THE # The Veliger 50(4):255–262 (December 16, 2008) CMS, Inc., 2007

A New Genus for Vesicomya inflata Kanie & Nishida, a Lucinid Shell Convergent with that of Vesicomyids, from Cretaceous Strata of Hokkaido, Japan

KAZUTAKA AMANO Department of Geoscience, Joetsu University of Education, Joetsu 943-8512, Japan (e-mail: [email protected])

ROBERT G. JENKINS Faculty of Education and Human Sciences, Yokohama National University, Yokohama 240-8501, Japan

YUKITO KURIHARA Department of Geology and Paleontology, National Museum of Nature and Science, Tokyo 169-0073, Japan

AND

STEFFEN KIEL Institute of Geosciences – Paleontology, Christian-Albrechts-University, 24118 Kiel, Germany

Abstract. Newly collected specimens of the large bivalve Vesicomya inflata Kanie & Nishida from the lower Cenomanian Tenkaritoge Formation reveal that it is not a vesicomyid but is instead an unusual lucinid. The new monotypic genus Ezolucina is herein proposed for this species, which is characterized by venerid or vesicomyid shell shape, large size, a smooth surface, a deeply impressed lunule, one cardinal and one anterior lateral tooth in the right , and two cardinal teeth in the left valve. Stable carbon isotope analyses and petrographic observations show that the carbonate concretions yielding this species do not represent ancient hydrocarbon-seep deposits as was suggested previously. Rather, Ezolucina inflata and the associated solemyid, lucinid, thyasirid, and manzanellid bivalves lived in organic- and sulfide-rich sediment.

INTRODUCTION and belong to a clade informally known as ‘‘small’’ vesicomyids, composed of the genera Vesicomya, Large fossil bivalves from the ‘‘Middle Yezo Group’’ in Waisiuconcha, Isorropodon, Callogonia and Pliocardia northern Hokkaido, Japan, were described by Kanie & (cf. Cosel & Salas, 2001; Krylova & Sahling, 2006). In Nishida (2000) as Vesicomya inflata, and listed as being contrast, the Cretaceous ‘‘Vesicomya’’ inflata reaches a the earliest record for the genus Vesicomya (Campbell, length of up to 157 mm. In a recent revision of fossil 2006; Kiel & Little, 2006). They were found in two North Pacific vesicomyids, Amano & Kiel (2007) large calcareous concretions surrounded by mudstone, pointed out that V. inflata has a deeply impressed and were associated with the solemyid Acharax asymmetrical lunule, a feature unknown in vesico- cretacea Kanie & Nishida, 2000 and the lucinid Miltha myids, and that its hinge structure had neither been sp. Extant members of these bivalve taxa harbor described nor illustrated. Consequently, Amano & Kiel chemoautotrophic endosymbionts, and because car- (2007) excluded V. inflata from the Vesicomyidae and bonate concretions bearing these taxa have repeatedly suggested lucinid affinities instead. been demonstrated to represent ancient hydrocarbon- Newly collected specimens from the type locality of seep deposits (Majima et al., 2005; Campbell, 2006), the ‘‘Vesicomya’’ inflata at Sanjussen-zawa Creek in concretions bearing Vesicomya inflata were interpreted northern Hokkaido possess hinge dentition, a pallial as ancient hydrocarbon-seep deposits (Kanie & line, and adductor scars that clearly place this species in Nishida, 2000; Kanie et al., 2000). a new genus of the Lucinidae. In addition, petrographic Living Vesicomya species have small shells that thin sections and stable carbon isotope analyses of the rarely exceed 13 mm in length (Cosel & Salas, 2001) fossil-bearing concretions are used to evaluate the Page 256 The Veliger, Vol. 50, No. 4

