THE NAUTILUS 133(2):48–56, 2019

Page 48

The Miocene to Recent biogeographic history of vesicomyidbivalves in Japan, with two new records of the family
Kazutaka Amano

Yusuke Miyajima

Robert G. Jenkins

Steffen Kiel

Department of Geoscience
Joetsu University of
Education
Joetsu 943-8512, Japan
amano@juen.ac.jp

Geochemical Research Center
Graduate School of Science
University of Tokyo
Bunkyo-ku, Tokyo 113-0033, Japan
yusukemiya@eqchem.s.u-tokyo.ac.jp

School of Geosciences andCivil Engineering
College of Science andEngineering
Kanazawa University
Kanazawa City, Ishikawa
920-1192, Japanrobertgj@staff.kanazawa-u.ac.jp

Swedish Museum of
Natural History
Department of
Palaeobiology
Box 50007, 10405
Stockholm, Sweden
steffen.kiel@nrm.se

ABSTRACTWe report on two fossil species of the chemosymbiotic bivalvefamily Vesicomyidae that were recently collected from Cenozoic
strata in Japan. The new species Pleurophopsis matsumotoi is
described from the upper Oligocene to lower Miocene Hioki
Complex in Kochi Prefecture, and the extant species Calyptogena paciﬁca Dall, 1891 is reported from the upper Miocene
Onnagawa Formation in Akita Prefecture. With these new records, vesicomyid bivalves in Japanese strata show the following
distribution pattern through the latter half of the Cenozoic Era:
during the early to middle Miocene Pleurophopsis was thedominant genus and is found in sediments of both the Japan Sea
and the Paciﬁc Ocean. From the late Miocene through to today
Archivesica and Calyptogena are the most common genera. Of
these, Calyptogena species including the extant C. paciﬁca, arethe most common vesicomyids in the cold waters of the Japan
Sea, whereas Archivesica species are mostly found in the warmer
waters of the Paciﬁc side of Japan. Since the late Miocene, thediversity of vesicomyids rapidly increased in the Japan Sea,
probably because of the semi-enclosed geographic situation withopened northern straits.
Additional Keywords: Pleurophopsis, Calyptogena, fossil,
paleobiogeography

INTRODUCTION
Vesicomyids are peculiar bivalves that derive their nutrition from symbiotic, sulfur-oxidizing bacteria (Fisher,1990). They include the iconic Giant White Clam“Calyptogena” magniﬁca that was discovered at deep-sea
hydrothermal vents in the late 1970s (Boss and Turner,1980). To date, more than 100 extant species have beenfound at hydrothermal vents, hydrocarbon seeps, andwhale falls (e.g., Taylor and Glover, 2010; Krylova et al.,2010). Vesicomyids also have a good fossil record, including more than 30 named fossil species, the oldest

being from the middle Eocene (Amano and Kiel, 2007;
Kiel and Taviani, 2017; and references therein).
Japan has a rich Cenozoic fossil record of hydrocarbon
seeps, whale falls, and organic-rich shales, from which
species belonging to ﬁve vesicomyid genera have beenreported: Adulomya Kuroda, 1931 (5 Pleurophopsis Van
Winkle, 1919, see discussion below), Archivesica Dall,
1908, Calyptogena Dall, 1891, Hubertschenckia Takeda,
1953, and Pliocardia Woodring, 1925 (Kanno et al., 1989;
Amano and Kiel, 2007, 2010, 2011, 2012; Amano andSuzuki, 2010; Amano, 2014). Of these genera, Hubertschenckia is an exclusively fossil genus known from upper
Eocene to lower Oligocene strata. Only two extant species
are known from the fossil record: Calyptogena paciﬁca
Dall, 1891 and Archivesica kawamurai (Kuroda, 1943)(Amano and Jenkins, 2011). Although numerous new
vesicomyid species have been discovered and described in
the last two decades, the history of this family in Japan has
never been thoroughly reviewed from a biogeographic
point of view.
Here we review and discuss the biogeographic historyof vesicomyid bivalves in the Miocene–Recent of Japan,
describe a new species of Pleurophopsis from the “Muroto
Formation” in Kochi Prefecture, Shikoku, and report the
new fossil record of Calyptogena paciﬁca from the
Onnagawa Formation in Akita Prefecture, Honshu.
MATERIALS AND METHODS
We carried out a comprehensive review of the literature on fossil vesicomyids in Japan to assess their paleobiogeographic distribution, including the two records
reported herein. Excluded from the review are the vesicomyids from the lower to middle Miocene Taishu Group(Ninomiya et al., 2014) in Tsushima Island at the westernentrance of the Japan Sea. This is so because most
specimens described by Ninomiya (2011) are deformed

