Cyclocardia dalek, 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 : 60-61

Cyclocardia dalek new species

Figure 3.7–9, 11

1907 Venericardia elegantoides (Ortmann) —Ihering, p. 284, pl. 14, fig. 98b.

Derivation of name. The specific name refers to Daleks, main villains of the “Doctor Who” TV series, characterized by an armour with prominent circles, similar to the nodular external sculpture of this species.

Type specimens. Holotype MACN-Pi 5770, one left valve. Paratypes MACN-Pi 5771, four right and three left valves. All from Playa La Mina (Santa Cruz Province, San Julián Formation ).

Other material. MACN-Pi 355; PRI.66396–66397, 66399 and 66407 (20 left and 31 right valves).

Diagnosis. Shell with subcircular to subtriangular outline. Right anterior tooth in contact with lunular margin. Left valve hinge with slightly concave ventral edge. Radial ribs (21 to 24) wide with well-developed nodes; intercostal spaces narrow and shallow with subrectangular transverse section.

Description. Shell small to medium-sized with subcircular to subtriangular outline; posterior, anterior and ventral margins rounded, dorsal margin slightly convex and elongate. Umbo placed at anterior third of the valve length. Lunule small, shallow, flat, longer than wide and bounded from remaining surface of shell by a groove.

Right valve hinge with slightly concave ventral edge; anterior tooth very small and laminar, straight, inclined forward, in contact with lunular margin; middle tooth triangular, with broad base, inclined posteriorly, slightly concave anterior and flat posterior sides, and higher towards apex on lateral view; posterior tooth thin and slightly curved. Left valve hinge with slightly concave ventral edge; anterior tooth triangular, elongate, wide, straight, slightly inclined posteriorly, higher towards base on lateral view, with flat anterior and posterior sides; posterior tooth elongate, high and inclined posteriorly.

External sculpture of 21 to 24 wide and low radial ribs with a subelliptic transverse section and covered with well-developed subcircular to subrectangular nodes; narrow and shallow intercostal spaces with a subrectangular transverse section; last 5 or 6 posterior radial ribs narrower than the remainder. Pallial line reaching between onequarter and one-fifth of total valve height. Inner ventral margin strongly crenulated; crenulations subrectangular, truncated, covering entire margin.

Remarks. Ortmann (1899) described Cardita elegantoides based on six shells collected by J. Hatcher from Punta Arenas (upper Eocene, Loreto Formation, Chile). He later also considered material from Monte Observación and the mouth of the Santa Cruz River (lower Miocene, Monte León Formation, Santa Cruz Province) as belonging in that species ( Ortmann 1902: pl. 26, figs. 5b–c). However, Ihering (1907) stated that material illustrated by Ortmann from the Santa Cruz River actually belongs to a different species. But, rather than proposing a new name for the Santa Cruz River material, he proposed Venericardia sylva for shells from Punta Arenas ( Ortmann 1902; pl. 26, fig. 5a). This single valve is the holotype of C. elegantoides (PRI.66394) designated by Ortmann (1899) and, therefore, V. sylva is a junior synonym of C. elegantoides . In this revision we agree with the criteria used by Ihering and propose a new species name for the Santa Cruz specimens. Ihering (1907) also mentioned shells of Ortmann’s species from Cañadón de los Artilleros (MACN-Pi 354) but this material is apparently lost.

Cyclocardia dalek n. sp. belongs in Cyclocardia because of the rounded umbo, entire, low and wide radial ribs and its broad hinge teeth with a very small right anterior tooth and a strongly curved right middle tooth.

Cyclocardia dalek n. sp. differs from C. cannada by having a subcircular to subtriangular outline, more pronounced hinge teeth, right anterior tooth in contact with lunular margin, left hinge with concave ventral edge, less and wider radial ribs covered by well-developed nodes.

Cyclocardia dalek n. sp. can be separated from C. granulata ( Say, 1824) (Miocene–Pleistocene of USA) ( Ward & Blackwelder 1987: pl. 15, figs. 4–7) by having more pronounced hinge teeth, left hinge with slightly concave ventral edge, less radial ribs with more strongly developed nodes.