Pliocardia, Woodring, 1925

Amano, Kazutaka, Saether, Kristian P., Little, Crispin T. S. & Campbell, Kathleen A., 2014, Fossil vesicomyid bivalves from Miocene hydrocarbon seep sites, North Island, New Zealand, Acta Palaeontologica Polonica 59 (2), pp. 421-428 : 423-424

Pliocardia ? sp.

Fig. 2A–C .

Material. — Four specimens from Moonlight North (locality Y16/f1059, collection AU19982 , specimen numbers UOA L4587 , UOA L4588 , UOA L4589 , UOA L4590 ); Lower to Middle Miocene, Bexhaven Limestone .

Description. —Shell medium size (L up to 36.2 mm), quadrate-ovate (H/L = 0.71–0.81), moderately inflated (W/L = 0.50–0.59), shell material partly to wholly absent in all specimens; surface sculpture unknown except for faint growth lines. Blunt ridge running from beak to postero-ventral corner. Beak prominent, prosogyrate, situated anteriorly at 24–27% along shell length from anterior margin.Antero-dorsal margin short and broadly arcuate; postero-dorsal margin straight to gently convex, continuing to subtruncated posterior margin; ventral margin usually moderately convex. Lunular incision weakly visible in dorsal view; large, deep grooves flanking central ligamental area along postero-dorsal margin, suggesting ligament external. Hinge plate of right valve narrow, with very thin ventral tooth (1); rather strong posterior tooth (3b); anterior cardinal tooth (3a?) obscure. Anterior adductor muscle scar ovate; posterior muscle scar subquadrate. Pallial sinus indistinct and shallow.

Dimensions.—See Table 2. Discussion. —From its size, blunt ridge, lunular incision, right valve hinge dentition and pallial sinus, this species probably represents a new species in the genus Pliocardia Woodring, 1925 , which was redefined by Krylova and Janssen (2006). However, until more details are known about the external shell and the left valve dentition we refrain from naming a new species.

Our species resembles Pliocardia kawadai ( Aoki, 1954) from the Lower to Middle Miocene in Japan (see Amano and Kiel 2012) in having a similar size and outline. However, P. kawadai has more inflated valves, a radial depression, and a V-shaped pallial sinus.

Genus Notocalyptogena nov.

Etymology: A combination of Greek notos, south and the genus Calyptogena .

Type species: Notocalyptogena neozelandica sp. nov.; see below. Species included: Only type species.

Diagnosis.—Shell medium size, moderately inflated, elongate; sculpture smooth except for growth lines; blunt external ridge running from beak to posterior corner. Radial internal ridge distinct, running from beak to postero-ventral corner and in contact with posterior adductor scar. Hinge plate narrow. Subumbonal pit absent. Hinge of right valve with three cardinal teeth; anterior tooth (3a) along postero-dorsal margin; middle tooth (1) strong, oblique anteriorly; posterior tooth (3b) vertical or slightly anteriorly inclined; deeply depressed or flat area behind posterior cardinal tooth. Hinge of left valve with three cardinal teeth; anterior tooth (2a) long, connecting with middle stout tooth (2b); posterior tooth (4b) also rather stout. Pallial line entire.

Discussion.— Notocalyptogena gen. et sp. nov. closely resembles Calyptogena Dall, 1891 in its size, elongate outline, lack of subumbonal pit, and integripalliate condition. However, this new genus differs from Calyptogena by having a narrow hinge plate without a U-shaped tooth overhanging a ventral tooth, and by having a depressed or flat area behind the 3b tooth in the right valve. Moreover, the strong internal ridge from the beak to posterior ventral corner of this new genus is not seen in Calyptogena . Notocalyptogena gen. nov. shares a narrow hinge plate, elongate shell shape, lack of subumbonal pit and an entire pallial line with Christineconcha Krylova and Cosel, 2011 . However, the right valve hinge of Christineconcha has a much shorter 3a tooth and teeth that radiate less around the umbo. Elenaconcha Cosel and Olu, 2009 can be easily distinguished from Notocalyptogena gen. nov. by having a lunule, a subumbonal pit and multiple “posterior nymphal ridges”. Although Hubertschenckia Takeda, 1953 has a similar arrangement of cardinal teeth to Notocalyptogena gen. nov., the former genus also has a subumbonal pit and pallial sinus, which are never seen in the new genus. Adulomya Kuroda, 1931 shares an elongate shell and a lack of a subumbonal pit with Notocalyptogena gen. nov. However, Adulomya differs from the new genus by having two radiating cardinal teeth in the right valve.

Stratigraphic and geographic range. —Lower Miocene Ihungia Limestone and Lower to Middle Miocene, Bexhaven Limestone, North Island, New Zealand.

Figures

Fig. 2. Vesicomyid bivalve Pliocardia? sp. from Moonlight North (Y16/ f1059), Early to Middle Miocene. A. UOA L4587. Dorsal view of articulated internal mould (A 1), arrow shows lunular incision. Internal mould of right valve (A 2), showing anterior adductor scar and posterior adductor scar. B. UOA L4588. Right valve of hinge plate (B 1), anterior tooth is not preserved. Left valve (B 2), internal mould, arrow shows pallial sinus.