Crenella washingtonensis, WEAVER, 1916

Hickman, Carole S., 2023, Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, Part II: The pteriomorphs, PaleoBios 40 (5), pp. 1-51 : 8-9

CRENELLA WASHINGTONENSIS WEAVER , 1916

FIGS. 3D, F ; 4B

Crenella washingtonensis Weaver (1916) . p. 37; pl. 3, fig. 40.

Crenella washingtonensis Weaver, Weaver (1943) . p. 116; pl. 25, fig. 4

Crenella porterensis Weaver, Tegland (1933) . p. 112; pl. 6, fig. 2.

Crenella washingtonensis Weaver, Durham (1944) . p. 140, unfigured hypotype UCMP 35164.

Discussion — Crenella washingtonensis was described from the Lincoln Creek Formation after description of Crenella porterensis porterensis and from a different type locality in the Lincoln Creek. It is distinguished by its smaller shell, ovate outline, and inconspicuous sculpture. It is recognized here from a single, distinctly ovate specimen from a locality in the upper part of the middle member of the Keasey Formation. The shell is a left valve with the exterior surface imbedded in matrix ( Fig. 3F ). The shell is considerably thinner and more fragile than that of C. porterensis . Portions of the opalescent interior layer remain intact, and thin exterior layer is sufficiently translucent that exceedingly fine radial sculpture is visible with a microscope under oblique lighting. The hinge plate is missing and probably was not well developed. The ovate shape may be primarily a correlate of smaller size, although the thin fragile shell and fine ornamentation serve to differentiate it from its more elongate, robust-shelled congener.

The stratigraphic range of this rarer species is unclear. It appears to extend into younger Oligocene strata. Close examination of Tegland’s (1933) hypotype of C. porterensis from the Oligocene type locality of Blakeley Formation at Restoration Point, Washington reveals an ovate shape and finer radial sculpture ( Fig. 3D ). It is here re-assigned to C. washingtonensis . In addition to finer and more numerous radial ribs, the Tegland specimen also shows numerous insertions of new ribs by intercalation ( Fig. 3B ).

Figured Keasey hypotype — UCMP 110727 (in matrix), approximate length 7.4 mm, approximate height 5.4 mm, Loc. UCMP IP7984.

Figures

Figure 3. Crenella porterensis Weaver (1912) (A–C, E) and C. washingtonensis Weaver (1916) (D, F). A. Articulated, left valve, hypotype UCMP 32410, Lincoln Creek Formation, height 18.2 mm. B. articulated, hypotype USNM 561803, Keasey Formation, height 13.5 mm. C. Exterior mold in matrix, hypotype UCMP 110725, Keasey Formation, height 10.8 mm. D. Right valve, hypotype UCMP 32250, Blakeley Formation, height 12.5 mm. E. Partial shell exterior in matrix, hypotype UCMP 110726, Keasey Formation, height 10.3 mm. F. Partial shell exterior in matrix, hypotype 110727, Keasey Formation, height 5.4 mm.

Figure 4. A. Crenella porterensis detail of coarser ribs and rib increase by splitting (yellow arrow) and by intercalation (black arrows) (enlarged from Fig. 3A). B. Crenella washingtonensis detail of finer ribs and rib increase by intercalation (black arrows) (comparable enlargement from Fig. 3D).