Figure 1. Type locality of Ezolucina inflata (Kanie & Nishida). environmental reconstruction of this site as an ancient trometer at EPUT. Carbon isotopic composition is hydrocarbon seep. expressed in the conventional d notation relative to the Vienna Peedee Belemnite standard (d13C, % vs. V- MATERIALS AND METHODS PDB, standard deviation ,0.03%). The type material was examined at the Yokosuka City MINERALOGY AND ISOTOPE COMPOSI- Museum, and nine new specimens of ‘‘Vesicomya’’ inflata were collected from four float carbonate TION OF CARBONATES concretions with molluscan fossils (HRK A–D) at the Thin-section observation and XRD analysis show that type locality in northern Hokkaido (Figure 1). Strati- the concretions are almost entirely composed of graphically the specimens are from the My 4 Member homogeneous micritic calcite and siliciclastic sediment. of the Tenkaritoge Formation, which is considered Structures typical for methane-induced carbonates, like early Cenomanian (Hashimoto et al., 1965; Nishida et clotted fabrics and stromatolitic laminae (cf. Greinert al., 1998). The figured specimens and additional new et al., 2002; Peckmann & Thiel, 2004), were not seen. material are housed at the Joetsu University of d13C values range from 27.7 to 26.3% (vs. V-PDB) Education (JUE). The mineralogy of the fossil-bearing concretions was Table 1 identified by thin-section observations and X-ray diffraction (XRD) analysis. Standard thin-section Stable carbon and oxygen isotope composition of the observations were performed by plane- and cross- carbonate concretions from the type locality of polarized and reflected light microscopy. XRD analy- Ezolucina inflata (Kanie & Nishida). ses were carried out on unoriented slurries using a 13 18 PANalytical X’Pert PRO at the Department of Earth Sample no. Texture Mineralogy d C d O and Planetary Science, the University of Tokyo HRK A micrite calcite 27.4 21.3 (EPUT). Carbon and oxygen isotopes were analyzed HRK A micrite calcite 27.3 21.2 using 2 to 10 mg powdered carbonate matrix. Carbon HRK B micrite calcite 26.3 23.8 dioxide was produced from each powdered sample by HRK C micrite calcite 26.7 24.9 HRK C micrite calcite 26.5 24.2 reaction with 100% phosphoric acid in vacuo (25uC), HRK C micrite calcite 27.7 25.2 and analyzed with a Finnigan MAT252 mass spec- K. Amano et al., 2007 Page 257

(Table 1) and are also not indicative of anaerobic Paratype: Articulated specimen, length 82.6 mm+, methane oxidation. Methane-derived carbonate usually height 59.8 mm, width 35.8 mm+, YCM-GP1174. shows d13C values lower than 240% (cf. Peckmann & Thiel, 2004). Thus, neither thin section observations Topotypes: Articulated specimen, length 157.5 mm, nor stable isotope analyses support the idea of Kanie & height 123.8 mm, width 74.0 mm+, YCM-GP1177; Nishida (2000) and Kanie et al. (2000) that ‘‘Vesico- right valve, length 33.7 mm, height 26.2 mm, JUE mya’’ inflata and associated mollusks lived at an no. 15853; right valve, length 28.9 mm, height ancient hydrocarbon seep. 21.9 mm, JUE no. 15854; articulated specimen, length 15.4 mm, height 13.5 mm, width 5.9 mm, JUE no. 15855. SYSTEMATICS Type locality: Bed of Sanjussen-zawa Creek, 6.5 km Family Lucinidae Fleming, 1828 upstream from where it flows into the Uryu River, Genus Ezolucina Amano, Jenkins, Kurihara & Horokanai Town, Hokkaido (44u149240N, 142u59260E); Kiel, gen. nov. My 4 Member of the Tenkaritoge Formation (locality R7203 by Nishida et al., 1998).

Type species: Vesicomya inflata Kanie & Nishida, 2000. Stratigraphic and geographic distribution: Late Creta- ceous (early Cenomanian); known only from the type Diagnosis: Large, inflated veneriform shell with smooth locality. surface except for rough, low commarginal lamellae; posterior radial sulcus weak, and lunule deeply Original description: ‘‘Large shell (length [L] L 5 impressed. Hinge of right valve with one stout cardinal 130 mm in holotype and probably gerontic stage) tooth and an anterior lateral tooth, left valve hinge probably of rounded triangular form (height [H] H/L with two cardinal teeth. Anterior adductor scar 5 0.8). (shell apex) situated almost centrally, quadrate and anteriorly detached from ; 45–46% from anterior margin. Strongly inflated valve pallial line entire and deeply impressed. ([H]/breadth [B], H/B?0.68). Shells of right and left are equivalved. Posterior end is truncated. In the juvenile Discussion: Here Gabb, 1866 is similar to Ezolucina to middle growth stage represented by paratype (L 5 gen. nov. by having one cardinal tooth, one anterior 88.6 mm), the shell height is shortened (H/L 5 0.68). lateral tooth in right valve and a deeply impressed Postero-dorsal end is truncated meeting with the lunule, but differs from Ezolucina by its smaller shell posterior margin. Lunule and hinge form is the same with fine ventral crenulations, and its much more as gerontic one. There is a lunule at antero-dorsal part. deeply impressed lunule. Another large Cretaceous The ventral margin is weakly arched. lucinid is Nipponothracia, which differs from Ezolucina probably external and long on the basis of morphology by having an edentulous hinge and a very elongate of the hinge area at postero-dorsal part. Test is very anterior adductor scar (cf. Kanie & Sakai, 1997; Kase thick about 7 mm at the ventral margin. Shell surface is et al., 2007; Kiel et al., 2008). The medium-sized North ornamented by concentric growth lines. The umbo American lucinid Nymphalucina Speden, 1970 from the bends strongly inside, where characteristic subumbonal Late Cretaceous Pierre Shale and Fox Hill Formation pit exists at the inside of the shell apex.’’ is oval in shape and lacks the sloping posterodorsal margin of Ezolucina. Trinitasia Maury, 1928, which Supplementary description: Shell large in size (up to was questionably placed in Lucinidae by Chavan 157.5 mm in length), thick, well inflated in adult (1969), is comparable in shell form and , but specimens, veneriform, rather equivalve, slightly in- based on internal shell features Woodring (1982) equilateral. Small specimens less inflated and Fela- showed that Trinitasia is not a lucinid but a mactrid. niella-like in shape. Umbo projecting above dorsal margin, prosogyrate, situated anteriorly at two-fifths of Etymology: A combination of the old name of shell length. Anterodorsal margin broadly arcuate, Hokkaido (Ezo) and the genus Lucina. grading into rounded anterior margin; ventral margin Ezolucina inflata (Kanie & Nishida, 2000) broadly arcuate; posterodorsal margin straight, grad- ually sloping, forming blunt angle with subtruncated (Figures 2–9) posterior margin. Surface smooth except for rough and low commarginal lamellae; coarse concentric ribs Vesicomya inflata Kanie & Nishida, 2000: p. 79–82, visible in right valve of a small specimen. Shallow figs. 1, 2. radial sulcus running from to posterior end in large specimens. Holotype: Articulated specimen, length 131.8 mm, Hinge plate wide; right valve with one stout and height 105.4 mm, width76.3 mm, YCM-GP1173. triangular central tooth (3b), one elongate anterior Page 258 The Veliger, Vol. 50, No. 4

