Giant fossil Acharax (Bivalvia: Solemyidae) from the Miocene of Japan

2011·3.6 MB·English

by  Kazutaka Amano

THE NAUTILUS 125(4):207-212, 2011 Page 207
Giant fossil Acharax (Bivalvia: Solemyidae) from
the Miocene of Japan
KazutakaAinano IlisaoAndo
DepartmentofGeoscience Departmentof Earth Sciences
Joetsu Universityof Education FacultyofScience
Joetsu943-8512,JAPAN Ibarald University
[email protected] Mito310-8512,JAPAN
[email protected]
ABSTRACT species. Eleven species have been recorded from depos-
its ofCretaceous to Pliocene age (Kanie and Kuramoehi,
SpecimensofAcharaxyokosukensisrecentlycollectedfromthe 2002; Kiel et ah, 2008). The largest known species is
upper lower Miocene Kokozura Formation of the Takaku A. gigas from the Miocene Haratajino Formation with a
Group in Ibarald Prefecture, centralJapan, served as the basis mm
length reaching 264.1 (Kanie et ah, 1999; Kurihara,
for a re-description ofthe species. These specimens include a
valve ofthe largest specimen of this genus known worldwide, 2000). Taylor and Clover (2010) cited a specimen of
exceeding 295.7 nun in length. The occurrences of the giant A. yokosukensis Kanie and Kuramoehi, 1995, from the
Acharaxspecies inJapanareconfined tolowertolowermiddle Miocene Hayama Group in Kanagawa Prefecture, cen-
Miocene sediments deposited in tropical shallow and deep tral Honshu, as the largest Acharax in the world, refer-
environments. ring to “a fossil Acharax from the Miocene [that]
measured a massive 300 mm”. However, Kanie and
Additionalkeywords: Chemosynthesis, bivalves, deep-sea
Kuramoehi (1995) only estimated that, based on a frag-
ment ofa shell.
We collectedan imperfectspecimen ofA. yokosukensis
mm
havingalength of295.7 from the Miocene Kokozura
INTRODUCTION Formation of the Takaku Group exposed along Pacific
coast in Kitaibaraki City, in the northern part of Ibarald
Solemyidae is the oldest group ol chemosymbiotic Prefecture, central Honshu. So far, this specimen is the
bivalves, ranging back to the Ordovician (Taylor and largest Acharax recorded worldwide. In this paper, we
Glover, 2010; Kiel, 2010). Among them, the oldest con- describeA. yokosukensis andspeculateaboutwhyitcould
firmed member of the extant genus Acharax is known have evolvedto such large size.
from the early Cretaceous in Hokkaido, northern Japan
(Kiel et ah, 2008), but there are many unidentified
solemyids from the oldergeological record that mayalso MATERIALS AND METHODS
belongtoAcharax. Two species ofAcharax are known to
live in Japanese waters: A. johnsoni (Dali, 1891) and The specimens described herein were collected from
A.japonicus (Dunker, 1882). The latter species is small boulders present in the uppermost lower Miocene
and iives in shallow water (0-20 m depth; Habe, 1977) Kokozura Formation that may constitute the base layer
and has been found in settling tanks of sea water at a underthe Iznra Kanlco Hotel (Figure 1). These boulders
m
marine laboratory (Yamanaka et ah, 2008). In contrast, with adiameterofaround 1 consistofsandylimestone
A.johnsoni has a large shell attaining 150 mm in length or calcareous fine-grained sandstone including many
(Kamenev, 2009), similar to other deep-water Acharax shells and trace fossils (Figure 2). Ueda et al. (2005)
species, and lives in deep water (the depth range of differentiated these calcareous concretions into six mor-
A. johnsoni is very wide; according to Kamenev (2009) phological types. Among them, type I concretions (large
the species may be found from 100-5379 m. However, irregularly shaped carbonates) show negative 51 ’C value
100 m seems to be an exceptional case. Kamenev’s data ranging from -29.4 to -20.9%o. These values indicate
and JAMSTEC data show this species usually lives in that the carbonates precipitated under the influence of
deeper than 370 m.) The largest living species of the oxidation of hydrocarbons such as crude oil or
Acharax is A. bartschii described by Dali (1908b) from methane with an influx of marine bicarbonate (Kiel and
the Philippines, with a shell length of 191 mm. Peckmann 2007). The boulders yielding the large
Notwithstanding problems with their identification, Acharax yokosukensis specimens resemble type I con-
fossil Acharax in Japan are more diverse than extant cretions in size and shape.
Page 208 THE NAUTILUS, Vol. 125, No. 4
Figure 1. Localitymap. Geological mapadaptedfrom Uedaetal. (2005).
