Cyclocardia cannada ( 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 : 57-59

Cyclocardia cannada ( Ihering, 1907)

Figure 3.1–3

.1902 Cardita patagonica Sowerby—Ortmann , p. 128, pl. 26, figs. 8a–c. v*1907 Venericardia cannada Ihering , p. 287, pl. 10, figs. 71a–f.

. 1907 Venericardia camaronesia var. cruzensis Ihering , p. 285.

v 2004 Pleuromeris cannada (Ihering) —del Río, p. 1102, Appendix 1–2. v. 2004 Pleuromeris cruzensis (Ihering) —del Río, figs. 10.11, 10.12, Appendix 1–2.

Type specimen. Holotype MACN-Pi 344, one left valve from Cañadón de los Artilleros (Santa Cruz Province, San Julián Formation ).

! + * Cyclocardia Darwinicardi * * Fasciculicardia Kolmeris * * Pleuromeris Purpurocardia Scalaricardita

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"! " " Other material. MACN-Pi 344–348, 5772, 5778–5780; CPBA 9373, PRI.66848 (lectotype of Venericardia camaronesia var. cruzensis ) (35 left valves, 68 right valves and one articulated shell).

Diagnosis. Shell with rounded anterior and posterior margins. Posterior area faint, defined by 5–6 radial ribs. Umbo placed near to anterior third of valve length. Lunule shallow. Right anterior tooth very reduced and middle tooth triangular with broad base. Left anterior tooth triangular and broad. Radial ribs (23 to 27) smooth or with weakly developed nodes, transverse section of ribs subelliptic.

Description. Shell small to medium-sized with subcircular outline; anterior, posterior and ventral margins rounded; dorsal margin slightly convex; posterior area faint and defined by 5–6 radial ribs slightly narrower than the remainder. Umbo placed at anterior third of valve length. Lunule small, shallow, flat, longer than wide and bounded from remaining surface of shell by a groove.

Right valve hinge with asymmetrically concave ventral edge; very small and laminar anterior tooth, straight, inclined forward; middle tooth triangular, with broad base, inclined posteriorly, slightly concave anterior and flat posterior sides, and higher towards apex on lateral view; posterior tooth very thin, slightly curved and shorter than middle tooth. Left valve hinge with a straight 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, wide and inclined posteriorly.

External sculpture of 23 to 27 wide and low radial ribs with subelliptic transverse section, smooth or covered with weakly developed nodes; narrow intercostal spaces with triangular transverse section. Pallial line one- fifth of total valve height. Inner ventral margin strongly crenulated; crenulations subrectangular, truncated, covering entire margin.

Remarks. Specimens of C. cannada display strong variation in shell outline and sculpture. While the outline varies from slightly subtriangular juvenile to subcircular adult shell, the sculpture can also vary from smooth radial ribs with weakly developed nodes.

Ihering (1907) placed this species in Venericardia Lamarck, 1801 . Later, del Río (2004) placed it in Pleuromeris Conrad, 1867 , but the small shell with subcircular outline, entire radial ribs and wide hinge teeth in C. cannada allow its inclusion in Cyclocardia .

Ihering (1907) also introduced Venericardia camaronesia var. cruzensis based on specimens illustrated by Ortmann (1902, pl. 36, figs. 8a–o) from the mouth of Santa Cruz River (Monte León Formation, Santa Cruz Province). However, Ortmann’s specimens (1902) belong in C. cannada according to their outline, external sculpture and hinge features. For this reason, Venericardia camaronesia var. cruzensis is here considered a junior synonym of C. cannada .

We include in this species valves from Lago Cardiel (lower Miocene, ‘Centinela’ Formation) and extend the geographic range of the taxon.

Cyclocardia cannada belongs in Cyclocardia because the shell has the umbo placed near to anterior third of valve length, wide hinge teeth with the right anterior tooth small, right middle tooth curved and broad and left anterior tooth large, and because it has 23 to 27 wide and low radial ribs.

Cyclocardia cannada resembles C. granulata ( Say, 1824) (Miocene–Pleistocene of USA) ( Ward & Blackwelder 1987: pl. 15, figs. 4–7) because of its outline and size. However, the former species has rounded posterior and anterior margins, a more reduced right anterior tooth, a more triangular and broader left anterior tooth, and the radial ribs have a subelliptic transverse section, being covered by weakly developed nodes.

Cyclocardia cannada shares with C. moniliata ( Dall, 1902) (Recent, Brazil) ( Huber 2010: p. 253) the presence of rounded anterior and posterior margins and hinge features. The first species has a larger shell, a posterior area, shallow lunule and radial ribs lacking nodes and with subelliptic a transverse section.