Scalaricardita camaronesia, (Ihering, 1907)

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 : 76

Scalaricardia camaronesia ( Ihering, 1907)

Figure 6.10–13

1902 Cardita patagonica Sowerby—Ortmann , p. 128, pl. 36, figs 8a–c. v*1907 Venericardia camaronesia Ihering , p. 285, pl. 10, figs 69a, d.

v 2004 Pleuromeris camaronesia (Ihering) —del Río, Appendix I and II.

Type specimens. Holotype MACN-Pi 349, one left valve from Camarones (Chubut Province, Camarones Formation ) . Paratypes MACN-Pi 349, 78 right and 98 left valves from Camarones (Chubut Province, Camarones Formation ).

Diagnosis. Shell very small (around 10 mm in length) with a subtriangular outline, dorsal and posterior margins straight. Umbo nearly anteriorly placed. Right middle tooth very broad. Radial ribs (23 to 25) with constant width along the whole valve.

Description. Shell small-sized, outline subtriangular; posterior margin slightly truncated with gentle angle between it and ventral margin, dorsal margin slightly convex with pronounced slope from beaks to posterior end, ventral and anterior margins rounded. Umbo placed between middle and anterior third of valve length. Lunule large, elongate, flat and shallow, bounded from remaining surface of shell by an incised groove.

Right hinge with concave ventral edge below middle tooth; anterior tooth very small, short, elongate, inclined forward, in contact with lunular margin; middle tooth triangular, inclined posteriorly with broad base, with straight anterior and slightly convex posterior sides, higher towards base on lateral view; posterior tooth very thin and straight. Left hinge with slightly curved ventral edge; anterior tooth elongate, triangular, high, slightly inclined forward with straight anterior and posterior sides; posterior tooth elongate, straight, as high as anterior one.

External sculpture of 23 to 25 entire radial ribs, low, with constant width along the whole valve with subelliptic transverse section; covered with small subrectangular and closely-spaced and very strong nodes; very narrow and very shallow intercostal spaces with subtriangular transverse section, sometimes reduced to a groove. Pallial line placed about a quarter of total valve height. Inner ventral margin strongly crenulated, crenulations subrectangular, truncated, covering entire margin.

Remarks. Ihering (1907) introduced this species based on shells collected in Camarones, stating that valves described by Ortmann (1902) from the Santa Cruz River area as Cardita patagonica Sowerby could be representatives of Venericardia camaronesia . However, as discussed below, the specimen described by Ortmann (1902: pl. 36, figs 8a–c) belongs to Cyclocardia cannada ( Ihering, 1907) , and S. camaronesia turns out to be a species restricted to the Camarones Formation.

The subtriangular outline, small and subcentrally placed umbo, large and flat lunule, 23 to 27 entire and wide radial ribs with closely-spaced subrectangular nodes and narrow intercostal spaces place this species in Scalaricardita .

S. camaronesia was placed by Ihering (1907) in the genus Venericardia Lamarck, 1801 , but the European genus has a larger umbo, the hinge has an anterior tooth inclined posteriorly, a narrow right middle tooth and shell is sculptured by more numerous tripartite radial ribs. Del Río (2004) placed this species in Pleuromeris , but this genus has a more triangular outline, fewer radial ribs covered by more widely spaced nodes and separated by wider intercostal spaces.

Scalaricardita camaronesia differs from S. compacta ( Tate 1886: pl. 2, fig. 13) by its more triangular outline, straight posterior margin and more anteriorly placed umbo.

Scalaricardita camaronesia can be distinguished from S. subcompacta ( Ludbrook 1954: pl. 2, figs 5 and 8) by having umbos more anteriorly placed, dorsal and posterior margins straight and a wider right middle tooth.

The Patagonian species can be distinguished from S. miniscula ( Bartrum & Powell 1928: fig. 12) by having straight posterior margin, more numerous and lower radial ribs with a homogenous width over the entire valve.