Lucinid bivalves from Miocene hydrocarbon seep sites of eastern North Island, New Zealand, with comments on Miocene New Zealand seep faunas

Three new fossil species of lucinids, Meganodontia haunuiensis, Elliptiolucina neozelandica, and Lucinoma saetheri , are described from lower to middle Miocene hydrocarbon seep carbonates from north and south of Hawke Bay, eastern North Island, New Zealand. Of these taxa Meganodontia haunuiensis is confined to seep sites south of Hawke Bay, while Elliptiolucina neozelandica comes only from the seep sites north of Hawke Bay. Using ecological information from modern bivalve species, we suggest the southern area seep sites formed in shallower waters than the northern sites. Among the lucinids, Meganodontia haunuiensis is one of the oldest records of the genus, and its distribution has shrunk from worldwide in the Miocene times to only around Taiwan today. Elliptiolucina neozelandica is the second oldest species in the genus. Since the Miocene the distribution of Elliptiolucina has narrowed but its habitat range has increased to both seeps and sandy environments, mainly around the Philippines.

Until now, only a few modern and fossil lucinid species have been recorded from hydrocarbon seep sites from North Island, New Zealand.Campbell et al. (2010) illustrated Lucinoma galathea (Marwick, 1953) from modern seeps off the Hikurangi Margin.Campbell et al. (2008) listed Lucinoma aff.taylori from the Miocene Rocky Knob seep site in the Gisborne region (Fig. 1).Saether et al. (2016) figured Meganodontia?sp. from several seep sites in the Hawke's Bay region east of Dannevirke (Fig. 1).Kiel et al. (2013) illustrated unidentified species belonging to the genera Ezolucina?and Nipponothracia?from Cenomanian to Campanian (Cretaceous) North Island seep deposits.Specimens of Nipponothracia?from the Cenomanian Port Awanui seep site on the Raukumara Peninsula were subsequently described as Amanocina raukumara by Kiel (2013).
The Miocene seep deposits of North Island occur in two main areas north and south of Hawke Bay, respectively, north of Gisborne and east of Dannevirke (Fig. 1).The majority of the molluscan taxa from these seep sites have been described by Saether et al. (2010aSaether et al. ( , b, 2012Saether et al. ( , 2016) ) and Amano et al. (2014Amano et al. ( , 2015)), except for the lucinids.In this paper, we describe and illustrate the Miocene seep lucinids from North Island, and briefly discuss the wider paleoenvironmental and regional paleobiogeographical context of the Miocene New Zealand seep communities.

Material
In the area east of Dannevirke, lucinids were collected from the Ugly Hill and Haunui seep sites of the lower Miocene Ihungia Limestone.In the area north of Gisborne, lucinid specimens were collected from the Bexhaven, Moonlight North and Rocky Knob seep sites of the middle Miocene Bexhaven Limestone.
All but four of the fossil specimens described and figured herein were collected in February 2012 by the authors, and these are deposited in the paleontology collections of the School of Environment, the University of Auckland (UOA).Additional fossils collected by UOA paleontologists between 1997-2010, also housed in the UOA collections, were examined.All the localities of the illustrated specimens are registered in the New Zealand Fossil Record File database (http://www.fred.org.nz),jointly administered by GNS Science and the New Zealand Geoscience Society.For com- parative purposes, modern seep lucinids were studied, comprising specimens dredged during the 2007 joint German-New Zealand NEW VENTS Hikurangi margin cruise with the R/V SONNE (stored in the UOA collections), and some Lucinoma specimens at the Museum of New Zealand, Te Papa Tongarewa (Wellington).Fig. 2 shows terminology for description.
Remarks.-Meganodontia acetabulum Bouchet and Cosel, 2004 has a very large shell (up to 150 mm in length).Bouchet and Cosel (2004) listed the generic characters: edentulous hinge, thick-shelled, large anterior adductor muscle scar broadening ventrally, less prominent umbo and maximum height located behind the umbo.However, based on our observations of published photographs of the Italian fossil species Meganodontia hoernea (Des Moulins, 1868), this latter character seems to be absent, so it may be a species rather than generic-level character.Within the subfamily Pegophyseminae, Pegophysema Stewart, 1930 differs from Meganodontia by having a smaller and thinner anterior adductor muscle scar, a more swollen umbo, a straighter hinge and some secondary pallial attachment scars (Bouchet and Cosel 2004;Taylor and Glover 2005).In addition, most Recent species of Pegophysema come from very shallow water sites, whereas fossil and Recent Meganodontia species come from deep-sea vent and seep sites (Bouchet and Cosel 2004;Taylor andGlover 2005, 2016;Kiel and Hansen 2015;Kiel and Taviani 2017).
