Pycnogryphaea, Kosenko, 2018

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

Genus Pycnogryphaea nov.

Etymology: Combination of two gryphaeid generic names Pycnodonte , and Gryphaea , since the new genus shares characteristics of both.

Type species: Gryphaea weberae Yanin in Tschelzova, 1969, Berriasian of Crimea , section Sarysu river .

Species included: Type species only.

Emended diagnosis.—As for the type species (by monotypy).

Description (microstructure).—Outer shell layer of left valve consists in furcate microstructure. Inner shell layer of left valve consists mainly in irregular complex crossed foliated microstructure with sublayers of simple regular foliated microstructure. Outer shell layer of right valve consists in prismatic microstructure. Inner shell layer of right valve consists in simple regular foliated and irregular complex crossed foliated microstructures with chomata influenced crossed foliated microstructure. Chambering absent. Vesicular microstructure absent.

Remarks.—Late Triassic to Late Jurassic (or even Early Cretaceous according to Zakharov 1966 and Kosenko 2017) Gryphaea Lamarck, 1801 , ( Gryphaeinae ) characterized by a gryphaeoid shell shape, orthogyrate beaked umbo, circular posterodorsally located posterior adductor muscle scar and radial grooves on the right valve, appears to be very similar to Pycnogryphaea . Pycnogryphaea differs from Gryphaea by chomata developed all along the circumferential curb. Due to the presence of chomata Pycnogryphaea is regarded here within the Pycnodonteinae . However, it differs from other genera of the Pycnodonteinae by the absence of vesicular structure in shell microstructure.

The Albian to Cenomanian Texigryphaea Stenzel, 1959 , appears to be similar to Pycnogryphaea by the lath chomata developed along the entire circumferential curb. However, Texigryphaea differs by commonly anterodorsally compressed left valve with median radial keel. Several genera of the Pycnodonteinae , such as Crenostrea Marwick, 1931 Paleogene of New Zealand), “ Pycnodonte ” gr. newberryi Stanton, 1893) (early Late Cretaceous of North America), Labrostrea Vialov, 1945 (Late Cretaceous–Paleogene, distributed worldwide), Phygraea Vialov, 1936 (Late Cretaceous–Paleogene, worldwide), and Costeina Vialov, 1936 Late Cretaceous of North America and Europe) had developed a gryphaeoid shell shape. However, all these taxa differ from Pycnogryphaea by the presence of a vesicular structure and chomata developed only near the ligament area. Another peculiar character of Pycnogryphaea is absence of simple herringbone regularly foliated structure typical for most Pycnodonteinae ( Aqrabawi 1993; Yakushin and Ivanov 2001).

The Late Jurassic Circunula Koppka, 2015 , questionably referred by Koppka (2015) to the Pycnodonteinae , recorded from the Oxfordian to Kimmeridgian of Switzerland and France, the Kimmeridgian of Germany and Crimea and the Tithonian of Czech Republic, differs from Pycnogryphaea by a large attachment area, the disappearance of chomata in adult growth stage and the non gryphaeoid shell shape.

Stratigraphic and geographic range.—Berriasian of Crimea.