Lucinoma saetheri, Amano & Little & Campbell, 2018

Amano, Kazutaka, Little, Crispin T. S. & Campbell, Kathleen A., 2018, Lucinid bivalves from Miocene hydrocarbon seep sites of eastern North Island, New Zealand, with comments on Miocene New Zealand seep faunas, Acta Palaeontologica Polonica 63 (2), pp. 371-382 : 377-379

Lucinoma saetheri sp. nov.

Fig. 5 .

2008 Lucinoma aff. taylori ; Campbell et al. 2008: 90.

Etymology: Named for Kristian P. Saether who studied the hydrocarbon seep fauna of New Zealand for his PhD at the University of Auckland.

Type material: Holotype, UOA L4780 ( Fig. 5D ) from locality U23/f266 (Ugly Hill), collection AU19663 . Paratypes: UOA L4772 ( Fig. 5B ) from locality Y16/f1174 (Moonlight North), collection AU 19923 ; UOA L4773 ( Fig. 5A ) from locality Y16/f1059 (Moonlight North), collection AU 19982 ; UOA L4779 ( Fig. 5F ) from locality U23/f266, collection 19663.

Type locality: Ugly Hill , North Island, New Zealand .

Type horizon: Ihungia Limestone , lower Miocene .

Dimensions.—See Table 3. Diagnosis.—Large, subcircular and moderately inflated Lucinoma with weak commarginal lamellate ribs, rather straight and gently oblique antero-dorsal margin, rounded anterior margin and weak posterior sulcus. Right valve hinge with very weakly bifid posterior cardinal tooth (3b) and relatively thick anterior cardinal tooth (3a). Ligament strong.

Description. —Shell large, up to 67.4 mm in length, rather thick, subcircular (H/L = 0.91–0.99), moderately inflated (W/L = 0.52–0.61), equivalve and slightly inequilateral. Antero-dorsal margin rather straight and gently oblique, making a near right angle turn at anterior margin end, and well-rounded ventral margin; postero-dorsal margin broadly curved, moving into posterior margin at obtuse angle; ventral margin semi-circular. Very weak sulcus present from umbo to postero-ventral corner in large specimens, including holotype and one paratype (UOA L4779). Surface sculptured with regular and widely spaced, thin commarginal lamellae; interspaces with two or three dense commarginal threads. In large specimens, these lamellae becoming low and nearly equal in strength with interstitial threads. Umbo prominent above dorsal line, prosogyrate and located at anterior one-third to two-fifths of shell length (AL/L = 0.31–0.48). Lunule lanceolate, sunken; demarcated by sharp ridge. Ligament strong, occupying about 70–80% of postero-dorsal length. Hinge of right valve with two cardinal teeth and one anterior lateral tooth; anterior tooth (3a) small, but relatively thick, oblique anteriorly; posterior tooth (3b) stout, very weakly bifid, oblique posteriorly; anterior lateral tooth (AI) very weak. Pallial line entire. Anterior adductor muscle scar elongate and long; posterior adductor muscle scar rather large and pyriform.

Remarks. —The shell size of specimens in the seep deposits south of Hawke Bay (such as at the type locality) is often larger than those in the seeps north of Hawke Bay, up to 67.4 mm in length in the former and up to 45.0 mm in length in the latter.

Campbell et al. (2008) listed Lucinoma aff. taylori from the Rocky Knob seep deposit. However, lucinid specimens from this site are not Lucinoma taylori ( Powell, 1935) , instead belonging to L. saetheri sp. nov.

As noted above, Lucinoma saetheri sp. nov. does somewhat resemble L. taylori ( Powell, 1935) from the lower Miocene Trikohua Formation of the Motutara area, west coast of Auckland, New Zealand. However, L. taylori is not as large as Lucinoma saetheri sp. nov. and has a shorter antero-dorsal margin, a broadly arched ventral margin, a more distinct posterior sulcus, and a less inflated shell (W/L = 0.46 in holotype; W/L = 0.52–0.61 in L. saetheri sp. nov.).

Lucinoma galathea ( Marwick, 1953) is a Recent species living in and around New Zealand, and differs from L. saetheri sp. nov. by having a distinctly bifid and more oblique posterior tooth (3b), a strong anterior lateral tooth, a subtruncated anterior margin, and being smaller (maximum size up to 51.5 mm; up to 67.4 mm in L. saetheri sp. nov.).

Another large, living Lucinoma species, L. taiwanensis Cosel and Bouchet, 2008 , can be easily separated from L. saetheri sp. nov. by having a distinct ridge from the beak to the anterior ventral corner.

Lucinoma perusinum ( Sacco, 1901) from the middle to upper Miocene seep carbonates of northern Italy was described as L. perucina by Kiel and Taviani (2017) who allocated this species to the genus Lucinoma rather than the original designation as Dentilucina perusina by Sacco (1901). We agree with their allocation, but as Lucinoma is a neutral noun, the species name should be changed to perusinum . This Italian species is similar to L. saetheri sp. nov. in having a similar condition of the umbo and outline. However, the Italian species has a straight postero-dorsal margin and a more elongate lunule than L. saetheri sp. nov.

Lucinoma acutilineatum ( Conrad, 1849) is a common species of Lucinoma from the lower Miocene seep sites of Japan. Specimens from the Kokozura Formation have a large shell (up to 58.5 mm in length; Kamada 1962). However, L. acutilineatum ( Conrad, 1849) has some regularly lamellate commarginal ribs, even in large specimens, and a distinctly truncated posterior margin, so differs from L. saetheri sp. nov.

Stratigraphic and geographic range. —Ugly Hill and Haunui; lower Miocene Ihungia Limestone. Bexhaven, Moonlight North and Rocky Knob; middle Miocene Bexhaven Limestone. North Island, New Zealand.

Figures

Fig. 5. Lucinid bivalve Lucinoma saetheri sp. nov. from the lower Miocene Ihungia Limestone at Ugly Hill (D, F, U23/266) and the middle Miocene Bexhaven Limestone at Moonlight North (A, C, E, Y16/1059; B, Y16/f1174), North Island, New Zealand. A. UOA L4773 (paratype), internal mould of right valve. B. UOA L 4772 (paratype), hinge plate of right valve. C. UOA L4785, left (C1) and right (C2) valves. D. UOA L4780 (holotype), dorsal view D1), right (D2) and left (D3) valves. E. UOA L4786, right valve. F. UOA L 4779 (paratype), left valve showing lamellated commarginal riblets (F1), dorsal view (F2). Scale bars 10 mm.