Nuculana acutilineata, Hybertsen & Kiel, 2018

Hybertsen, Frida & Kiel, Steffen, 2018, A middle Eocene seep deposit with silicified fauna from the Humptulips Formation in western Washington State, USA, Acta Palaeontologica Polonica 63 (4), pp. 751-768 : 757-758

Nuculana acutilineata sp. nov.

Fig. 12 .

Etymology: From Latin acuti, sharp and lineae, lines; in reference to the fine increments on the outer shell.

Type material: Holotype: NRM Mo 185009 consisting of a right valve with interior features . Paratypes: NRM Mo 185008, left valve ; NRM Mo 185010, young articulated specimen; NRM Mo 185011, adult articulated specimen; NRM Mo 185012, right valve.

Type locality: The Satsop Weatherwax seep deposit, Washington State, USA .

Type horizon: Basal Humptulips Formation, middle Eocene.

Material.— More than 300 specimens at NRM , including type material, mostly single valves along with numerous shell fragments. Five specimens are deposited in the Burke Museum ( UWBM 108856–60 ) .

Diagnosis. —Elongated, oval and equivalve shell with slightly oval and wrinkled prodissoconch.

Description.—Prodissoconch slightly oval; sculptured by up to seven commarginal wrinkles; transition to dissoconch marked by thick prominent rib; single valve dimensions up to length 6 mm, height 2 mm and width 1.2 mm. Elongated oval dissoconch, equivalve; more than 25 fine commarginal growth lines; pointed beaks of fair size; umbo anterior (at 25% of total length); angular hinge line with taxodont teeth, at least 12 anterior and at least 18 posterior teeth found in three specimens (see Fig. 12B ); anterior muscle scar pear-shaped, pallial line parallel to dorsal margin (see Fig. 12E ).

Remarks.— Nuculana acutilineata does not have the elongated posterior end seen in the type taxon Nuculana pernula (Pliocene of North America and Japan to Recent). Nuculana sp. as described by Goedert and Campbell (1995) has thicker and fewer commarginal ribs on the shell than N. acutilineata . Both Nuculana elenensis ( Sowerby, 1833) (Pleistocene of North America to Recent) and Nuculana taphira Dall, 1896 (Oligocene of northwestern North America to Recent) have fine and many commarginal growth lines similar to N. acutilineata , however, N. acutilineata has a more rounded posterior end and straighter anterodorsal margin than N. elenensis , and the posterior end on N. taphira is narrower than on N. acutilineata . The growth lines on Nuculana minuta Müller, 1776 (Pliocene of North America to Recent) are fine and about the same number as on N. acutilineata , but the prominent, pointy prodissoconch is different as well as the wide, marginal ridge defining the escutcheon on N. minuta . N. acutilineata does resemble the Recent Nuculana grasslei Allen 1993 , possibly also associated with cold-seep carbonates of the Lincoln Creek Formation ( Peckmann et al. 2002; Kiel 2006) which is characterized by a large, ornamented prodissoconch. Nuculana grasslei was re-assigned to “ Tindariopsis ” grasslei by Coan and Valentich-Scott (2012). The prodissoconch can be seen in our specimens with fairly strong wrinkles; however N. acutilineata has over 25 commarginal growth lines at a narrower distance than at least 20 growth lines on T. grasslei .

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 12. The nuculanid bivalve Nuculana acutilineata sp. nov. from the middle Eocene Satsop Weatherwax seep deposit, Washington State, USA. A. NRM Mo 185010, almost complete young specimen with initial part intact (A1); close-up on initial part (A2). B. NRM Mo 185008, left valve in lateral view. C. NRM Mo 185009 (holotype), right valve in lateral view showing interior features. D. NRM Mo 185012, right valve specimen showing interior with hinge, faint muscle scar and pallial line. E. NRM Mo 185011; almost complete adult specimen (E1); close-up of escutcheon and dentition (E2).