Darwinicardia, Pérez & Del Río, 2017

Pérez, Damián E. & Del Río, Claudia J., 2017, Systematics of the family Carditidae (Bivalvia: Archiheterodonta) in the Cenozoic of Argentina, Zootaxa 4338 (1), pp. 51-84 : 63

Genus Darwinicardia new genus

Type species. Cardita patagonica G. B. Sowerby I, 1846 (pl. 2, fig. 17). San Julián , Monte León , Carmen Silva and Barranca Final formations, upper Oligocene–upper Miocene, Argentina.

Diagnosis. Carditid with subtriangular outline and recurved umbo. Posterior area defined by a change in convexity starting from the sixth rib. Right valve hinge with ventral edge ‘V’-inverted shaped. Right anterior tooth curved and left anterior tooth broad. External sculpture of 22 to 26 tripartite and high radial ribs with subtriangular transverse section covered with small subcircular nodes. Radial rib tips raising above crenulations on internal view.

Included species. Darwinicardia angusticostata ( Deshayes, 1824) (Eocene, Paris Basin, France).

Derivation of name. The name honours Charles Darwin, who collected the first specimen of the type species, followed by the suffix ‘- cardia ’, that means ‘heart’ (in reference to its outline when viewed anteriorly); the suffix is often used for carditid genera.

Remarks. This new genus is placed along with the members of the alticostate group of carditids ( Harris 1919; Heaslip 1968), which are characterized by the presence of tripartite radial ribs usually covered with nodes, scales or tubercles and narrow hinges.

Darwinicardia resembles Glyptoactis Stewart, 1930 (type species Venericardia hadra Dall, 1903 , Miocene of Florida, USA) ( Heaslip 1968: pl. 27–29) by having a similar outline and sculpture. However, Darwinicardia has a stronger and rectangular left anterior tooth, higher radial ribs with weaker paracostal ribs separated by wider intercostal spaces with a subtriangular transverse section.

Darwinicardia differs from Venericardia Lamarck, 1801 by its subtriangular outline, umbo more convex and recurved, a larger lunule, a smaller number of higher radial ribs with weaker paracostal ribs separated by wider intercostal spaces and covered by smaller comarginal nodes.

The shell outline and external sculpture of Darwinicardia resemble those of Fasciculicardia Maxwell, 1969 (type species Venericardia subintermedia Suter, 1917 : lower–middle Miocene of New Zealand) ( Maxwell 1969: pl. 23, fig. g) but the new genus has a right anterior tooth with a very convex anterior side, a stronger left anterior tooth, a larger lunule and a smaller number of radial ribs, standing out from crenulations on internal view.

Darwinicardia can be distinguished from Cardites Link, 1807 (type species Chama antiquata Linnaeus, 1758 ; Miocene–Recent of Europe and North Africa) by having a right anterior tooth, higher tripartite radial ribs standing above crenulations on internal view and separated by wider intercostal spaces.

Darwinicardia differs from Cardiocardita Anton, 1839 (type species Cardita ajar Bruguière, 1792 ; Recent, West Africa) by having a more clearly subtriangular outline, umbo more convex and recurved, a larger lunule, a right valve hinge with ‘V’- shaped ventral edge, and higher radial ribs with weaker paracostal ribs separated by wider intercostal spaces.

We also include in this new genus the species Venericardia angusticostata Deshayes, 1825 (= V. granulata Defrance, 1828 ) (Eocene, Paris Basin, France). Pacaud & Le Renard (1995) placed the species in Venericardia . However, the subtriangular outline, convex umbo, the posterior area defined by a change in convexity starting from the sixth rib, the right valve hinge with a ‘V’- shaped ventral edge, the strong and rectangular left anterior tooth and the 22 to 23 tripartite and high radial ribs covered by small subcircular nodes and intercostal spaces with transverse section places this species in the new genus Darwinicardia .