Meganodontia haunuiensis, 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 : 373-375

Meganodontia haunuiensis sp. nov.

Fig. 3 .

2016? Meganodontia sp. ; Saether et al. 2016: 16, table 1.

Etymology: Named for the type locality (Haunui) in North Island, New Zealand.

Type material: Holotype, UOA L4781 ( Fig. 3B ) from locality U23/ f270 (Haunui), collection AU19880 . Paratypes: UOA L4777 ( Fig. 3A ) from locality U23/f280 (Ugly Hill), collection AU 21299 ; UOA L4782 ( Fig. 3E ) , UOA L4791 ( Fig. 3C ) , UOA L4792 ( Fig. 3F ) from locality U23/f270, collection AU 21298 .

Type locality: Haunui seep site, North Island, New Zealand .

Type horizon: Ihungia Limestone , lower Miocene .

Dimensions.—See Table 1.

Diagnosis.—Moderate-size, subcircular and strongly inflated Meganodontia with narrow lunule demarcated by sharp ridge in both valves and deeply sunken posterior lanceolate dorsal gape. Hinge rather broad and edentulous.

Description. —Shell moderately thick, moderate size up to 84.6 mm in length, subcircular but slightly longer than high

H/L = 0.87–1.00), strongly inflated (W/L = 0.43–0.68), equivalve, inequilateral. Postero-dorsal margin straight making obtuse angle with subtruncated posterior margin; ventral margin well rounded, graduating subcircular posterior margin into nearly straight dorsal margin. Umbo rather prominent but not swollen, situated at anterior one-fourth to two-fifths of shell length (AL/L = 0.26–0.44). Maximum height slightly behind umbo in holotype but located at umbo in paratype specimens. Surface sculptured by fine commarginal growth lines and some irregular commarginal undulations. Very weak groove running from umbo to postero-ventral corner. Lunule narrow, demarcated by sharp ridge in both valves and occupying postero-dorsal margin observed in right valve; posterior lanceolate dorsal gape deeply sunken and probably once occupied by ligament. Scar trace of pallial blood vessel observed in one specimen. Hinge rather broad for genus and edentulous, except for nymph separating ligament and subumbonal flat area. Anterior adductor scar rather large, elongate and stout; posterior adductor scar indistinct and subquadrate. Pallial line entire with no secondary pallial attachment scars. Inner surface of shell covered by fine radial grooves.

Remarks. —Despite having comparatively smaller shell size compared to other species in the genus, we placed these specimens into Meganodontia Bouchet and Cosel, 2004 because they have a large anterior adductor scar, a less swollen umbo than Pegophysema , a curved hinge area and no secondary pallial attachment scar. However, the broad hinge plate with a distinct posterior ridge has not been observed in other species of Maganodontia. Meganodontia hoernea ( Des Moulins, 1868) from the middle to upper Miocene seep deposits of Italy is the most similar species to M. haunuiensis sp. nov. in that it also has a large shell and similar inflation (W/L = 0.31–0.71), but differs from the New Zealand species by being even larger (up to 180 mm in length) and possessing a narrower hinge plate.

Meganodontia sp. was illustrated from the Cuban early Miocene Cantera Portugalete seep site in Kiel and Hansen (2015). These specimens have similar outlines (especially their fig. 7C) to M. haunuiensis sp. nov. However, the Cuban specimens are larger (up to 120 mm in length) than M. haunuiensis sp. nov. A lack of information on the hinge area of the Cuban specimens prevents us from comparing these features.

Meganodontia haunuiensis sp. nov. can be easily distinguished from the type species of the genus, M. acetabulum , by its smaller shell (up to 84.6 mm in M. haunuiensis ; up to 150 mm in M. acetabulum ), more prominent umbo and much broader hinge plate.

“ Anodontia (Anodontia) ” waharoaensis Eagle, 1992 from Motutara, North Island, New Zealand, does not belong to Anodontia owing to its large size (L = 123 mm), but rather to the Pegophyseminae (genus uncertain). This species differs from M. haunuiensis sp. nov. by having a larger (L = 123 mm), thinner shell that is more elongated (H/L = 0.82; H/L = 0.87–1.00 in M. haunuiensis ), with a narrowly rounded anterior margin.

Meganodontia haunuiensis sp. nov. resembles the Recent Indo-Pacific species, Pegophysema philippiana ( Reeve, 1850) in its shell outline (H/L = 0.85–0.94 in P. phillipiana from the Kawamura Collection and NSMT-Mo 84906 stored at the National Science Museum, Tsukuba, Japan), size (max L = 71.0; Taylor and Glover 2005) and inflation (W/L = 0.52–0.74 from the Kawamura Collection and NSMT-Mo 84906 stored at the National Science Museum, Tsukuba). However, the broader hinge and larger, but narrower lunule of M. haunuiensis sp. nov. are very different from the same shell characters in P. philippiana .

Meretrix sphericula Basedow, 1902 (= Anodontia sphericula by Darragh, 1970) was described from the Miocene crystalline limestone around Edithburg, Australia. Ludbrook (1978) also described a well-preserved specimen from the Pliocene Roe Formation, Australia. This species is large in size (up to 105 mm) and has a strongly inflated shell (W/L = 0.75). However, the Australian species differs from M. haunuiensis sp. nov. in possessing a thinner shell with narrow hinge plate, a more horizontal postero-dorsal margin and a more protruding umbo.

Loripes goliath Yokoyama, 1928 , from the Miocene of Taiwan has a very large shell (up to 150 mm in length) and was questionably allocated to Anodontia by Taylor and Glover (2009). This species differs from M. haunuiensis sp. nov. by having a more inflated shell and steeply sloping and straight postero-dorsal margin. Nonetheless, poor preservation and a lack of information about the internal structure of L. goliath prevent us from comparing the species further.

Stratigraphic and geographic range. —Haunui, Ugly Hill, Ngakawa, and Wanstead; Lower Miocene Ihungia Limestone, North Island, New Zealand.

Figures

Fig. 3. Lucinid bivalve Meganodontia haunuiensis sp. nov. from the lower Miocene Ihungia Limestone at Haunui (B–G, U23/270) and Ugly Hill A, U23/280), North Island, New Zealand. A. UOA L4777 (paratype), edentulous hinge plate of left valve (A1,), left valve (A2). B. UOA L4581 (holotype), dorsal view (B1) and frontal view (B2) of right valve. C. UOA L4791 (paratype), edentulous hinge plate of right valve. D. UOA L4790, right valve of small specimen. E. UOA L4782 (paratype), dorsal view of slightly compressed specimen (E1), left valve (E2). F. UOA L4792 (paratype), dorsal view showing symmetric lunule in both valves (F1), right valve showing internal mould (F2), enlargement of the anterior adductor muscle scar of F2 (F3). G. UOA L 4789, right valve, showing internal mould of small specimen. Scale bars 10 mm.