Darwinicardia patagonica ( Sowerby, 1846 )

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 : 64-66

Darwinicardia patagonica ( Sowerby, 1846)

Figure 4.1–10

v*1846 Cardita patagonica Sowerby : p. 251, pl. 2, fig. 17.

.1887 Cardita inaequalis Philippi : p. 167, pl. 37, fig. 5.

1897 Cardita patagonica Sowerby—Ihering , p. 244–245.

1897 Cardita inaequalis Philippi—Ihering , p. 245.

1899 Cardita pseudopatagonica Ihering , p. 16.

1900 Cardita inaequalis Philippi—Ortmann , p. 380.

v1902 Cardita volckmanni Philippi—Ortmann , p. 126–127, pl. 26, figs. 7a–d. v1902 Cardita inaequalis (Philippi) —Ortmann, p. 127–128, pl. 26, figs. 6. v1907 Venericardia patagonica (Sowerby) —Ihering, p. 283–284, pl. 10, fig. 67a–b. 1907 Venericardia inaequalis (Philippi) —Ihering, p. 284.

1907 Venericardia pueyrredona Ihering , p. 286–287.

1978 Cardiocardita inaequalis (Philippi) —Malumián et al., p. 275, pl. 3, fig. 5. 1995 Venericardia inaequalis (Philippi) —del Río, p. 110, fig. 8: 1–5. v 2004 Fasciculicardia inaequalis (Philippi) —del Río, p. 2004, Appendix 1–2. 2008 Fasciculicardia patagonica (Philippi) —Griffin & Nielsen, p. 277, pl. 13, figs. 6–7.

Type specimens. Syntype BHNM-L 27955-6 , right and left valves from ‘ Santa Cruz’ (Santa Cruz Province, Monte León Formation ).

Other material. SGO.Pi 469 (holotype of Cardita inaequalis ); MACN-Pi 357–359, 362, 365 (holotype of Cardita pseudopatagonica ), 3578, 3583, 3602, 3611, 5768–5769, 5781–5782, 5803; CIRGEO-PI 2358, 2370, 2831, 2856, 2888; CPBA 6543, 8639–8640, 9328, 9364, 9373, 15946–15949, 15942–15960; MLP 1837, 1838, 10507, 18334; PRI.66324, 66402–66403, 66405–66406, 66408–66410, 66702 (116 left valves, 126 right valves, six articulated shells, 22 internal and one external moulds).

Diagnosis. Dorsal margin slightly convex with a pronounced slope from beaks to posterior end. Umbo large. Lunule deep and separated by a deep groove from remaining surface of shell. Radial ribs (23 to 26) lower and less developed towards ventral margin.

Description. Shell medium-sized, thick, subtriangular; anterior and ventral margins rounded; dorsal and posterior margins slightly convex, dorsal margin sharply sloping from beaks to posterior end; posterior area defined by a change in convexity starting from the sixth rib. Umbo large, convex, recurved, placed at anterior third of the valve length. Lunule medium-sized, deep and sharply separated by an incised groove from remaining surface of shell.

Right valve hinge with a ‘V’- shaped ventral edge below middle tooth; anterior tooth very small and curved, inclined posteriorly with broad base and with convex anterior and concave posterior sides; middle tooth triangular, inclined posteriorly, with broad base, 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 a slightly concave ventral edge; anterior tooth strong, large, rectangular, inclined posteriorly, with slightly concave anterior and straight posterior sides; posterior tooth elongate, curved with broad base and acute apex.

External sculpture of 23 to 26 high and tripartite radial ribs, with triangular transverse section and lower and less developed towards ventral margin; central costal cord covered with aligned nodes of subcircular transverse section; poorly developed smooth paracostal ribs, narrower than central cord; wide intercostal spaces with triangular transverse section; first six posterior ribs narrower and lower than the remainder. Pallial line at a quarter of total valve height. Inner ventral margin strongly crenulated; crenulations subrectangular, truncated, covering entire margin to top of posterior adductor muscle scar. Radial rib tips reaching above crenulations on internal view.

Remarks. Darwinicardia patagonica is one of the most common carditids recorded in the Patagonian Neogene and was the first to be described. Sowerby (1846) described and illustrated shells collected by Charles Darwin from Miocene beds at the mouth of the Santa Cruz River (Monte León Formation, Santa Cruz Province).

Philippi (1887) introduced Cardita inaequalis based on an internal cast coming from the same locality as Darwin’s specimens. He distinguished C. inaequalis by its smaller and less convex shell and its less pronounced umbo. Later, Ortmann (1900, 1902) separated those species by the better developed commarginal nodes in C. inaequalis . Ihering (1907) suggested that both species could be synonyms, an idea endorsed by Malumián et al. (1978), del Río (1995; 2004) and Griffin & Nielsen (2008). Del Río (1995) suggested sexual dimorphism within this species, which was supported by a morphometric geometrical analysis carried out by Pérez et al. (2017b).

This species was initially known from the lower Miocene of the mouth of Santa Cruz River ( Ihering 1897, 1907; Ortmann 1900, 1902) and middle Miocene of Tierra del Fuego ( Malumián et al. 1978), but new records expand its stratigraphical and geographica range into the lower Miocene of Chubut (Cerro Cabeza Papagayo, lower Chenque Formation) and the late Miocene of northern Patagonia (Barranca Final Formation, Río Negro Province).

Darwinicardia patagonica can be separated from D. angusticostata (Eocene, France) by having a less convex and steeply sloping dorsal margin, larger umbo, a deeper lunule, a higher number of radial ribs that are less pronounced towards ventral margin.