Squiresica knapptonensis ( Amano and Kiel, 2007 ) Hybertsen & Goedert & Kiel, 2022

Hybertsen, Frida, Goedert, James L. & Kiel, Steffen, 2022, A new genus of chemosymbiotic vesicomyid bivalves from the Oligocene of western North America, Acta Palaeontologica Polonica 67 (3), pp. 703-709 : 705-707

Squiresica knapptonensis ( Amano and Kiel, 2007) comb. nov.

Figs. 2 , 3 .

1993 Calyptogena (Calyptogena) chinookensis Squires and Goedert, 1991 ; Goedert and Squires 1993: 74, fig. 4.

2007 Archivesica knapptonensis sp. nov.; Amano and Kiel 2007: 284, figs. 30–43.

Type material: Fragment of right valve with preserved hinge dentition, length 13.5 mm +, height 7.0 mm, USNM 534954 .

Type locality: LACMIP loc. 5843, beach terrace of the Columbia River near Knappton, Washington State, USA .

Type horizon: Upper Oligocene part of the Lincoln Creek Formation .

Material.— Numerous specimens (figured: NRM PAL Mo 195149–195155) from the West Fork Satsop seep deposit, lower Oligocene , Lincoln Creek Formation, western Washington, USA .

Description. —Elongated, weakly inflated, small shell, reaching little over 20 mm in length, H/L-ratio 0.46–0.60, W/H-ratio 0.61–0.65; umbo slightly elevated and moderately coiled, positioned at anterior 20–25% of shell length; external ligament opisthodetic, occupying>60% of postero-dorsal margin; anterior adductor scar D-shaped, its posterior margin thick; posterior adductor scar nearly round; pallial line impressed anteriorly, commarginal, forming a small indentation right beneath posterior adductor scar.

Remarks.— Goedert and Squires (1993) reported Calyptogena chinookensis Squires and Goedert, 1991 from the Pysht and Makah formations along the northern coast of Washington state, and from the upper Oligocene part of the Lincoln Creek Formation on the southern side of Washington state. Among their illustrated specimens, that from the Lincoln Creek Formation ( Goedert and Squires 1993: fig. 4) shows a lunular incision and a different hinge morphology than C. chinookensis and instead belongs to Squiresica knapptonensis (i.e., Amano and Kiel 2007). The newly collected material from the West Fork of Satsop River shows all characters possessed by specimens from the upper Oligocene type locality of S. knapptonensis and thus extends the stratigraphic range of this species down into the lower Oligocene.

The second species assigned here to the new genus, Squiresica marincovichi ( Kiel and Amano 2010: 80–81, figs. 17–25) from the Oligocene Kulthieth Formation in Alaska, differs from S. knapptonensis by being less elongated, having a broader escutcheon, lacking irregular growth lines on the outer surface, and a less distinct subumbonal pit (see Fig. 4 for a re-illustration of the type material). Squiresica marincovichi reaches almost 40 mm in length and it thus notably larger than S. knapptonensis .

Stratigraphic and geographic range.—Oligocene, Lincoln Creek Formation, western Washington State, USA.

Figures

Fig. 2. The vesicomyid bivalve Squiresica knapptonensis (Amano and Kiel, 2007) from the West Fork Satsop seep deposit, lower Oligocene, Lincoln Creek Formation, western Washington, USA. A. NRM PAL Mo 195149, articulated specimen with partially preserved shell, in lateral (A1, A3) and dorsal (A2) views. B. NRM PAL Mo 195150, left valve embedded in matrix, with partially preserved shell; B2, close-up showing pallial line and anterior adductor muscle scar (dotted line). C. NRM PAL Mo 195151, left valve embedded in matrix, with partially preserved shell (C2) and with posterior adductor muscle scar barely visible (dotted line in C1). D. NRM PAL Mo 195153, articulated specimen showing inflation and ligament. E. NRM PAL Mo 195152, small specimen with fully preserved shell, view on outer shell surface. F. NRM PAL Mo 195154, right valve embedded in matrix, with partially preserved shell, showing pallial line and anterior adductor muscle scar. G. NRM PAL Mo 195155, left valve with mostly preserved shell.

Fig.3. The vesicomyid bivalve Squiresica knapptonensis (Amano and Kiel, 2007) from the upper Oligocene part of the Lincoln Creek Formation at Knappton, along the Columbia River, western Washington, USA. A. Holotype USNM 534954, left valve showing external surface (A1) and hinge (A2). B. Paratype USNM 534956, left valve showing external surface (B1) and hinge (B2). C. USNM 534955, posterior part of a left valve showing external surface (C1) and the posterior end of the nymphal ridge (C2). All images (except C2) from Amano and Kiel (2007).

Fig. 4. The vesicomyid bivalve Squiresica marincovichi (Kiel and Amano, 2010), from the Oligocene Kulthieth Formation, Alaska, USA. A. Rubber peel of holotype UCMP 555221 showing internal features including hinge dentition of the right valve. B. Paratype UCMP 555223, internal mold of a left valve, showing adductor muscle scars and small pallial sinus. C. Paratype UCMP 555224, an articulated specimen showing inflation, lunular incision, and escutcheon (C1) and external sculpture of right valve (C2). All images from Kiel and Amano (2010).