Glycymeris andersoni, 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 : 15

GLYCYMERIS ANDERSONI DICKERSON, 1917

FIGS. 8A–E , 9A, B

Glycymeris andersoni Dickerson (1917) . pp. 166–167; pl. 27, figs. 3a–c.

Glycymeris andersoni Dickerson, Effinger (1938) . p. 366.

Glycymeris andersoni Dickerson, Weaver (1943) . p. 59;

pl. 10, figs. 5, 11. Discussion — Glycymeris andersoni is easily recog-

nized by its ovate, moderately inflated, nearly equilateral shell with broad flat-topped radial ribs and narrow inter- spaces accentuated by chalky preservation ( Fig. 8B, D ). The commissural margin is strongly crenulate ( Fig. 8D ) and the interlocking of the valves is further reinforced by interior crenulation ( Fig. 8A, C ). Maximum shell size is large (length and height> 25 mm) relative to species in the older tropical Eocene faunas of the Pacific North- west and California. It is abundant in the Gries Ranch Formation and represented by both juvenile and adult specimens of equal linear dimension, consistent with isometric growth. The dorsal slope angle does not differ between large and small shells ( Fig. 9A, B ), indicating isometric growth.

A second Gries Ranch glycymeridid species, described subsequently by Effinger (1938), is treated below, and figured for comparison.

Material examined — 39 specimens from Loc. UCMP IP2289 .

Type information —Figured hypotypes, all from Loc. UCMP IP2289 : UCMP 110729 , length 19.5 mm, height 19.2 mm; UCMP 110730 , length 22.1 mm, height 21.6 mm; UCMP 110731 , length 29.0 mm, height 27.7 mm; UCMP 110732 , length 19.0 mm, height 17.1 mm; UCMP 110733 , length 8.2 mm, height 7.6 mm.

Stratigraphic occurrence —This species is known only from the Gries Ranch Formation. It is not present in the tropical Eocene fauna of the underlying Cowlitz Formation or in the recovery fauna of the overlying Pittsburg Bluff and Eugene Formations.

Figures

Figure 8. Glycymeris andersoni Dickerson (1917), Gries Ranch Formation. A. Interior of right valve, hypotype UCMP 110730. B, C. Exterior and interior of right valve, hypotype UCMP 110729. D, E. Ventral and do2rsal of articulated valves, hypotype UCMP 110731. Scale bars=1 cm.

Figure 9. Comparison of dorsal slope angles in Juvenile and adult shells of Glycymeris andersoni Dickerson (1917) and Glycymeris winlockensis Effinger (1938), Gries Ranch Formation. A. Adult G. andersoni hypotype UCMP 110732, scale bar=1 cm. B. Adult G. winlockensis hypotype UCMP 110734, scale bar=1 cm. C. Juvenile G. andersoni hypotype UCMP 110733, scale bar=5 mm. D. Juvenile G. winlockensis hypotype UCMP 110735, scale bar=5 mm.