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.rior 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.
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 = 0.52-0.74from 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.Subfamily Myrteinae Chavan, 1969 Genus Elliptiolucina Cosel andBouchet, 2008 Type species: Elliptiolucina magnifica Cosel and Bouchet, 2008;Recent, central Philippines.Remarks.-Thisgenus is characterized by having a subquadrate-ovate and weakly inflated shell, an edentulous hinge except for small cardinal and anterior lateral teeth in E. williamsae Glover and Taylor, 2016, and a short anterior adductor scar.Other edentulous lucinids with a weakly inflated, elongate shell include Nipponothracia Kanie and Sakai, 1997, Elongatolucina Gill and Little, 2013, and Amanocina Kiel, 2013.According to Okutani (2011), Elliptiolucina can be separated from Nipponothracia by its smaller and thicker shell and possession of a characteristic dorsal concavity in the posterior adductor muscle scar.Elongatolucina has an elongate shell with straight ventral margin, which is not seen in Elliptiolucina (see Kiel 2013).Amanocina can be easily distinguished from the other genera as it lacks a subumbonal triangular excavation and has a longer adductor muscle scar detached at a narrower angle from the pallial line (Kiel 2013).
Diagnosis.-Small and compressed Elliptiolucina having a thin shell without a sulcus.Antero-dorsal margin gently sloping to subtruncated anterior margin.Lunule narrow and escutcheon deeply sunken, demarcated by sharp ridge.Hinge of right valve shallowly sunken and edentulous with nymph.
Description.-Shellsize rather small for genus, up 34.9 mm in length; rather thin-shelled, elongate ovate (H/L = 0.75), compressed, equivalve and inequilateral.Antero-dorsal margin gently sloping, making right angle with subtruncated anterior margin; ventral margin broadly arched and subparallel with dorsal margin; postero-dorsal margin nearly straight, leading into subtruncated posterior margin.Distinct but shallow sulcus present from beak to an-tero-ventral corner; shallow depression from beak running in postero-ventral direction.Surface largely smooth, sculptured by growth lines and about four very low and dense commarginal threads between growth lines.These growth lines turning into lamellated ribs near anterior and posterior dorsal margins.Umbo not prominent, prosogyrate and located at anterior one-third of shell length in one small entire specimen.Lunule narrow, lanceolate, shallowly sunken.Escutcheon lanceolate, deeply sunken and demarcated by sharp ridge.Hinge of right valve shallowly excavated and edentulous with nymph.Pallial line entire.Anterior adductor muscle scar elongate, long and slightly detached from pallial line; posterior adductor muscle scar large, subquadrate and rather deeply depressed on dorsal side.
Remarks.-The outlines, hinge characters, and shape of the posterior adductor scar show that the specimens belong to the genus Elliptiolucina, except for having rather thin shells.This is the first record of fossil Elliptiolucina in New Zealand.
Elliptiolucina neozelandica sp.nov. is most similar to the Recent Philippines Sea species, E. labeyriei Cosel and Bouchet, 2008 in having a relatively small, rather thin shell, a narrow lunule and a similar outline.However, E. labeyriei has a distinct and wide sulcus from the beak to the posteroventral corner, which is not seen in E. neozelandica sp.nov.
Elliptiolucina williamsae Glover and Taylor, 2016 from the central Philippines lives in rather shallow water (280-679 m depth) compared to other species in the genus.This  species also has a small, thin shell (up to 44.1 mm), as in E. neozelandica sp.nov.However, the Recent species has a well-rounded ventral margin with a cardinal tooth and an anterior lateral tooth.
Elliptiolucina washingtonia Kiel, 2013 from the upper Oligocene Lincoln Creek Formation in Washington State (Kiel 2013) is up to 140 mm long, larger than any of the extant species, and larger than E. neozelandica sp.nov.