Figures 2–4. Type material of Ezolucina inflata (Kanie & Nishida). Figure 2. Holotype, length 5 131.8 mm, YCM-GP1173. Figure 2a. Lateral view of right valve. Figure 2b. Dorsal view showing a posterior radial sulcus and deeply impressed lunule. Figure 3. Topotype, length 5 157.5 mm+, YCM-GP1177. Figure 3a. Dorsal view. Figure 3b. Lateral view of left valve. Figure 4. Paratype, length 5 82.6 mm, YCM-GP1174. Figure 4a. Lateral view of right valve. Figure 4b. Dorsal view showing a posterior radial sulcus and deeply impressed lunule. Figure 4c. Lateral view of left valve. K. Amano et al., 2007 Page 259

Figures 5–8. Additional specimens of Ezolucina inflata (Kanie & Nishida). Figure 5. Topotype, length 5 28.9 mm, JUE no. 15854. Figure 5a. Hinge of right valve showing one strong cardinal and distinct anterior lateral tooth, length of hinge plate 5 21.3 mm. Figure 5b. Dorsal view showing deeply depressed lunule. Figure 5c. Lateral view of right valve. Figure 6. Topotype, length 5 39.9 mm+, JUE no. 15856. Figure 6a. Dorsal view of left valve showing deeply impressed lunule. 6b. Hinge of left valve showing two strong cardinal teeth, length of hinge illustrated here 5 21.8 mm. 6c. Lateral view of left valve. Figure 7. Topotype, length 5 15.4 mm, JUE no. 15855. Figure 7a. Lateral view of left valve. Figure 7b. Lateral view of right valve having rude concentric ridges. Figure 8. Topotype, length 5 33.7 mm, JUE no. 15853. Figure 8a. Lateral view of right valve stressing anterior adductor scar. Figure 8b. Normal lateral view of right valve. lateral tooth (AI) parallel to hinge base, and a weak developed, entire, and quite distant from shell margin. blunt node just below deeply impressed lunule; left Inner ventral margin smooth. valve with strong anterior cardinal (2), touching deeply impressed lunule, posterior tooth (4b) rather thin. Remarks: Very similar to Ezolucina inflata in shell Ligament occupying two-thirds of posterodorsal mar- shape and size is ‘Lucina’ colusaensis Stanton, 1895, a gin. Anterior adductor scar elongate quadrate, moder- species that is apparently restricted to Upper Jurassic ate in size (adductor length 5 9.1 mm in JUE (Tithonian) to Lower Cretaceous (Hauterivian) cold- no. 15853; adductor length/shell length 5 0.27) and seep carbonates in northern California, USA (Stanton, about 75% of its length detached from pallial line 1895, p. 60, pl. 11, figs. 4, 5; Campbell, 2006; Kiel et al., (maximum distance from pallial line 5 1.9 mm in JUE 2008). Compared to our material of Ezolucina inflata no. 15853); posterior adductor scar pear-shaped. Inner the thickness of ‘Lucina’ colusaensis resembles that of surface covered by coarse and distinct radial striations the less inflated specimens of Ezolucina inflata. (Figure 9). Lunule broadly lanceolate, well defined, Unfortunately, ‘Lucina’ colusaensis is usually poorly deeply impressed, slightly asymmetrical, slightly larger preserved and features of the interior of this shell are in right valve than in left valve, and occupying one- unknown (Stanton, 1895; SK, pers. observation; K.A. third of anterodorsal margin. Pallial line narrow, well- Campbell, personal communication 2007); thus a Page 260 The Veliger, Vol. 50, No. 4

