Squiresica yooni, Kiel & Amano & Goedert, 2023

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 : 309-310

Squiresica yooni sp. nov.

Fig. 9 .

1976 Calyptogena cf. elongata Dall ; Yoon 1976a: 13, pl. 3: 3, 7, 9.

Zoobank LCID: urn:lsid:zoobank.org:act:74536D5A-3B34-4763-B4E0-89FC7DD33241 .

Etymology: In honor of Sun Yoon, who donated the illustrated specimens to the late Professor Kanno, who in turn passed them on to KA. Type material: Holotype: partially damaged internal mold, showing hinge characters (NRM Mo 204829). Paratypes: three articulated specimens, mostly internal molds (NRM Mo 204830–32) from the type locality and horizon.

Type locality: Loc. 18 of Yoon (1976b), hillside cut for the foundation of house building, about 400 m southeast of Goedong-dong , Daesong-myeon, Yeongil-gun, South Korea .

Type horizon: Duho Formation, uppermost part of Yeonil Group, Middle Miocene.

Material.—The type material only.

Dimensions.—See Table 6.

Diagnosis.—Small Squiresica having some rough commarginal ribs in posterior surface, umbo strongly displaced anteriorly, convex ventral margin, very indistinct lunular incision, and rather strong cardinal tooth 1 of RV.

Original description.—“Shell small to medium in size, ovate, transversely elongated, thin. Beaks low, small, situated at about one-fourth of shell length anteriorly. No lunule, but escutcheon long, excavated; ligament external. Antero-dorsal margin short; anterior end rather narrowly rounded; posterodorsal margin long, almost straightly sloping down and passing into posterior end which is more narrowly rounded than the anterior one; ventral margin almost straight except for the both ends. Surface sculptured-with fine, somewhat rough, concentric growth lines.” ( Yoon 1976a).

Supplemental description.—Hinge plate narrow, RV hinge with strong, elongate, forward-directed cardinal tooth 1, cardinal tooth 3a very thin, long, parallel to dorsal margin, cardinal tooth 3b short, bifid with raised edges, radiating posteroventrally, subumbonal pit elongate-triangular, very distinct.

Slight dimorphism in shell outline: posterior margin either evenly rounded or with sloping posterodorsal margin.

Remarks.—According to Yoon (1976b), specimens of Squiresica yooni sp. nov. occur in dense aggregations of articulated valves in the siltstone of the type locality, associated with specimens of a thyasirid identified as Conchocele bisecta . Yoon’s (1976a) largest specimen was 34.5 mm long.

The main differences between Squiresica yooni and S. knapptonensis ( Amano and Kiel, 2007) are the shorter anterior portion of the shell, the stronger cardinal tooth 1, and the virtual lack of a lunular incision in S. yooni . The Eocene– Oligocene Squiresica plana sp. nov. described above differs from S. yooni by being smaller (length up to 20.8 mm in S. plana vs. up to 30 mm in P. yooni ), possessing a more distinct lunular incision and having more parallel dorsal and ventral shell margins.

Notably similar to Squiresica yooni regarding size, elongation, and inflation is the early Oligocene age Pleurophopsis lithophagoides Olsson, 1931 , from Peru, which is also similar to the Oligocene Squiresica knapptonensis . With the pallial sinus and lunule of P. lithophagoides being unknown, its relation to Squiresica remains uncertain (i.e., Kiel et al. 2020b; Hybertsen et al. 2022).

Stratigraphic and geographic range.—Middle Miocene Duho Formation, South Korea.

Figures

Fig. 9. The vesicomyid bivalve Squiresica yooni sp. nov., from the Middle Miocene Duho Formation, loc. 18 of Yoon (1976b), South Korea. A. Holotype, NRM Mo 204829, an articulated specimen with preserved cast of RV hinge area; close-up on hinge area (A1), view on right valve and exposed hinge of left valve (A2), and view on left valve (A3). B. Paratype, NRM Mo 204832, articulated specimen, mostly an internal mold; dorsal view showing inflation (B1); view on exterior of RV (B2); close-up on anterior part of LV showing the lack of a lunular incision (B3). C. Paratype, NRM Mo 204831, articulated specimen with some shell preserved; dorsal view showing inflation (C1); lateral view on RV showing fine growth increments (C2).