Crenella porterensis, WEAVER, 1912

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 : 6-8

CRENELLA PORTERENSIS WEAVER , 1912

FIGS. 3A–C, E ; 4A

Crenella porterensis Weaver (1912) . p. 15; pl. 14, fig. 115.

Crenella porterensis Weaver, Weaver (1916) . pp. 36–37; figs. 41, 42.

Crenella porterensis Weaver, Weaver (1943) . p. 115; figs. 3, 4.

Crenella porterensis Weaver, Durham (1944) . p. 140. (unfigured hypotype UCMP 35163).

Crenella porterensis Weaver, Vokes in Warren, Norbisrath and Grivetti (1945).(checklist)

Crenella porterensis Weaver, R.C. Moore and Vokes (1953) . p. 119.

Crenella porterensis Weaver, Kanno (1971) . p. 47. pl. 1, fig. 27.

Crenella porterensis Weaver, Armentrout (1973) . pp. 387–389; pl. 8, fig. 23.

Crenella porterensis Weaver, E.J. Moore (1976) . p. 45; pl. 12, fig. 12.

Solamen porterensis (Weaver) , Addicott (1976b). p. 29; pl. 4, figs. 3,11.

Crenella porterensis Weaver, Allison and Marincovich (1981) . pp. 4, 8; pl. 1, figs. 5, 6.

? Crenella cf. C. porterensis Weaver, E.J. Moore (1963) . p. 63; pl. 25, fig. 6.

? Crenella sp. ? Hickman (1969). pp. 33-34.? Solamen snavelyi Addicott (1976b) . p. 29; pl. 4, figs.

6,9. not Crenella porterensis Weaver, Tegland

(1933). p. 112; pl. 6, fig. 2.

Discussion — Crenella porterensis is distinguished by its thin and strongly inflated shell, ovate-elongate shape, prominent prosogyrous umbones, external elongate posterior ligament groove, and sculpture of numerous (>100) very fine radial ribs with distinct narrow inter- spaces and weaker commarginal ribs. Ribs terminate at the ventral margin to produce fine interlocking crenula- tions. Increase in the number of radial ribs is primarily by intercalation ( Fig. 4A , black arrows) of new ribs, although bifurcation also occurs ( Fig. 4A , yellow arrow). Both modes of rib numerical increase may occur on a single specimen. Rib increase is most prominent late in growth on the medial portion of the shell. Shell growth is strongly allometric with an initially ovate shape increasing to elongate and clearly visible at growth checks on a single adult shell ( Fig. 2A ).

The finely decussate shell surface layer is translucent, and the thin underlying (middle) nacreous layer is visible as a pattern that is enhanced as light passes through fine interruptions of crossing radial and concentric elements of surface sculpture. A third non-nacreous layer is present interior to the middle nacreous layer in some of the Keasey shells. This is consistent with observations of complex crossed lamellar aragonite and other aragonitic microstructures in the interior layer of crenellids ( Carter 1990). The interior of well-preserved dry shells of living Crenella decussata (Hickman, personal observation) suggests a patchy, discontinuous distribution of mineralogy and microstructure that includes some nacre.

Although none of the specimens examined are suffi- ciently well preserved for excavation of the hinge, offset of the valves below the beaks reveals part of a thickened denticulate plate ( Fig. 3A ). A denticulate hinge plate is characteristic of type Crenella , distinguishing it from edentulous living species assigned to the genus Solamen . This is probably a long-ranging species complex that includes the Miocene crenellid described by Addicott (1976b) as Solamen snavelyi . Armentrout (1973) report- ed it from six molluscan zones extending from the late Eocene into the early Miocene. The Miocene specimen from the Astoria Formation, illustrated by E.J. Moore (1963, pl. 25, fig. 6) has a denticulate hinge, and the mode of duplication of radial sculpture is consistent with the two modes documented here in Crenella porterensis .

Geographically, this is a high-latitude, cool-water species complex, occurring from the Keasey Formation in Oregon to the Poul Creek and Narrow Cape Formations in the Gulf of Alaska ( Kanno 1971, Allison and Marincovich 1981). The fauna of the Narrow Cape Formation is noted for its mixture of Asiatic, North American, and endemic high-latitude North Pacific taxa ( Allison and

Marincovich 1981).

The Danian Crenella kannoi Marincovich (1993) from the Prince Creek Formation on the Arctic coastal plain of Alaska is strikingly similar in size and ovate-elongate shape to C. porterensis , but differs in a distinctive pattern of concentric undulations and wavy radial threads. As noted above, Arctic and Northeastern Pacific faunas were not connected at this time.

Material examined — 13 specimens.

Figured Keasey hypotypes — UCMP 110725 (in matrix), length 7.9 mm, height 10.8 mm, Loc. UCMP IP7984 (= USGS 25031); UCMP 110726 (in matrix), length 7.9 mm, height 9.8 mm, Loc. UCMP IP2286 (= USGS 15265); USNM 561803, length 8.9 mm, height 9.5 mm. Loc. USGS 15282.

Figured Lincoln Creek hypotypes — UCMP 32410, length 10.18 mm, height 18.2 mm, Loc. UCMP A9 (= UW 256, Weaver’s type locality); UCMP 35163, length 11.7 mm, height 18.0 mm, Loc. UCMP A1811.

Keasey Formation localities —Lower member: UCMP IP2286, USGS 15265. Middle member: UCMP IP7984, USGS 15282, 15276, 15277, 15279, 15525. Upper member: USGS 13515, 15518.

Other occurrences —In Oregon this species also occurs stratigraphically higher in the overlying Pittsburg Bluff Formation and Miocene Astoria Formation. In

Washington it occurs in the Eo-Oligocene Lincoln Creek Formation, Quimper Sandstone, and Blakeley Formation .

Figures

Figure 2. Alternative reconstructions of the life habit of Crenella decussata. A. Completely encased in crypt of cemented sedimentary grains, living below sediment-water interface in North Atlantic. B, C. Byssus attached to shell fragments, living at sedimentwater interface in North Pacific.

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).