Cyclocardia

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 : 61-63

‘ Cyclocardia ’ nortensis (del Río, 1986)

Figure 3.10, 12–13

v*1986 Venericardia nortensis del Río, p. 115, pl. 1, fig. 3.

v1994 Venericardia (‘ Cyclocardia ’) nortensis (del Río)—del Río, p. 104–105, pl. 1, fig. 10.

Type specimens. Holotype CPBA 13242, one left valve. Paratype CPBA 13247 , one right valve. All from Punta Norte (Chubut Province, Puerto Madryn Formation ).

Other material. CPBA 9326, 13223–13241, 13243–13246, 13248–13273, MLP 1883 (26 left valves, 43 right valves and one articulated shell).

Diagnosis. Umbo placed at the anterior third of valve. Right anterior tooth vertical or inclined posteriorly. Left anterior tooth triangular, thin, in contact with lunular margin. Radial ribs (21 to 22) covered by weakly developed nodes subrectangular in cross-section and separated by intercostal spaces with subrectangular transverse section (modified from del Río 1986: p. 115).

Description. Shell medium-sized with subcircular outline; anterior and ventral margins rounded, posterior margin slightly truncated and dorsal margin very convex. Umbo placed at anterior third of valve length. Lunule small, deep, flat, vertical or slightly inclined posteriorly, longer than wide, separated by a groove with sculpture of fine commarginal growth lines.

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

External sculpture of 21 to 24 wide and entire radial ribs, low, with subelliptic transverse section, becoming obsolete towards ventral margin, covered with weakly developed and subrectangular nodes; narrow and shallow intercostal spaces with subrectangular transverse section. Pallial line one-fifth of total valve height. Inner ventral margin strongly crenulated; crenulations subrectangular, truncated, covering entire margin to ventral edge of posterior adductor muscle scar, vanishing from here.

Remarks. Del Río (1986) related ‘ Cyclocardia ’ nortensis to the New Zealand ‘ awamoensis ’ group ( Maxwell 1969; Beu & Maxwell 1990) and may thus represent a lineage separate from Cyclocardia s.s. ( Maxwell 1969). It shares with that group the posteriorly truncated shell, prominent umbo, small lunule and radial ribs becoming obsolete towards the ventral margin.

‘ Cyclocardia ’ nortensis differs from ‘ C. ’ awamoensis ( Harris, 1897) (lower Miocene, New Zealand) ( Beu & Maxwell 1990: pl. 17, figs. h,i) by having a smaller shell, umbo placed at the anterior third of valve length, lunule more vertical, left anterior tooth thinner, more numerous radial ribs with nodes weakly developed and intercostal spaces narrower and with subrectangular transverse section.

‘ Cyclocardia ’ nortensis can be separated from ‘ C. ’ christei ( Marwick, 1929) (upper Oligocene, New Zealand) ( Beu & Maxwell 1990: pl. 10, figs. g, i) by having a smaller shell, left anterior tooth more triangular, thinner and in contact with the lunular margin. Also, its intercostal spaces have a subrectangular transverse section.

‘ Cyclocardia ’ nortensis differs from ‘ C. ’ marama Maxwell, 1969 (upper Oligocene, New Zealand) ( Maxwell 1969: pl. 3, figs. 35–37) by having a more rounded umbo placed at the anterior third of valve length, left anterior tooth in contact with lunular margin and radial ribs covered by weakly developed nodes.

‘ Cyclocardia ’ nortensis can be distinguished from ‘ C. ’ pseutella ( Marwick, 1929) (upper Oligocene, New Zealand) ( Marwick 1929: figs. 29, 31) by its smaller shell, lunule more vertical, left anterior tooth in contact with lunular margin and a smaller number of radial ribs separated by intercostal spaces with subrectangular transverse section.