Pleurophopsis thieli, Kiel & Amano & Goedert, 2023

Kiel, Steffen, Amano, Kazutaka & Goedert, James L., 2023, New taxa, records, and data for vesicomyid bivalves from Cenozoic strata of the North Pacific region, Acta Palaeontologica Polonica 68 (2), pp. 297-320 : 311-314

Pleurophopsis thieli sp. nov.

Figs. 11 , 12.

2002 Vesicomya (Calyptogena) sp.; Peckmann et al. 2002: 861, fig. 3C. 2003 Vesicomya (Calyptogena) sp.; Goedert et al. 2003: pl. 42: 6, 7.

Zoobank LCID: urn:lsid:zoobank.org:act:C3DAEBBE-2844-4BDB-BEA4-3C1C01A76A69 .

Etymology: In honor of Bruce Thiel (Portland, Oregon), in recognition of his help with fieldwork related to this project.

Type material: Holotype: articulated internal mold showing pallial line and muscle scars ( NRM Mo 204761) . Paratypes: mostly internal molds ( NRM Mo 204762–65, 204826) from Jansen Creek Member ; articulat- ed or semi-articulated specimens with shell material ( Mo 204794–797) and internal molds ( Mo 204798–802 from Whiskey Creek seep deposit. Upper Eocene to lower Oligocene, Washington State, USA.

Type locality: Shoreline exposures approximately 800 m southeast of the mouth of Rasmussen Creek , Clallam County, Washington State, USA .

Type horizon: Jansen Creek Member of the Makah Formation, lower Oligocene .

Material.—About 25 specimens and fragments from the Whiskey Creek site, Pysht Formation (NRM Mo 204794– 204802, UWBM 97305, 06). About 34 specimens from the Jansen Creek Member of the Makah Formation, including four from near Rasmussen Creek (NRM Mo 204761–64) and about 30 from near the mouth of Jansen Creek, many in butterfly preservation (NRM Mo 204765–68, 204828). Nine specimens from UWBM loc. B6702 (= SR 2 in Peckmann et al. 2002), Lincoln Creek Formation (NRM Mo 204826, 27). Upper Eocene to lower Oligocene, Washington State, USA. UWBM 97038 was previously reported as Vesicomya (Calyptogena) sp. by Peckmann et al. (2002).

Diagnosis.—Shell average size for genus, inflation slight to moderate, posterior side tapering, often bent downward due to concave ventral margin; anterior adductor muscle scar bean or D-shaped; posterior adductor muscle scar rather small, displaced anteriorly, near posterodorsal margin; hinge with two strong, radiating teeth in right valve and three in left valve; subumbonal pit large and deep.

Description.—Shell elongate, up to 57.6 mm long; umbones anterior, at about 20% of total shell length; blunt, low ridge running from umbones to posterior margin; no lunular incision or escutcheon; anterior margin evenly to acutely rounded; anterior half of ventral margin straight, posterior half slightly concave, giving shell curved appearance; posterodorsal margin either straight and slightly tapering ( Fig. 11D ), or anterior half straight and parallel to ventral margin, and posterior half more strongly inclined toward the ventral (Fig. 12E); posterior margin evenly rounded. Anterior adductor muscle scar bean to D-shaped with posterior edge, pallial line starting at its posteroventral corner, extending subparallel to ventral shell margin, and bending upward and anteriorly near the posterior shell margin, to merge into ventral margin of posterior adductor muscle scar; posterior adductor muscle scar positioned at about 30% of total shell length, relatively small, with broad trapezoid area including the pedal retractor scar to its anterior; hinge plate short and moderately broad, with two strong, radiating cardinal teeth in each valve; RV with short cardinal tooth 1 originating under umbo, running oblique to anterodorsal shell margin, cardinal tooth 3a reduced, 3b originating under umbo, short, running oblique to posterodorsal shell margin; LV with short, thin cardinal tooth 2a running subparallel to anterodorsal shell margin, 2b thick, triangular or bifid, directly under umbo, 4b very short and thin, or reduced; oval subumbonal pit present in each valve.

Dimensions.—See Table 8.

Remarks.—Most similar to Pleurophopsis thieli sp. nov.

