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

publication ID

https://doi.org/ 10.5070/P940561331

publication LSID

lsid:zoobank.org:pub:1756B24A-813B-423F-896F-91B21FF58A79

DOI

https://doi.org/10.5281/zenodo.11505149

persistent identifier

https://treatment.plazi.org/id/C23987DD-FFDF-2916-FC24-FA9BEF3CBB58

treatment provided by

Felipe

scientific name

Acesta oregonensis
status

 

ACESTA OREGONENSIS ( CLARK, 1925)

FIGS. 25A–D View Figure 25 ; 26A, B View Figure 26 ; 27A, B View Figure 27

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 View Figure 25 ) and paratype ( Fig. 25C View Figure 25 ) 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 View Figure 25 ) 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 View Figure 26 ). A peculiar disjunct pattern of interrupted ribbing develops at intersections with commarginally bunched growth lines ( Fig. 26B View Figure 26 ). The glossy, pale brown exterior calcitic layer is underlain by a white chalky inner aragonitic layer ( Fig. 27A, B View Figure 27 ).

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 View Materials , 15315 View Materials , 15518 View Materials , 15601 View Materials , UCMP 4118 View Materials (Clark’s type locality) , UCMP IP2540 View Materials , 2549 View Materials , 2550 View Materials . 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.

R

Departamento de Geologia, Universidad de Chile

UCMP

University of California Museum of Paleontology

Kingdom

Animalia

Phylum

Mollusca

Class

Bivalvia

Order

Limida

Family

Limidae

Genus

Acesta

Loc

Acesta oregonensis

Hickman, Carole S. 2023
2023
Loc

Lima (Plagiostoma) oregonensis

Clark, Hertlein 1952
1952
Loc

Lima oregonensis

Clark, Durham 1944
1944
Loc

Lima (Plagiostoma) oregonensis

Clark, Schenck 1936
1936
Loc

Lima (Plagiostoma) oregonensis Clark (1925)

Clark, Vokes 1925
1925
Loc

Lima oregonensis

Clark, Vokes 1925
1925
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