Squiresica cf. knapptonensis ( Amano and Kiel, 2007 )

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 : 307-309

Squiresica cf. knapptonensis ( Amano and Kiel, 2007)

Fig. 7 .

Material.—Eleven numbered specimens (NRM Mo 204773– 78; 204805–09) and numerous additional fragments (NRM

Mo 204810) from the Eocene Siltstone of Unit B in the Grays River area, western Washington State, USA.

Dimensions.—See Table 4.

Remarks.—The visible features of the late Eocene age specimens from the Siltstone of unit B fall within the variation seen in the early to late Oligocene age specimens of S. knapptonensis from various deposits of the Lincoln Creek Formation Amano and Kiel 2007; Hybertsen et al. 2022). However, their hinge dentition remains unknown and therefore we assign these specimens only preliminarily to S. knapptonensis .

Stratigraphic and geographic range.—Upper Eocene, Siltstone of unit B, southwestern Washington State, USA.

Squiresica plana sp. nov. Fig. 8 .

1993 Calyptogena chinookensis Squires and Goedert, 1991 ; Goedert and Squires 1993: fig. 5.

1995 Calyptogena (Calyptogena) chinookensis Squires and Goedert, 1991 ; Goedert and Campbell 1995: 24, table 1.

Zoobank LCID: urn:lsid:zoobank.org:act:82DEC36A-45E3-438D-9AF0-99BBD8F1A5D4 .

Etymology: From Latin plana , flat; in reference to the minimal inflation of the species.

Type material: Holotype: articulated specimen with shell material NRM Mo 204704 ). Paratypes: single valves or semi-articulated specimens embedded in rock matrix ( NRM Mo 204705, 06, 204770–72) from the type locality and horizon .

Type locality: UWBM loc. B7452, Canyon River , Washington State, USA .

Type horizon: The Lincoln Creek Formation , Oligocene .

Material.— Four specimens ( NRM Mo 204769–72) and numerous fragments from Canyon River ( UWBM loc. B7452) , five specimens from LACMIP loc. 17426 ( NRM Mo 204704–08); all Lincoln Creek Formation. About ten fragmentary specimens showing external features only ( NRM Mo 204834) from LACMIP loc. 15621, Pysht Formation. Oligocene, Washington State, USA .

Diagnosis.—Small for genus, minimally inflated, lunular incision very indistinct, ligament short, anterior adductor muscle scar circular.

Description.—Small, elongate, little inflated shells reaching 21 mm in length; umbones only slightly elevated, slightly prosogyrate, pointed, situated anteriorly at about 20% total shell length; surface covered by fine, dense growth increments; escutcheon and ligament short, narrow, no lunular incision. Anterior adductor muscle scar roundish with acute dorsal side; pallial line starting just anterior of posteroventral corner of anterior adductor muscle scar, running parallel to ventral margin, at posterior end curving upward into circular posterior adductor muscle scar. Hinge plate moderately wide, LV with three radiating teeth, cardinal tooth 2a thin, pointing anteriorly, 2b thick, short, pointing posteroventrally, cardinal tooth 4b short, thin, subparallel to 2b; subumbonal pit small, triangular.

Dimensions.—See Table 5.

Remarks.— Squiresica plana sp. nov. differs from Squiresica knapptonensis mainly by having a smaller, less inflated shell with posteriorly directed 2b tooth and very indistinct, elongate and narrow lunular incision. The Miocene Squiresica yooni sp. nov. from Korea described below is more inflated and has a wider escutcheon. Oligocene specimens previously reported as Calyptogena chinookensis Squires and Goedert, 1991 , from the Pysht and Makah formations in northwestern Washington State ( Goedert and Squires 1993; Goedert and Campbell 1995) have the general shape and size of S. plana and are here considered as belonging to this species. They are quite different from the large type specimens of Pleurophopsis chinookensis (discussed below) that reach 80 mm in length.

Stratigraphic and geographic range.—Seep deposits in the Oligocene parts of the Pysht, and Lincoln Creek formations, western Washington State, USA.

Figures

Fig. 7. The vesicomyid bivalve Squiresica cf. knapptonensis (Amano and Kiel, 2007) from upper Eocene seep deposits of the Siltstone Unit B along the West Fork of Grays River in western Washington State, USA. A. NRM Mo 204773, internal mold of RV showing adductor muscle scars and pallial line. B. NRM Mo 204808, internal mold of semi-articulated specimen; view on RV (B1); close-up on lunular incision (B2). C. NRM Mo 204776, internal mold of LV showing anterior adductor muscle scar, onset of pallial line, and the short ligament. D. NRM Mo 204775, internal mold of RV showing adductor muscle scars and pallial line.E. NRM Mo 204809, anterior half of RV showing external shell sculpture.F. NRM Mo 204805, view on internal mold of posterior adductor muscle scar.G. NRM Mo 204807I, internal mold of semi-articulated specimen, view on LV.H. NRM Mo 204806, close-up on lunular incision of LV.

Fig. 8. The vesicomyid bivalve Squiresica plana sp. nov., from Oligocene seep deposits in the Lincoln Creek Formation in western Washington State, USA. Specimens from UWBM loc.B7452 (A–C) and LACMIP loc.17426 (= SR4) (D–F). A. Paratype, NRM Mo 204772, exterior of LV showing fine growth increments. B. Paratype, NRM Mo 204771, exterior of LV showing shell outline and fine growth increments. C. Paratype, NRM Mo 204770, close-up on anterior part of RV showing near absence of a lunular incision. D. Holotype, NRM Mo 204704, articulated specimen with preserved shell; lateral view on RV (D1) and dorsal view, showing inflation and ligament (D2). E. Paratype, NRM Mo 204705, specimen with partially preserved shell, showing anterior adductor muscle scar and onset of pallial line. F. Paratype, NRM Mo 204706, semi-articulated specimen; F1, close-up on LV hinge area; F2, lateral view on LV.