Pliocardia, Woodring, 1925

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 : 304-307

Pliocardia ? guthrieorum sp. nov.

Fig. 6 .

ZooBank LCID: urn:lsid:zoobank.org:act:EEDC380D-91CD-44E9-A722-6096971A8D9C .

Etymology: In honor of Lloyd S. and Kay Guthrie (Olympia, USA), for their help over several fieldtrips that produced some of these specimens.

Type material: Holotype: NRM Mo 204785 ( Fig. 6G ); paratypes: NRM Mo 204779–82, 204784, 204813, 14, UWBM IP 117930 , 31 . All specimens from the type locality and horizon.

Type locality: UWBM loc. B 7452 in the Canyon River in western Washington State, USA .

Type horizon: Lincoln Creek Formation , Oligocene .

Material.— 73 specimens from UWBM loc. B7452 ( NRM Mo 204779–89, 92, 93, 204813, 14, UWBM IP 117930–31 ), and four from UWBM loc. B6702 ( NRM Mo 204790), all Lincoln Creek Formation . One specimen from the Makah Formation , west of Whiskey Creek ( NRM Mo 204791). Oligocene, Washington State, USA .

Dimensions.—See Table 3.

Diagnosis.—Shell oval to elongate-oval, moderately inflated, reaching nearly 50 mm in length; blunt, submarginal posterior ridge; umbones prominent, elevated, prosogyrate; lunular incision very elongate, often indistinct; ligament short.

Description.—Shell medium-sized, broadly oval-elongate, moderately inflated (max. L = 48.6 mm, H = 37.5 mm, W = 22.7 mm); umbones elevated, prosogyrate, positioned anterior at about 21% of total shell length; blunt ridge close to posterodorsal margin; elongate lunular incision present but not very deep; short and narrow ligament, no escutcheon. Anterior adductor muscle scar D-shaped, posterior margin shallow; pallial line starting at posteroventral corner of anterior adductor scar, extending mostly parallel to ventral shell margin, near posterior end of shell bending upward to continue nearly straight into the base of the posterior adductor muscle scar; posterior adductor muscle scar round, with weak but distinct ridge running from umbo to ventral margin where it meets the pallial line. Hinge plate narrow, with three teeth in each valve; teeth in RV subparallel to dorsal shell margin, 1 thin, elongate, just anterior of umbo, 3a and 3b both short, thin, originating underneath umbo; LV with thin 2a, subparallel to dorsal shell margin, 2b nearly perpendicular to 2a, thick and short.

Remarks.—All specimens are from the Oligocene part of the Lincoln Creek Formation in the Canyon and Satsop rivers area. One specimen from limestone deposits found in upper Eocene rocks mapped as Makah Formation, just west of Whiskey Creek, may also belong here. It is a small (c. 25 mm long) right valve that externally resembles Pliocardia ? guthrieorum, but it is embedded in rock matrix so neither hinge nor pallial line or muscle scars are visible, hampering a reliable identification.

We hesitantly assign this species to Pliocardia because it resembles some of the larger-sized extant species assigned to this genus, as well as for example P. kawadai reported above. However, virtually all species of Pliocardia are more strongly inflated than Pliocardia ? guthrieorum. Furthermore, our interpretation of the hinge dentition is based on x-ray scanning of a specimen with an only moderately preserved hinge, and some uncertainty remains, in particular about cardinal 3a. Extant species of the genus Wareniconcha Cosel and Olu, 2009 , have shells with similar minimal inflation, but they have more convex ventral margins and less elevated umbones, and they have an escutcheon ( Smith 1906; Prashad 1932; Cosel and Olu 2009; Krylova and Sahling 2010), which Pliocardia ? guthrieorum lacks.

Stratigraphic and geographic range.—Upper Eocene? to Oligocene seep deposits; Olympic Peninsula, western Washington State, USA.

Figures

Fig. 6. The vesicomyid bivalve Pliocardia? guthrieorum sp. nov., from the Oligocene seep deposits at UWMB loc. B7452, Lincoln Creek Formation, western Washington State, USA. A. Paratype, NRM Mo 204779, large specimen showing outline, inflation, and ligament; left valve (A1) and dorsal (A2) views. B. Paratype, NRM Mo 204784, showing outline, inflation, and anterior adductor muscle scar; left valve (B1) and dorsal (B2) views. C. Paratype, NRM Mo 204780, small, elongate specimen; right valve (C1) and dorsal (C2) views; digital sections through the hinge area, with our interpretation of the hinge teeth indicated (C3, C4). D. Paratype, NRM Mo 204782, showing ligament. E. Holotype, NRM Mo 204785, showing inflation (E1) and both adductor muscle scars and the pallial line in the RV (E2). F. Paratype, NRM Mo 204781, large specimen with particularly large and blunt umbo. G. Paratype, NRM Mo 204814, small specimen in dorsal view, showing inflation and lunular incision (G1) and the lack of an escutcheon (G2). H. Paratype, NRM Mo 204786, showing lunular incision. I. Paratype, NRM Mo 204813, showing lunular incision.Abbreviation: pams, posterior adductor muscle scar.