Fig. 12. The vesicomyid bivalve Pleurophopsis thieli sp. nov., from lower Oligocene , western Washington State, USA. Jansen Creek Member , Makah → Formation (A–C, G); “Big slide”, Jansen Creek Member , Makah Formation (D–F); UWBM loc. B6702, Lincoln Creek Formation (H). A. Paratype, NRM Mo 204762, internal mold of articulated specimen, showing anterior adductor muscle scar (aams), inflation, and long ligament; left valve (A 1) and dorsal (A 2) views. B. Holotype, NRM Mo 204761, internal mold of articulated specimen, showing muscle scars and pallial lin; dorsal view of posterior part (B 1) and right valve (B 2). C. Paratype, NRM Mo 204764, internal mold of LV, showing anterior adductor muscle scar and course of anterior part of pallial line. D. Paratype, NRM Mo 204768, RV with preserved shell material, showing external sculpture. E. Paratype, NRM Mo 204765, RV with preserved shell material, showing external sculpture. F. Paratype, NRM Mo 204828, block with numerous specimens in butterfly preservation. G. Paratype, NRM Mo 204763, rubber cast of

LV hinge area. H. Paratype, NRM Mo 204826, anterior part of RV showing hinge dentition. B and C not coated with ammonium chloride.

is Pleurophopsis chitanii ( Kanehara, 1937) from Lower to Middle Miocene strata in Japan ( Kanehara 1937; Hirayama 1973; Amano and Jenkins 2007; Amano and Kiel 2011). The main difference is in the posterior adductor muscle scar, which is smaller and more anteriorly positioned in P. thieli , whereas P. chitanii has a very large posterior adductor muscle scar in an almost terminal posterior position.

Specimens resembling P. thieli have been reported as P. chitanii from presumably Lower to Middle Miocene strata on Kayak Island, Alaska ( Kanno 1971). In our view, the identity and relation of the Kayak Island specimens to either P. chitanii or P. thieli are somewhat uncertain. The size and morphological range of the specimens sketched and illustrated by Kanno (1971) suggests that they perhaps belong to more than one species. One illustrated specimen Kanno 1971: pl. 7: 6) shows a pams that appears to be in a somewhat intermediate position between those seen in P. chitanii and P. thieli . Confusingly, although Kanno (1971) lists P. chitanii only from USGS loc. M 1769 in the upper part of the Yakataga Formation on Kayak Island, Plafker 1974) lists only one mollusk from that locality, a sareptid bivalve. Plafker (1974) does, however, list P. chitanii from the upper and lower Yakataga Formation at four other localities on Kayak Island. Plaf ker (1974) lists Calyptogena n. sp.? from five additional localities, one of which (USGS loc. 15861), approximately 1.8 km east of Cape St. Peter is apparently the same, or very near loc. M1769 according to the map. Additional collecting effort is needed to clarify how many species of Pleurophopsis (or possibly other vesicomyid genera) are present in the strata on Kayak Island, including the upper and lower Katalla Formation because Plafker (1974) also recorded? Hubertschenckia and a “vesicomyacid” from those strata. We also do not at present know anything about the local detailed geologic context for each of the sites listed and mapped by Plafker (1974); much more could be learned if these are discreet ancient seep sites. According to Addicott (1976: 123), the age of the Yakataga Formation on Kayak Island is Early Miocene, whereas the exposures on the mainland are Middle Miocene.

Pleurophopsis thieli differs from the late Eocene P. chinookensis by having a lower shell, more curved outline, resulting from its often slightly concave ventral margin and its sloping posterodorsal margin, whereas P. chinookensis has nearly straight and subparallel posterodorsal and posteroventral margins. The Middle Miocene type species P. unioides also has straighter and subparallel posterodorsal and posteroventral margins compared to P. thieli , and furthermore has larger and more round anterior and posterior adductor muscle scars ( Van Winkle 1919; Kiel 2007; Krylova et al. 2010).

Stratigraphic and geographic range.—Seep deposits within the Pysht and Makah formations, upper Eocene to lowermost Oligocene, western Washington State, USA.

Figures

Fig. 11. The vesicomyid bivalve Pleurophopsis thieli sp. nov., from the upper Eocene Whiskey Creek seep deposit in the Pysht Formation in western Washington State, USA. A. NRM Mo 204797, small articulated specimen, showing inflation; left valve (A1) and dorsal (A2) views. B. NRM Mo 204795, semi-articulated specimen; left valve (B1) and right valve (B2) views. C. NRM Mo 204796, small left valve, showing lack of lunular incision. D. NRM Mo 204794, LV showing shell outline. E. NRM Mo 204799, hinge area and anterior adductor muscle scar of LV. F. NRM Mo 204800, hinge plate of right valve. G. NRM Mo 204801, hinge plate and ligament nymph of RV. H. NRM Mo 204802, hinge plate and ligament nymph of LV. All specimens are paratypes.