Pliocardia kawadai ( Aoki, 1954 )

Kiel, Steffen, Amano, Kazutaka & Goedert, James L., 2023, New taxa, records, and data for vesicomyid bivalves from Cenozoic strata of the North Pacific region, Acta Palaeontologica Polonica 68 (2), pp. 297-320 : 303-304

Pliocardia kawadai ( Aoki, 1954)

Figs. 4 , 5 .

1954 Lamelliconcha kawadai sp. nov.; Aoki 1954: 36–37, pl. 2: 1–13, 22.

1962 Vesicomya kawadai (Aoki) ; Kamada 1962: 88–89, pl. 8: 2a–b.

2001 Vesicomya kawadai (Aoki) ; Amano et al. 2001: 192, figs. 3–5, 8–11.

2007 Vesicomya kawadai (Aoki) ; Amano et al. 2007: figs. 3D, E, G, J.

2012 Pliocardia kawadai (Aoki) ; Amano and Kiel 2012: 80–83, figs. 2–4, 6, 7, 9–12.

2019 Pliocardia kawadai (Aoki) ; Amano et al. 2019b: fig. 4 C, E, J–O.

2022 Pliocardia kawadai (Aoki) ; Amano et al. 2022: fig. 10.9 f, g.

Material.— Eight specimens from LACM 6132 ( NRM Mo 204715–18, 21, LACMIP 6132.2 – 6132.4 ) ; seven specimens from northeast of Rocky Point ( NRM Mo 204709–13, 19, 20); 27 mostly fragmentary specimens from west of Rocky Point ( NRM Mo 204714 [1 specimen], 204812 [26 specimens]). Miocene , Astoria Formation , Bald Ridge unit, north shore of the Columbia River, Pacific County, Washington State, USA .

Dimensions.—See Table 2.

Remarks.—The specimens of Pliocardia kawadai from the Lower to Middle Miocene Columbia River seep deposits resemble those from the Lower Miocene Honya Formation of Joban coalfield in Japan in every aspect ( Aoki 1954; Kamada 1962). The topotype specimens subsequently reported ( Amano and Kiel 2012) show no discernable difference in hinge structure compared with the specimens from Washington, though they appear slightly less inflated and have sharper (more angular) posterodorsal ridges. The Japanese specimens of Pliocardia kawadai reach 43.1 mm in length (i.e., Amano et al. 2019b), which is similar to the average size of the specimens from Washington, with the exception of one specimen that is nearly 50 mm long.

As pointed out previously ( Amano and Kiel 2012), the most similar extant species is the West Pacific P. crenulomarginata ( Okutani, Kojima, and Iwasaki, 2002) , which differs from P. kawadai mainly by having less prosogyrate umbones and fine crenulations on the posterior margin.

Stratigraphic and geographic range.—Lower–Middle Miocene of Hokkaido and Honshu, Japan, and seep deposit in southwestern Washington State, USA.

Figures

Fig. 4. The vesicomyid bivalve Pliocardia kawadai (Aoki, 1954), from Middle Miocene seep deposits of the Astoria Formation in western Washington State, USA. A. NRM Mo 204714, Rocky Point, internal mold showing muscle scars and pallial line. B. NRM Mo 204719, Frankfort, specimen with partially preserved shell (LV) showing the fine, irregular growth lines and the anterior adductor muscle scar. C. NRM Mo 204709, Frankfort, mostly an internal mold, showing the anterior adductor muscle scar and the posterior ridge. D. NRM Mo 204720, Frankfort, mostly an internal mold, note naticid drill hole on posterodorsal margin. E. LACMIP 6132.2, LACMIP loc. 6132, specimen with partially preserved shell; view of RV showing muscle scars E1); anterodorsal view showing elongate lunular incision (E2); posterodorsal view showing ligament (E3). F. NRM Mo 204711, Frankfort, internal mold of left valve, showing pallial line and posterior adductor muscle scar. G. NRM Mo 204712, Frankfort, specimen with partially preserved shell (RV) showing the fine, irregular growth lines and the anterior adductor muscle scar.

Fig. 5. The vesicomyid bivalve Pliocardia kawadai (Aoki, 1954), from Middle Miocene seep deposit at Frankfort of the Astoria Formation in western Washington State, USA.NRM Mo 204713, polished sections of the hinge area in the RV (A)and LV (B). Photographs (A1, B1)and their interpretation (A2, B2).