Elliptiolucina hetzeli (Martin, 1933) from the upper Miocene formation of Buton Island, Indonesia, redescribed by Kiel (2013), can be separated from E. neozelandica sp.nov.by its larger shell (more than 48 mm) and acutely rounded posterior margin.
Another Indonesian species, the living E. virginiae Cosel and Bouchet, 2008, can be easily separated from E. neozelandica sp.nov.by its larger size (up to 78 mm long), truncated posterior margin and narrow hinge plate.
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.-Shelllarge, 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 twofifths 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  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.  4 shows the molluscan taxa described so far from the Miocene seep sites in eastern North Island of New Zealand, based on this study and data from Saether et al. (2010aSaether et al. ( , b, 2012Saether et al. ( , 2016) ) and Amano et al. (2014Amano et al. ( , 2015)).The species composition differs to the north and south of Hawke Bay.Bathymodiolus heretaunga, Gigantidas coseli, and Provanna marshalli are shared by both areas, but are predominant in the northern area.Some of the taxa intimately linked to seepage are confined to the northern seep sites: solemyids, Elliptiolucina neozelandica, Thyasira beui, Pliocardia? sp., and "Serradonta" kimberleyae, together with vagrant species: the taxodont bivalves Lamellinucula and Nuculana, the limid Acesta, the mud pecten Parvamussium and the cardiid Pratulum.In contrast, lucinids are predominant members of the southern seep sites.Of these, Maganodontia haunuiensis has been found only from the southern sites; whereas Lucinoma saetheri is common to both areas.
The distribution pattern of the lucinids suggest that there was likely a palaeobathymetric difference between the northern and southern areas of Miocene seepage in eastern North Island.Elliptiolucina and some vagrant specimens occur only at the northern Moonlight North locality.Most modern Ellliptiolucina species live in the deep sea (520-2570 m) around the Philippines, Indonesia, and southern Japan (Cosel and Bouchet 2008;Okutani 2011;Kuhara et al. 2014).Of the vagrant species, Lamellinucula and Pratulum now live in shallow water around New Zealand, 5-46 m and 15-180 m, respectively (Powell 1979).However, the former lives in deeper seas of Japan (50-3000 m; Higo et al. 1999).The living depth of Nuculana ranges from 0-980 m around New Zealand (Powell 1979).Acesta saginata and Parvamussium maorinum are confined to the deep sea 1000-1400 m and 168-1300 m, respectively (Powell 1979).

Species
The large lucinid Meganodontia occurs only in the southern seep sites.The sole Recent species, Meganodontia acetabulum, has been recorded from 246 m to 472 m water depth as dead shells (Bouchet and Cosel 2004).These shells were plausibly derived from shallower water.In addition, as described above, Lucinoma saetheri sp.nov. is larger in the southern sites than the northern ones and, according to Tunberg (1984), off Norway the Recent species Lucinoma borealis grows larger in shallower water than deeper sites.Moreover, one vagrant species from the Haunui seep site, Leptomya, now lives in shallow water (4-216 m) around New Zealand (Powell 1979).
Using the above depth ranges of living bivalves as a comparative tool, we suggest the seep sites north of Hawke Bay formed in deeper water than the seep deposits to the south of Hawke Bay, the latter inferred to have developed over a depth range of approximately 250-472 m (based on Meganodontia).This palaeobathymetric estimation is consistent with the lithological differences between seep deposits in the northern and southern areas previously noted by Campbell et al. (2008).Specifically, the southern seep deposits are sandier than those in northern areas.This lithological difference also reflects the occurrence of the two deposit feeding bivalves taxa Lamellinucula and Nuculana, which are confined to the finer-grained, northern seep deposits.
Ancient and Recent distributions of Lucinoma, Meganodontia, and Elliptiolucina.-Lucinomafirst appeared in seep sites of early Oligocene age (33.9-27.8Ma) in western Washington, USA (Goedert and Campbell 1995;Peckmann et al. 2002;Kiel 2010), and today is widely distributed among seep sites at upper bathyal depths on the Louisiana slope, off Chile, West Africa, Taiwan, Japan, and New Zealand, as well as in Mediterranean mud volcanoes and the oxygen minimum zones off California and Oman (e.g., Taylor and Glover 2010;Campbell et al. 2010).Thus, the worldwide distribution of Lucinoma indicates its adaptability to different environments, and may be one of the reasons why L. saetheri is found regionally, and in different lithologies, in Miocene seeps of New Zealand, unlike the other two lucinid genera in North Island seep deposits.
