Plicacesta bela, (DICKERSON, 1917)

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 : 41-43

PLICACESTA BELA ( DICKERSON, 1917)

FIGS. 28A–C

Lima bela Dickerson (1917) . pp. 172–173; pl. 29, fig.

11.

Lima (Radula) oakvillensis Clark (1925) . p. 84; pl. 14,

figs. 1, 3.

Lima bela Dickerson, Effinger (1938) . pp. 368–369.

Lima bela Dickerson, Weaver (1943) . pp. 97–98; pl. 21, fig. 3.

Lima oakvillensis Clark, Weaver (1943) . p. 98; pl. 21, fig. 1; pl. 22, fig. 7.

Acesta (Plicacesta) oakvillensis Clark, E.J. Moore (1984a) . pp. 27–28; figs 118, 132, 134.

Not Acesta cf. A. oakvillensis Clark, Addicott (1976a) . p. 441; fig. 6ac.

Discussion —The resilifer is clearly posterior to the beak. The posterior ear is well-developed on the rounded posterior dorsal margin. The anterior dorsal margin is straight, anterior to the beaks, and lacking evidence of a discrete ear. The radial ribs are strongly developed, with flat-bottomed interspaces that are wider than the radial ribs on the central portion of the shell. The ribs become scabrous at intersections with concentric growth increments. Shells are more similar in shape, ornamentation, and cardinal area to those of the Japanese type species of Plicacesta than to other species that have been assigned to the genus. The robust sculpture is more typical of the Cretaceous (Maestrichtian) species of Costellacesta illustrated by Kauffman (1964), although interspaces lack the fine secondary radial threads characteristic of the geologically older genus. The monotypic and endemic Antarctic genus Antarcticesta is distinguished by its thin shell and sculpture of very broad, scaly, and widelyspaced radial ribs. Recognition of four distinct genus group taxa has considerable morphological support as a working hypothesis.

Dickerson described Lima bela from a small, left valve. Specimens of large adults collected subsequently from the type locality ( Fig. 28A, B ) cannot be distiguished from Plicacesta oakvillensis , described by Clark (1925) from the coeval beds in the lower part of the Lincoln Creek Formation at the Oakville Quarry (Fig. C). Unfortunately, both Dickerson and Clark confused anterior and posterior in stating that there was well-developed anterior ear and a straight posterior margin. This is clearly not the case with the specimens illustrated here.

Co-occurrence of Plicacesta bela and Porterius gabbi in Gries Ranch assemblages is consistent with a deep-water depositional environment as discussed above in the treatment of Porterius .

Figured Gries Ranch hypotype — UCMP 110743 (left valve), length 8.4 cm, height 10.4 cm., Loc. UCMP IP2289.

Figured Lincoln Creek hypotype — UCMP 32405 (right valve), length 8.9 cm, height 11.3 cm, Loc. UCMP A-368.

Stratigraphic range —Although this species has not been collected in the Keasey Formation, it is narrowly restricted to the Gries Ranch Formation and coeval beds in the Lincoln Creek Formation, where it co-occurs with Porterius gabbi . As noted above, the narrow restriction of P. gabbi includes the upper member of the Keasey Formation.

Figures

Figure 28. Plicacesta bela (Dickerson, 1917). A, B. Gries Ranch Formation hypotype UCMP 110743, length 8.4 cm. A. Left valve exterior calcitic layer, B. Left valve interior with calcitic layer (white) and interior portion of aragonitic layer (gray). C. Lincoln Creek Formation hypotype UCMP 32405. Right valve exterior, length 8.4 cm.