Acesta oregonensis, (CLARK, 1925)

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 : 38-41

ACESTA OREGONENSIS ( CLARK, 1925)

FIGS. 25A–D ; 26A, B ; 27A, B

Lima (Plagiostoma) oregonensis Clark (1925) . p. 84; pl. 24, figs. 3, 4.

Lima (Plagiostoma) oregonensis Clark, Schenck (1936) . p. 62.

Lima (Plagiostoma) oregonensis Clark, Woodring (1938) . p. 48.

Lima oregonensis Clark, Weaver (1943) . p. 99; pl. 22, figs. 2, 6.

Lima oregonensis Clark, Durham (1944) . p. 139.

Lima oregonensis Clark, Vokes in Warren et al. (1945) (checklist).

Lima oregonensis Clark, Vokes in Warren and Norbisrath (1946). p. 227.

Lima (Plagiostoma) oregonensis Clark, Hertlein (1952) . p. 379.

Lima (Acesta) oregonensis (Clark) , R.C. Moore and Vokes (1953). pp. 115, 118.

Lima (Acesta) oregonensis (Clark) , Vokes (1963), pp.

85–86.

Discussion —In the original description, Clark assigned this species to Plagiostoma J. Sowerby (1814), a genus based on an Early Jurassic species with a thicker shell, a broad cardinal area, and a triangular resilifer located beneath the beaks. The diagnostic features of Plagiostoma are in marked contrast to the thin shell, narrow cardinal area and posteriorly oblique ligament pit of the Paleogene species, which here is assigned to Acesta . Clark’s original illustrations are misleading photographs in which the holotype ( Fig. 25A ) and paratype ( Fig. 25C ) were trimmed to eliminate the matrix in which they are imbedded, and fail to show the distribution of adhering, poorly-preserved shell fragments. New photographs of the types ( Figs. 25B, D ) are provided here. Clark also reversed the orientation of the ears in his description, describing the larger posterior ear as anterior and the smaller anterior ear as posterior.

Subsequent specimens collected by Hans Norbisrath, R.M. Grivetti and Harold Vokes as well as specimens collected in preparation of this monograph are all from the upper member of the Keasey Formation, where original shell material is typically well preserved and the dominant (occurring at most localities) element in assemblages along with basal root tufts of hexactinellid sponges, less frequent specimens of Porterius gabb i, and poorly-preserved terebratulid brachiopods. Failure to collect thin, delicate shell fragments undoubtedly accounts for under-representation of the species in both museum and private collections.

Careful examination of all available material is most informative in a composite characterization of the exterior ornamentation. Shells are smooth or with faint radial lines on the central part of the shell but pronounced on the anterior and posterior dorsal margins where numerous, finely incised radial lines are irregular in width and spacing ( Fig. 26A ). A peculiar disjunct pattern of interrupted ribbing develops at intersections with commarginally bunched growth lines ( Fig. 26B ). The glossy, pale brown exterior calcitic layer is underlain by a white chalky inner aragonitic layer ( Fig. 27A, B ).

The Keasey species is most similar in shell shape and ornamentation to Acesta hamlini ( Dall, 1900) from the Pliocene of the Los Angeles Basin where it was carefully documented and figured by Woodring (1938). Durham (1944) described Lima twinensis and Lima robertsae from younger Oligocene horizons in the Twin River and Blakeley formations in northeastern Washington, but preservation is inadequate to permit detailed comparisons with the Keasey species other than to note that they are large-shelled species more appropriately assigned to Plicacesta Vokes (1963) . Durham’s species are more similar to Plicacesta wilsoni E.J. Moore (1984a) from the uppermost exposed portion of Lincoln Creek Formation along with the Keasey-equivalent giant limid treated and figured separately below.

Material examined —11 partial to nearly-complete valves and numerous shell fragments suitable for investigation of shell microstructure.

Figured hypotypes — UCMP 110740 (left valve), length 7.3 cm, height 7.8 cm., Loc. UCMP IP2540. UCMP 110741 (right valve), length 7.8 cm, height 7.7 mm, Loc. UCMP 110740.

Unfigured hypotype — UCMP 110742 (incomplete righty valve in matrix), Loc. UCMP IP2540.

Localities — USGS 15269 , 15315 , 15518 , 15601 , UCMP 4118 (Clark’s type locality) , UCMP IP2540 , 2549 , 2550 . The type locality is an abandoned railroad tunnel with outcrops that are no longer accessible, but the beds fall within the stratigraphic interval represented by the upper member.

Stratigraphic range —Restricted to the upper member of the Keasey Formation.

Figures

Figure 25. Holotype and paratype of Acesta oregonensis (Clark, 1925). A. Clark’s trimmed and retouched original figure of Keasey holotype, right valve, UCMP 30303. B. Unmodified color image of holotype in matrix. C. Clark’s original figure of the paratype, left valve, UCMP 30312. D. Unmodified color image of paratype in matrix. Scale bars=1 cm.

Figure 26. Acesta oregonensis (Clark, 1925), Keasey hypotype UCMP 110740. A. Original shell, left valve exterior, fitted together from two siltstone blocks, matrix digitally removed. Scale bar=5 cm. B. Detail of posterior ear.

Figure 27. Acesta oregonensis (Clark, 1925). Hypotype, UCMP 110741. A. Right valve in matrix with adhering interior (aragonitic) and exterior (calcitic) shell layers. Scale bar=5 cm. B. Fragments of exfoliated aragonitic layers from A.