Generic lucinid compositions are similar between the southern seeps of New Zealand and the middle to late Miocene seep faunas in Italy (Kiel and Taviani 2017), containing both Meganodontia and Lucinoma.The paleobathymetry was estimated as 200-800 m for the Italian "Calcari a Lucina", based on benthic foraminifers and oxygen isotopes (Aharon and Sen  Bouchet and Cosel (2004), Cosel and Bouchet (2008), and Glover and Taylor (2016).The fossil data are based on Kase et al. (2007), Kiel (2013), Kiel and Hansen (2015), Kiel and Taviani (2017), and this study.Gupta 1994).Rather shallow settings allowed Meganodontia to live in the seeps of southern New Zealand and Italy.M. haunuiensis is one of the oldest records of the genus, contemporaneous with Meganodontia sp. from the early Miocene (20.44-15.97Ma) of Cuba.All fossil species of Meganodontia are found in hydrocarbon seep sites, while the sole modern species, M. accetablum, inhabits waters off Taiwan.The distribution of the genus has shrunk from worldwide in the Miocene to only around Taiwan today (Fig 6).
Until now, all fossil Elliptiolucina species have only been found from hydrocarbon seep sites in the Pacific.The oldest species, E. washingtonia, comes from an upper Oligocene (37.8-33.9Ma) seep site in Washington State (Kiel 2013).
The second oldest species is E. neozelandica from the middle  in New Zealand.E. hetzeli is from upper Miocene (11.63-5.333Ma) deposits of Buton Island, Indonesia (Kiel 2013), and probably also from the lower Pliocene cold-seep deposits in Leyte, Philippines (Kase et al. 2007).In comparison, modern species of Elliptiolucina are found mainly in the Philippines, and also occur in Indonesia and Amami-Oshima, southwestern Japan (Cosel and Bouchet 2008;Okutani 2011;Kuhara et al. 2014;Glover and Taylor 2016).Of these, E. magnifica, E. labeyriei, and E. ingens were collected with vesicomyid bivalves, probably from seep sites (Cosel and Bouchet 2008;Kuhara et al. 2014).The species living in shallowest water is E. williamsae, collected from sandy substrates (Glover and Taylor 2016).The habitat of E. virginiae is unknown.The species of this genus once lived in hydrocarbon seep sites around the Pacific; since then their distribution has narrowed and their habitat has expanded to include both seep sites and also sandy environments, mainly around the Philippines (Fig. 6).

Conclusions
This paper presents a study of lucinid bivalves from the lower to middle Miocene hydrocarbon seep carbonates in the north and south of Hawke Bay in North Island of New Zealand.Three new fossil species belonging to each of the genera Meganodontia, Elliptiolucina, and Lucinoma are described.Using modern bivalve depth ranges as a comparison, we suggest the southern area seep sites formed in shallower waters than the northern sites.Among the lucinids, the genera Meganodontia and Elliptiolucina were distributed worldwide or Circum Pacific area in the Miocene but their distribution has since narrowed to around the Philippines, Indonesia and southern Japan.
Institutional abbreviations.-AU,Paleontology Collection, School of Environment, UOA; NSMT, National Science Museum of Tokyo; UOA L, bivalve specimen number, Paleontology Collection, School of Environment, UOA; UOA, University of Auckland, Auckland, New Zealand.Other abbreviations.-AL,anterior length; H, height; L, length; W, width; Y and U, map codes in New Zealand.

Fig. 2 .
Fig.2.Schematic drawing of a model lucinid bivalve with explanation of the main morphological terms used in this paper.
Stratigraphic and geographic range.-UglyHill and Haunui; lower Miocene Ihungia Limestone.Bexhaven, Moonlight North and Rocky Knob; middle Miocene Bexhaven Limestone.North Island, New Zealand.Discussion Characteristics of the Miocene New Zealand seep faunas.-Table
Kuhara et al. 2014ima, southwestern Japan, differs from E. neozelandica sp.nov. in having a larger (up to 98.4 mm in length;Kuhara et al. 2014) and thick shell with a rounded posterior margin.Type species: Lucina filosa Stimpson, 1851; Recent, east coast of North America.