Protobranchia Pelseneer, 1889

Kiel, Steffen & Goedert, James L., 2007, New mollusks associated with biogenic substrates in Cenozoic deep-water sediments of Washington State, Acta Palaeontologica Polonica 52 (1), pp. 41-52 : 46

Subclass Protobranchia Pelseneer, 1889 Family Nuculanidae H. and A. Adams, 1858 Genus Nuculana Link, 1807

Type species: Arca rostrata Bruguière, 1789 ; Recent, Indo−Pacific.

Remarks.— Nuculana is used here in a broad sense. We are aware that many genera and subgenera are in use for nuculanids and Nuculana based on characters like shell shape, hinge dentition and muscle attachment scars ( Coan et al. 2000; La Perna 2003; La Perna et al. 2004). The internal features of Nuculana posterolaevia sp. nov. are unknown and hence a more precise classification of the species than to “ Nuculana sensu lato ” is impossible at present. However, Nuculana posterolaevia has distinct external features on which it can readily be distinguished from other Recent and fossil nuculanids of this region.

“ Nuculana ” posterolaevia sp. nov.

Fig. 5 .

1996 Nuculana sp. ; Goedert and Campbell 1996: 25, fig. 2.

2003 “ Nuculana ” sp. aff. “ N.” grasslei Allen ; Goedert et al. 2003: 226, pl. 43: 8.

Derivation of the name: From Latin laevis, smooth, for its smooth posterior part.

Holotype: LACMIP 12310 . Specimen with damaged central part of shell.

Paratype: UWBM 97315 , from UWBM loc. B6753.

Type locality: USGS loc. 26895, Shipwreck Point, Clallam County,

Washington State, USA.

Type horizon: Lower Oligocene, Makah Formation.

Material.— The type material and several specimens from the wood−fall sites USGS locs. 26896−D, 26897−A, 26898−B, and from a whale specimen ( USNM 314545 ) from USGS loc. 26898. These specimens are deposited in the Cenozoic mollusk collection of the USNM.

Diagnosis.—A weakly rostrate Nuculana with orthogyrate umbo in anterior position (approx. 38% total length); sculpture of strong commarginal ribs and equally sized interspaces on anterior and central part of shell, ribs fade on posterior part of shell; rostrum smooth.

Description.—Shell moderately elongate, height/length ratio approx. 1.52–1.57; anterodorsal margin straight, anterior margin rounded and slightly pointed; posterodorsal margin convex near the umbo and becomes straight on the rostrum; lunule long, narrow, and only slightly excavated; ventral margin well−rounded; rostrum with growth lines only. Hinge unknown. The holotype is 7.0 mm long and 4.6 mm high, the paratype is 6.0 mm long and 3.8 mm high.

Discussion.—“ Nuculana ” posterolaevia sp. nov. is the most frequently found taxodont protobranch at cold seeps, whale− and wood−falls in the Cenozoic deep−water sediments in Washington. However, despite the overall similarity between these fossil communities and their modern analogs, this or a similar species has not yet been reported from extant seeps, whale− or wood−falls. The extant vent inhabiting Nuculana grasslei Allen, 1993 differs significantly by its sculptured posterior part of the shell.

The extant North Pacific Nuculana (Jupiteria) taphira ( Dall, 1896) is similar to “ Nuculana ” posterolaevia in also having a smooth posterior part, but N. (J.) taphira differs by having more prosogyrate beaks, its smooth posterior part is shorter and more slender, and the posteroventral margin is concave ( Coan et al. 2000: pl. 7), whereas it is convex in “ N. ” posterolaevia . Nuculana ochsneri var. elmana Etherington, 1931 from the Miocene Astoria Formation in Oregon and Washington State is more elongate and its smooth posterior part is much narrower than in “ N. ” posterolaevia .

Stratigraphic and geographic range.—Uppermost Eocene to lowermost Miocene, Washington State, USA, associated with wood, whale bones, and in cold−seep carbonates at Shipwreck Point, Makah Fm. ( Goedert and Campbell 1995) and Whiskey Creek, Pysht Fm. ( Goedert et al. 2003).

Figures

Fig. 5. Nuculanid bivalve “Nuculana” posterolaevia sp. nov. from Eocene and Oligocene seep carbonates, whale− and wood−falls in Washington State. A. Paratype UWBM 97315, from a uppermost Eocene seep−carbonate at Whiskey Creek (UWBM loc. B6753). B. Holotype LACMIP 12310, from a lower Oligocene seep−carbonate at Shipwreck Point (USGS loc. 26895).