Pleuromeris sulcolunularis ( 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 : 71-72

Pleuromeris sulcolunularis ( Ihering, 1907)

Figure 5.10–14

v*1907 Venericardia sulcolunularis Ihering , p. 285–286, pl. 10, fig. 70a–f. 1996 Venericardia sulcolunularis Ihering—Parodiz p. 271.

v 2004 Pleuromeris sulculonularis (Ihering) —del Río, Appendix 1–2.

Type specimens. Holotype MACN-Pi 367, right valve from Bajo de las Flechas (Santa Cruz Province, San Julián Formation ) . Paratype MACN-Pi 366, two left, three right valves and an articulated shell from Cabo Tres Puntas (Santa Cruz Province, San Julián Formation ).

Other material. MACN-Pi 368–370, 5773–5777, PRI.66398 (16 left valves, 18 right valves, and two articulated shells).

Diagnosis. Shell thick and very convex with very recurved umbo. Lunule very deep and strongly concave, delimited by a deep groove. Left anterior tooth broad and inclined posteriorly. High radial ribs (18 to 20) separated by wide intercostal spaces.

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

Right valve hinge with a 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 broad base, slightly concave anterior and gently convex posterior sides, higher towards apex on lateral view; posterior tooth thin and slightly curved. Left valve hinge with a slightly concave ventral edge; anterior tooth strong, broad, triangular, slightly inclined posteriorly, with straight anterior and posterior sides and higher towards apex on lateral view; posterior tooth elongate, broad, and slightly curved.

External sculpture of 18 to 20 entire radial ribs, high, narrow, with triangular transverse section, widening towards ventral margin; covered with closely-spaced nodes of subrectangular transverse section; intercostal spaces as wide as radial ribs with ‘V’-shaped transverse section. Pallial line one-fifth of total valve height. Inner ventral margin strongly crenulated; crenulations subtriangular, truncated, covering entire margin to posterior adductor muscle scar.

Remarks. Ihering (1907) and Parodiz (1996) placed this species in Venericardia , and del Río (2004) in Pleuromeris without discussion. Because of its small-sized shell, subtriangular outline, large and wide lunule, wide right middle tooth and the presence of 18 to 20 radial ribs, P. sulcolunularis should be assigned to Pleuromeris .

Ihering (1907) stated that P. sulcolunularis is related to P. volckmanni ( Frassinetti & Covacevich 1995; pl. 1, figs 10–17, Miocene, Navidad Formation, Chile) but it differs from the Chilean species by having a less recurved umbo, more concave and deeper lunule and narrower intercostal spaces.

Pleuromeris sulcolunularis can be distinguished from P. decemcostata , the type species of the genus ( Gardner 1943, pl. 13, figs 1–4, lower Pliocene–lower Pleistocene, USA), by its thicker and more convex shell, a more concave lunule, left anterior tooth inclined posteriorly and by having a higher number of radial ribs.

Pleuromeris sulcolunularis differs from P. tridentata ( Say, 1826) ( Conrad 1838, pl. 43, fig. 11, Pliocene– Recent, USA) by the presence of more convex shell with a larger and recurved umbo, a concave instead of a flat lunule, left anterior tooth broader and inclined posteriorly and a higher number of radial ribs set apart by wider intercostal spaces.

Pleuromeris sulcolunularis can be separated from P. zelandica ( Deshayes, 1854) ( Huber 2010, p. 255, lower Pliocene–Recent, New Zealand) because of its more convex shell, larger and more recurved umbo, deeper and more concave lunule and left anterior tooth inclined posteriorly.

This species can be distinguished from P. fueguina ( Steinmann & Wilckens, 1908) by having a deeper and more concave lunule, broader left anterior tooth and taller radial ribs.