Figure 9. Muscle scars and pallial line of Ezolucina inflata (Kanie & Nishida) based on JUE no. 15853. Abbreviations: aa, anterior adductor scar; pa, posterior adductor scar; pl, pallial line. confirmation of its generic position must await the p. 161, pl. 14: 5) from a potential Hauterivian seep site availability of well-preserved specimens. in eastern Czech Republic lacks the strongly sloping Another large lucinid with prominent umbos is posterodorsal margin of Ezolucina inflata. Saxolucina (Megaxinus) matsushitai Matsumoto (1971, p. 663–665, pl. 1, fig. 1, pl. 2, figs. 1–3) from DISCUSSION the Oligocene Setogawa Group [now considered as Miocene in age; see Watanabe (1988)] in central Japan, Examination of the type and additional specimens but this species is clearly distinct from Ezolucina inflata clearly shows that Vesicomya inflata is a member of by having a less inflated shell with an edentulous Lucinidae because it has the hinge structure and the dentition. adductor muscle scar and pallial line patterns typical of Ezolucina inflata resembles Here excavata (Carpen- Lucinidae (i.e., a lucinoid hinge dentition, a broadly ter, 1857) in having a deeply depressed lunule, one lanceolate, asymmetric, sunken lunule, and an elongate cardinal tooth and anterior lateral tooth in the right anterior adductor muscle scar detached from the pallial valve, and two cardinal teeth in the left valve. However, line). The previous assignment of this species to the Here excavata differs from Ezolucina inflata by its Vesicomyidae was due to the superficial resemblance in subcircular shell with concentric lamellae, many fine shell outline and the lack of the information on the ventral crenulations and more deeply sunken lunule. Its shell interior. The new monotypic genus Ezolucina veneriform shape with the strongly sloping postero- based on V. inflata is more elongate than most lucinids dorsal margin sets Ezolucina inflata apart from most and has moderately prominent umbones somewhat other Cretaceous lucinids, which mostly have a nearly suggestive of a vesicomyid or an eomiodontid. How- round outline (e.g., Discoloripes septentrionalis Kelly, ever, shells of these families can easily be distinguished 1992; ‘‘Lucina’’ spp. in Stephenson, 1952; Callucina from those of the Lucinidae by their hinge and muscle olea Vokes, 1946; ‘‘Lucina’’ aquensis Holzapfel, 1889, scar patterns. p. 188, pl. 19, fig. 4). A lucinid with similar hinge In their compilation of the stratigraphic ranges of dentition was described and figured as Lucina sub- mollusks at cold seeps, Kiel & Little (2006) listed V. nummismalis d’Orbigny, 1850 from the Campanian inflata as the oldest fossil occurrence of Vesicomya, Vaals Greensand of Germany (Holzapfel, 1889, p. 187– based on the available literature (Kanie & Nishida, 8, pl. 19, figs. 1–3). This species is distinct from 2000). When Amano & Kiel (2007) subsequently Ezolucina inflata because it is very flat, has distinct revised the North Pacific fossil record of the Vesico- commarginal ribs, and lacks the strongly sloping myidae, they could not confirm any of the previous posterodorsal margin of Ezolucina inflata.Also‘‘Luci- records of this genus. One of the oldest ‘‘small’’ na aff. valentula de Lor.’’ described by Ascher (1906, vesicomyids related to Vesicomya (cf. Cosel & Salas, K. Amano et al., 2007 Page 261

Table 2 deep-water environments of the Late Cretaceous were inhabited by a number of exceptionally large taxa like Faunal list based on fossils that we collected from the bivalve Inoceramus or the capulid limpet Gigantocapu- type locality of Ezolucina inflata (Kanie & Nishida). All lus giganteus (cf. Takahashi et al., 2007). Thus, the of these species possibly harbor symbiotic bacteria. paleoenvironment of Ezolucina inflata remains enig- Numbers indicate the number of recovered individuals, matic. capital letters are sample designations. Acknowledgments. We are very grateful to James L. Goedert Species Carbonate no. (HRK -) A B C D (Burke Museum, Seattle) for his critical reading of this Acharax cretacea Kanie and Nishida 2 manuscript. We thanks John D. Taylor (Natural History Nucinella ?sp. 1 Museum) and Geerat J. Vermeij (University of California at Ezolucina inflata (Kanie and Nishida) 3 2 4 Davis) for their review. We also thanks Tamio Nishida (Saga Miltha? sp. 1 University) for his offering some fossil specimens. Thyasira sp. 1 LITERATURE CITED

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