Lucinoma acutilineatum (Conrad) and Nipponothracia?
sp., the thyasirid Conchocele bisecta (Conrad), and the
vesicomyids Callogonia? sp. andAdulomya sp. The pred-
atory gastropod species are represented by a few spec-
imens of the naticid Cryptonatica clausa (Broderip and
Sowerby) and theturridMegasurcula yokoyamai (Otuka).
For comparing the shell proportion ol Acliarax
yokosukensis, some Recent specimens of A. johnsoni
from northern Pacific, stored at National Science
Museum in Tokyowere examined.
Abbreviations used in text: |UE=Joetsu University of
Education; YCM-Gp = Yokosuka City Museum.
SYSTEMATICS
Figure 2. Occurrence of Acharax yokosukensis in a calcare- Family Solemyidae
ous concretionblock. GenusAcharax Dali, 1908
Type Species: Solemyajohnsoni Dali, 1891
The age of the Kokozura Formation of the Takaku
Group in this areawas assigned to the uppermost lower Acharax yokosukensis Kanie and Kuramochi, 1995
Miocene Crucidenticula kanayae zone (NPD 3A zone; (Figures 3-10)
NPD = Neogene North Pacific Diatom) by Yanagisawa —
(1996), based on diatom fossils. From the sedimentary Acharax aff. tokunagai (Yokoyama). Ogasawaraetah,
facies and the occurrence ol heterotrophic bivalves like 1994: 34-35,—figs. 3-la-e.
Mizuhopecten kobiyamai (Kamada) and Cylocardia Acharax n. sp. Kanieetal., 1995: 57-58, figs. 1, 2-1-3, 3-2-4.
Acharaxyokosukensis Kanieand Kuramochi, 1995: 52,
siogamensis (Nomura), these sedimentswereconsidered
to be deposited on the muddy sand shelf (Ueda et ah, 56, figs. 1—4; Kanieand Kuramochi, 2002: 56, figs. 2-13.
2005). Holotype: YCM-Gp Ig36.
The fauna associated with Acliarax yokosukensis is a
blend of chemosymbiotic bivalves and predatory gastro- Material Examined: Eight specimens including one
pods. The chemosymbiotic species include the lucinids almostperfect specimen. |UE nos. I5887-1~8.
K. Amano and II. Ando, 2011 Page 209
Figures 3—10. Acharax yokosukensis (Kanie and Kuramochi). 3—5, 7-10. Specimens from the Kokozura Formation. 3—5. JUE
no. 15887-2, right valve. 3. Rubbercast ofhinge ofthe specimen illustrated in Figure 5, hinge length, 82.9 mm. 4. Rubber cast of
outer shell surface of the specimen illustrated in Figure 5, length 168.4 mm. 5. Inner mold, length 155.1 mm. 7. Rubber cast of
outer shell surface, JUE no. 15887-3, length 195.2+ mm, right valve. Most posterior part and some ventral part are missing.
8. Rubber cast of inner shell surface, JUE no. 15887-4, length 109.9 mm, left valve. 9, 10. JUE no. 15887-1, left valve. 9. Inner
mold, length 244.7 mm. 10. Rubber cast ofoutershell surface, length 295.7+ mm. 6. Holotype from the Hayama Group. YCM-Gp
Iog36, lenogth 144.1 mm, rioghtvalve.
Description: Shell exceptionally large for genus, central or slightly posteriorly (anterior length: AL/ shell
exceeding 295.7 nun in length (Table 1), elongate quad- length-ratio = 0.52-0.61). Anterior margin subtruncate;
rate, height/length-ratio = 0.40-0.44, equivalve and ventral margin nearly straight; antero-dorsal margin
inequilateral, moderately inflated. Umbo situated nearly nearly straight; postero-dorsal margin very broadly
Page 210 THE NAUTILUS, Vol. 125, No. 4
Table 1 Measurements (mm) ofvalves ofAcharax yokosukensis.
.
Specimens Length Height H/L AL AL/L Valve
JUE no. 15887-1 left
(rubbercastofoutersurface) 295.7+ 108.5
(inner mold) 244.7 107.3 0.44 127.6 0.52
JUE no. 15887-2 right
(rubbercastofoutersurface) 168.4 66.7 0.40 102.2 0.61
(innermold) 155.1 60.9 0.39 92.7 0.60
JUE no. 15887-3
(rubbercastofoutersurface) 195.2+ - - - right
arched, continuing into well-rounded posterior margin. (latest early to earliest middle Miocene), based on the
Hinge edentulous and inner surface ofsubumbonal part examination of planktonic foraminiferans (Takahashi,
radiallygrooved. Nymph opisthodetic. Anterioradductor 1992; Oishi and Takahashi, 1990). This Miocene age
muscle scar indistinct; posterior adductor muscle scar corresponds to the nrid-Neogene Climatic Optimum
large, quadrate, deeplyimpressedandcrenulatedby dis- (Tsuchi, 1987). At that time, a tropical climate prevailed
tinct radial grooves. Surface sculptured by fourteen in wh1)at is now the Kanto District (Ogasawara, 1994).
