Aperiploma bainbridgensis ( Clark, 1925 )

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 : 10

Aperiploma bainbridgensis ( Clark, 1925) Figs. 3A–E, G

Cochlodesma bainbridgensis Clark, 1925, p. 86 ; pl. 13, figs. 3, 4.

Cochlodesma bainbridgensis Clark, Tegland 1933, p. 112 ; pl. 6, figs. 3, 4.

Cochlodesma bainbridgensis Clark, Weaver 1942, p. 117 , pl. 25, fig. 1, pl. 29, fig. 2.

Cochlodesma bainbridgensis Clark, Durham 1944, p. 141 . Cochlodesma bainbridgensis Clark, Moore 1976, p. 53 , Pl. 16, figs. 6–11.

Discussion— Twelve specimens from eight localities in the lower and middle members of the Keasey Formation are assigned to this species. Allocation to Aperiploma is clear based on the combination of thin fragile shell, interior nacre, irregularly-spaced and weak concentric sculpture, absence of surface granules or pustules, opisthogyrous beaks, fissured umbones, and distinctive opisthodetic chondrophore attached to the shell by a prominent supporting buttress.Variability in outline and proportions of the Keasey specimens is at least in part a consequence of deformation, but the shells were inequivalve, with a nearly flat left valve slightly more convex right valve ( Fig. 3D). Moore (1976) noted a similar difficulty in characterizing the outline of specimens from the overlying Pittsburg Bluff Formation. The shell is ovate in outline and nearly equilateral, with an acutely-rounded to sub-truncate posterior margin and more broadly-rounded anterior margin.

Although Grant and Gale (1931, p. 255) did not illustrate C. bainbridgensis , they suggested that it be referred to Cyathodonta Conrad, 1849 . However, specimens from the Eocene and Oligocene of the Pacific Northwest all have weaker and non-undulating sculpture as well as lacking the “minute, very closely arranged, granulated radiating ribs” described by Conrad (1849, p. 156) in the type species, C. undulata . Cyathodonta is also posteriorly truncate and lacks the characteristic buttressing ridge that supports the chondrophore in Aperiploma . The chondrophore of a topotype of C. bainbridgensis from the Blakeley Formation ( Tegland, 1933, pl. 6 fig. 3) is refigured here ( Fig. 3F).

This is the earliest reported occurrence of the species and the earliest unequivocal occurrence of the genus in the Cenozoic of the Eastern Pacific.

Material examined — Fourteen specimens from the Keasey Formation and specimens from younger Paleogene formations noted below.

Hypotypes— USNM 561793, length 26.4 mm, height 18.6 mm, Loc. USGS 15318; UCMP 110673, length 21.5 mm, height 19.5 mm, Loc. UCMP IP1432 (= USGS M3865); UCMP 110674, length 22.0 mm, height 19.5 mm, Loc. UCMP IP1432 (= USGS M3865); UCMP 110675, length 20.6 mm, height 15.5 mm, Loc. UCMP IP1431 (= USGS M3862); UCMP 110676, length 14.9 mm, height 10.5, Loc. UCMP IP7983 (= USGS 25026). Unfigured hypotype: USNM 561794, length 10.7 mm, height 8.2 mm, Loc. USGS 15318.

Keasey Formation localities — USGS 15309, UCMP IP7983 (lower member); USGS 2713, 15279, 15314, 15318, UCMP IP7984 (middle member); USGS M3862, M3865 (middle member).

Other occurrences — Lincoln Creek Formation, Twin River Formation, and Blakeley Formation (Washington); Pittsburg Bluff Formation and Alsea Formation (Oregon). The species is also reported from the Poul Creek and Stepovak Formations in the Gulf of Alaska ( Burk 1965) but specimens were not examined in the course of this study. Collections from strata of probable middle Eocene age at USGS locality 15289 contain fragments that have an outline and ornamentation suggestive of Aperiploma . The fragments are in a shale lens within the Tillamook Volcanic Series in the Oregon Coast Range and are overlain by strata containing a late Eocene Cowlitz fauna.

Four of the five California Cenozoic species described under Periploma Schumacher, 1817 are inadequately preserved to confirm their generic or familial assignment. Specimens of a fifth species, Periploma cryphia Woodring, 1938 (and subspecies P. cryphia stenopa Woodring, 1938 ), from the deep-water facies of the Pliocene Repetto Formation in the Los Angeles Basin, preserve the distinctive chondrophore buttress. They are similar to Keasey specimens in compressional crushing of the shells and exfoliation of the thin exterior layer, and Woodring (p. 56) noted their similarity to specimens from the Blakeley Formation in Washington. However, there are no other records of A. bainbridgensis from the Neogene, and there is nothing similar in the living fauna of the eastern Pacific. Although shell morphology may be conservative, there is no basis for suspecting evolutionary stasis.