongated chomata located near ligament area, simple lath chomata developed below elongated chomata.

Description.—Left valve gryphaeoid, moderately to highly convex (convexity coefficient Conv/L in studied samples varies from 0.36 to 0.80; see Fig. 5C View Fig ), triangular to oval in outline, sometimes with opisthogyrate beak, larger than the right valve; right valve flat or slightly concave. Examined specimens up to 66 mm high and up to 72 mm long ( Fig.5A View Fig ); elongation coefficient (L/H) varies in studied sample from 0.66 to 1.22 ( Fig. 5B View Fig ). Left valve covered by commarginal growth lines; well-defined posterior sulcus. Attachment area very small (diameter up to 10 mm in shell with 42 mm high and 47 mm long; usually not distinguishable). Right valve with well-defined commarginal growth lines and antimarginal grooves.

Ligament area: small, triangular; initially prosogyrate in some specimens; resilifer triangular, weakly concave with slightly convex bourrelets on both valves, less elevated on the right valve. Plane of ligament area depends on the degree of beak development: from almost perpendicular to the commissure plane in shells with a well-developed beak, to subparallel to the commissure plane in shells having a non-developed beak.

Internal shell characters: Umbonal cavity absent. Posterior adductor muscle rounded, relatively small, posterodorsal. Quenstedt muscle scar well distinguishable on the right valve, approximately located approximately under the resilifer (Fig. 4A–C, E, F). Commissural shelf narrow, well-defined, delimited by the circumferential curb. Chomata developed on commissural shelf all along the circumferential curb. Elongated chomata located near the ligament area, simple lath chomata developed below the elongated chomata.

Microstructure: Outer shell layer of left valve characterized by furcate microstructure (around 300 μm) ( Fig. 6D View Fig ). Inner shell layer of left valve characterized by different types of irregular complex crossed foliated microstructure: homogeneous to “mosaic” irregular complex crossed foliated ( Fig. 6D View Fig ), low angle irregular complex crossed foliated ( Fig. 6B, E, F View Fig ) and dominated by high angle irregular complex crossed foliated ( Figs. 6C, C, E View Fig , 7A View Fig 2 View Fig ). Myostracum of left valve with irregular simple prismatic structure ( Fig. 7A View Fig 2 View Fig ). Outer layer of right valve consists of coarse simple prismatic microstructure (around 100 μm) ( Fig. 7B View Fig 3 View Fig ). Inner layer consists of simple regular foliated microstructure ( Fig. 7A View Fig 3 View Fig , A 4, B 2, B 3) with areas of chomata-influenced crossed foliated microstructure ( Fig. 7B View Fig 2 View Fig ) lapsing to low angle irregular complex crossed foliated ( Fig. 7A View Fig 3 View Fig , A 4); inflated parts of right valve consist of high angle irregular complex crossed foliated ( Fig. 7A View Fig 4).

Remarks. — The species Pycnogryphaea weberae was for the first time described by Yanin in the book by Tschelzova (1969), and was attributed to the genus Gryphaea . Tschelzova (1969) studied the microstructure of Pycnogryphaea weberae and showed that inner shell layer was defined by a regular foliated microstructure with rare empty chambers and subrhombic microstructure, which includes various types of irregular complex crossed foliated microstructure. Furcate microstructure was observed on the outer shell layer. Such microstructures are typical for the genus Gryphaea , and the species was included into Gryphaea by Yanin (in Tschelzova 1969). The current study of microstructure of Pycnogryphaea weberae agrees with the original description of Tschelzova (1969) ( Figs. 6 View Fig , 7 View Fig ). The presence of chomata mentioned on its original description (Yanin in Tschelzova, 1969), later encouraged Tamara N. Bogdanova (Arkadiev et al. 1997) and Boris T. Yanin (Arkadiev et al. 2012) to assign this species to the genus Pycnodonte .