radial ribs; five distinct, low andwide radial ribs in ante- Other than the occurrence ofthe Kokozura specimens,
riorpart; five fine andround-toppedradial ribswithvery these giant Acharax were from deep seep sites. Excep-
wide interspaces in middle part; four round-topped tionally, the largest specimen of A. yokosukensis was
radial ribs with narrowerinterspaces in posteriorpart. recovered from the shallow seep deposits ol the
2K0o1k0o)zura Formation. Thus, regardless of depth, the
Remarks: AspointedbyKanieandKuramochi (1995), warm climate and methane seep environment might
Acharax aff. tokunagai (Yokoyama) described by affect the size offossil Acharax.
Ogasawara et al. (1994), from the lower Miocene Like solemyids, lueinid bivalves are chemosymbiotic,
Aokiyama Formation in Chiba Prefecture, is a synonym deep burrowers, and geologically old members of the
ofA. yokosukensis. These authors reached this conclu- seep fauna, which first appeared in the Jurassic (Kiel,
sion because of the species centrally situated beak and Occurrences oflarge fossil lueinids were summa-
.
similarnumberofribs (II in the Aokiyama specimens). rized by Taylor and Glover (2009) who described the
largest lueinid, Superlucina megameris (Dali) that
Comparison: Acharax gigas (Kanno, 1960) from the reached 280 mm in length and 311 mm in height.
Miocene Hiranita and Haratajino formations remsemmble According to their list, the large lueinids appeared in
A. yokosukensis in its huge shell size (264.1 in geological ages withwarm climates like the late Jurassic-
length). However, A. gigas can be distinguished from early Cretaceous, middle Eocene, and middle Miocene.
A(.moyroekoesluoknegnasties sihnelhla,viHn/gLa=sma0l.l2e5r-0.h3e4i)g,ht/alnedngtah-mraotrieo rInendceepsosairtes ymooustnlgyerctohnafnintehdeteoartlryopPilciaolceanree,asthelikoeccturh-e
posteriorly situated beak (AL= 0.65-0.78) (Figures 11, Philippines and Taiwan. Moreover, large lueinids were
12). The Recent species, Acharaxjolmsoni (Dali, 1891) recoveredfrom bothshallowanddeep sites likethe giant
differs from A. yokosukensis by its smaller shell and Acharax.
more posteriorly situated beak (AL = 0.70-0.77) Generally speaking, the maximum size of largest spe-
(Figures 11, 12).' cies ofeach guildin the coastal marine tropics correlates
Distribution: Lower Miocene Aokiyama Formation or correlated with the marine productivity (Vermeij,
in Chiba Prefecture; upper lower Miocene Kokozura 201 . Solemyids are well known to have a reduced gut
Formation of Takaku Group in Ibaraki Prefecture; or to lack an alimentary tract, and to depend on their
lower middle Miocene Hayama Group in Kanagawa chemoautotrophic symbionts for nutrition (Stewart and
Cavanaugh, 2006; Taylor et al., 2008). It seems thus
Prefecture.
plausible that chemosynthetic species need to take up a
DISCUSSION lot ofsulfide to achieve a large size (Taylor and Glover,
2009). Assumingthatwarm temperatures facilitate phys-
The giant specimens of Acharax yokosukensis reported iological process like sulfide uptake, the warm tem-
here were recovered from probable hydrocarbon seep peratures of the latest early to earliest middle Miocene
deposits in the lower to lower middle Miocene sedi- might have played a role in the gigantism oftheAcharax
ments in the northeastern Kanto District, central Hon- specimens reported here, although the exact mecha-
shu (Figure 13). Another giantAcharax species,A. gigas nisms are unknown. At modern seeps, Acharax often
has also been recorded from ancient seep sites in the lives at the periphery away from the highest sidfide con-
Hiranita and Iaratajino Formations in the northwestern centrations (Sahling et ah, 2002). Thus, for obtaining
I
Kanto District, correlated with zone N8 of Blow (1969) sulfides, the lackof competition might also play a role.
/ 1 .
K. Amano and H. Ando, 201 Page 21 I
• A. johnsoni
A A gigas
A yokosukensis (t)
111 A yokosukensis
A A. bartschi
a>
20
0L
0 50 100 150 200 250 300
11 Length (mm)
4. HiranitaF.; and5. Haratajino F.
study was partly supported by a Grant-in-Aid for
Scientific Research from the Japan Society for Promo-
tion ol Science (C, 20540456, 2008-2010).
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