Thracia keaseyensis, Carole & Hickman & Ca, 2014

Carole, Hickman, S. & Ca, 2014, Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, part IV: The anomalodesmatans, PaleoBios 31 (3), pp. 1-21 : 8

Thracia keaseyensis n. sp.

Figs. 2A–H

Thracia n. sp. Vokes in Warren et al. 1945 (checklist).

Thracia n. sp. Vokes in Warren and Norbisrath 1946, p. 227. Thracia n. sp. Moore and Vokes 1953, p. 115, 118.

Diagnosis— Shell large for genus (up to 62.5 mm length x 41 mm width); strongly elongate-ovate; inequivalve but nearly equilateral; valves compressed, right valve more inflated; shell thin and fragile, typically articulated but usually diagenetically crushed or deformed; beaks opisthogyrate, left beak penetrating right and abraded right beak showing iridescence; sculpture of fine co-marginal growth lines and occasional irregular low undulations; surface granulation strongly developed posteriorly, with abundant granules aligned in co-marginal rows; ventral margin shallowly convex; anterior dorsal slope rounded and merging with evenly-rounded anterior end; posterior dorsal margin shallowlyconvex and posterior end broadly truncate; posterior dorsal slope set off by shallow radial sinus and ridge; ligament external, with well-developed nymph plates and deep ligament groove, hinge plate apparently edentulous but inadequately exposed.

Description —Shell size, extent of surface granulation and organization of granules variable; muscle scars and interior shell features unknown.

Discussion — Thracia keaseyensis is readily distinguished from T. condoni Dall, 1909 in the overlying Pittsburg Bluff Formation, Eugene Formation, and other coeval Oligocene formations by its more elongate form and prominent co-marginal surface granulation. Thracia schencki Tegland, 1933 from the younger Oligocene Blakeley Formation is smaller and also relatively shorter. Thracia t rapezoidea C onrad, 1 849, d escribed f rom t he Miocene Astroria Formation and still living in the North Pacific, also has surface granulation. However, it is more inflated and also less elongate. Thracia dilleri Dall, 1898 , from the late Eocene Coaledo Formation in Oregon, is similar in outline and surface granulation, although it is inequilateral, with a more pronounced posterior end. Material of other Cenozoic thraciid species is inadequately preserved to permit meaningful comparison.

Etymology— For the geologic formation from which the species is described.

Material examined —The description is based on 28 almost complete double-valved specimens from 12 localities as well as crushed or fragmental specimens with diagnostic features such as the distinctive surface granulation. The species is restricted to the middle member where it is frequently associated with well-preserved double-valved specimens of Nemocardium weaveri (Anderson and Martin, 1914) and Moerella n. sp.

Holotype — USNM 561781 , length 45.7 mm, height 29.7 mm, Loc. USGS 15282 .

Paratypes — USNM 561782 , length 43.6 mm, height 29.5 mm, Loc. USGS 15282 ; USNM 561783 , length 45.3 mm height 22.5 mm, Loc. USGS 15267 ; UCMP 110672 , length 39.3 mm, height 21.6 mm, Loc. UCMP IP1984 (= USGS 25031 ) . (Measurements cannot be corrected for distortion, crushing, and broken margins.)

Keasey Formation localities — Type locality: USGS 15282 (middle member). Other Localities: USGS 15267 , 15274 , 15276 , 15277 , 15281 , 15283 , 15284 , 15285 , 15508 , 15525 , 25031 (middle member) .