Pleuromeris fueguina ( Steinmann & Wilckens, 1908 ), 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 : 70-71

Pleuromeris fueguina ( Steinmann & Wilckens, 1908)

Figure 5.7–9

v*1908 Cardita elegantoides var. fueguina Steinmann & Wilckens , p. 39–41, pl. 3, figs 5a–c, pl. 4, figs 2a–b. 1978 Pleuromeris elegantoides (Ortmann) —Malumián et al., p. 275, pl. 4, figs 6–8.

Type specimens. Holotype PZ-NHRM Mo. 117903, right and left valves from Sierra Carmen Silva (Tierra del Fuego Province, Carmen Silva Formation ).

Other material. MACN-Pi 2619, 3747, 3748, CIRGEO-PI 2382, 2877, CPBA 9349 (35 left valves, 30 right valves, three articulated shells and one internal mould).

Diagnosis. Shell slightly convex with slightly recurved umbo. Lunule flat. Left anterior tooth narrow and inclined posteriorly. Low radial ribs (18 to 21).

Description. Shell small-sized (less than 30 mm in length), slightly convex, subtriangular; dorsal and ventral margins straight and posterior and anterior ones slightly rounded. Umbo large, rounded, slightly recurved, placed between first and second anterior third of valve length. Lunule large, flat, vertical, bounded from remaining surface of shell by an incised groove.

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

External sculpture of 18 to 21 entire radial ribs, low, narrow with triangular transverse section, widening towards ventral margin; covered with closely-spaced nodes with subrectangular to subquadrate transverse section; wide ‘V’-shaped intercostal spaces of. Pallial line one-fifth of total valve height. Inner ventral margin strongly crenulated, crenulations subrectangular, truncated, covering entire margin to posterior adductor muscle scar.

Remarks. The original Cardita elegantoides Ortmann, 1899 , from the Eocene of the Loreto Formation in Chile, is here described under Purpurocardia . Steinmann & Wilckens (1908) introduced P. fueguina as a variety of this species, pointing out that illustrations provided by Ortmann (1902, pl. 26, fig. 5a) show differences between the specimens from Santa Cruz and Tierra del Fuego. Ortmann illustrated the holotype of C. elegantoides from Punta Arenas, and of other shells collected in the Santa Cruz River area on plate 26, figures 5b–c (herein described as Cyclocardia dalek n. sp.).

Pleuromeris fueguina is placed in this genus because of its small-sized shell with subtriangular outline, large and wide lunule, broad right middle tooth and the development of 18 to 21 radial ribs covered with nodes.

The species can be distinguished from P. decemcostata by the development of a flat lunule, left anterior tooth inclined posteriorly and a higher number of radial ribs.

It is separated from P. tridentata by having a larger and less recurved umbo, narrower and inclined posteriorly left anterior tooth and a higher number of radial ribs.

Pleuromeris fueguina differs from P. zelandica by its more convex shell, more recurved umbo and left anterior tooth inclined posteriorly.