K. Amano et al., 2019and internal shell features were not illustrated, hence theirgeneric assignments remain uncertain.
The four specimens described here as Pleurophopsis
matsumotoi new species were collected from a limestone
nodule from mudstone of the Hioki Complex of the Nabae
Group. The nodule included also some specimens of athyasirid bivalve of the genus Conchocele. It was found about
1,500 m north of Mitsu Maruyama, Muroto City, in Kochi
Prefecture, Shikoku, Japan (at 33°180150 N, 134°110230 E;
Figure 1, Loc. 1). The vesicomyid specimens were previously reported as “Akebiconcha uchimuraensis” by Matsumoto and Hirata (1972). The age of the Hioki Complex
was considered as late Oligocene based on planktonic foraminifera (Taira et al., 1980) and as late Oligocene to early
Miocene based on radiolarian fossils (Suyari et al., 1989).
Seven specimens identiﬁed as the extant species
Calyptogena paciﬁca Dall were recovered from a calcareous concretion derived from shales of the Onnagawa
Formation, found as ﬂoat under the bridge at Ashigafuchi,
Chokai-cho, Yurihonjo City in Akita Prefecture, Honshu,
Japan (at 39°110150 N, 140°11039.80 E; Figure 1, Loc. 2).
The age of the Onnagawa Formation around this locality is
considered as late Miocene (around 8.4 Ma) based onradiolarian fossils (Tsuji et al., 1991).
All specimens described and illustrated here are deposited in National Museum of Nature and Science,
Tsukuba, Japan (NMNS PM for fossils and NSMT-Mo for
Recent specimens).
SYSTEMATICS
Family Vesicomyidae Dall and Simpson, 1901
Genus Pleurophopsis Van Winkle, 1919

Figure 1. Localities of the new vesicomyid fossils described inthis paper.

Page 49

Type Species: Pleurophopsis unioides Van Winkle, 1919
(monotypy), middle Miocene, Trinidad (Van Winkle,
1919; Kiel, 2007).
Remarks: The genus Pleurophopsis was introduced andused for elongated, fossil vesicomyids from the Caribbean
region, Peru and Ecuador (Van Winkle, 1919; Olsson
1931, 1942). Its shell characters are virtually identical tothose of Adulomya Kuroda, 1931, including an elongate
shell, two cardinal teeth in the right valve, the anterior
original point of the pallial line at the posterior side ofanterior adductor muscle scar, and the lack of a pallial
sinus and a subumbonal pit (Kiel, 2007; Krylova et al.,2010). Kiel (2007) considered the genus doubtful because
the type material of Pleurophopsis unioides is lost and thethen available material lacked critical hinge features tofully characterize the species; he suggested using Adulomya instead. Krylova et al. (2010) essentially followed
this view, but Krylova and Sahling (2010) included
Pleurophopsis in their list of vesicomyid genera. Ongoingwork on new material of Pleurophopsis unioides from thetype locality (SK and CTS Little) indicates that Pleurophopsis is a valid genus and that Adulomya should besynonymized with it. The name Adulomya has long beenused for elongated fossil vesicomyids in Japan (Kanno
et al. 1998; Majima et al. 2005; Amano and Kiel, 2007,2011; Isaji, 2013; Miyajima et al., 2017) and virtually all
species previously assigned to Adulomya match thecharacteristics of Pleurophopsis. Only two species show aslight deviation: in Adulomya chitanii Kanehara, 1937, thepallial line bends backwards before reaching the posterior
adductor scar; this is unlike other Adulomya/Pleurophopsis species but we consider this not to represent an
actual pallial sinus. Adulomya hokkaidoensis Amano andKiel, 2007 has a very small subumbonal pit, which,however, is very unlike the large and elongate-triangularsubumbonal pit of Ectenagena elongata (Dall, 1916)
which is otherwise very similar.
Abyssogena Krylova, Sahling, and Janssen, 2010 is anextant vesicomyid genus with very elongated shells. In
particular, the deep-sea living species Abyssogena phaseoliformis (Métivier, Okutani, and Ohta, 1986) from theJapan, Kurile, and Aleutian Trenches, resembles the typespecies of Adulomya, A. uchimuraensis (Kuroda, 1931) inhaving a large and very elongated shell (Figure 9).
However, as Krylova et al. (2010) observed, Abyssogena
can be easily distinguished from Pleurophopsis in having
an anterior original point of the pallial line located at
ventral part of the anterior adductor muscle scar andpossessing an indistinct and irregular shaped pallial sinus(Figures 8, 11, 12).
Like Pleurophopsis and Abyssogena, Ectenagena
Woodring, 1938 also has an elongate shell and two cardinalteeth in the right valve. Ectenagena also shares with Pleurophopsis the anterior point of origin of the pallial line locatedat the posterior part of anterior adductor muscle scar (Krylova
et al., 2010). However, Ectenagena has a small (up to 50 mm;
Coan et al., 2000), very thin, compressed shell with ashort nymph. Further, unlike Abyssogena and Pleurophopsis

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THE NAUTILUS, Vol. 133, No. 2

Figures 2, 3, 5–7, 10, 11, 13, 14. Pleurophopsis matsumotoi new species. 2, 3. Right-valve hinges, NMNS PM13233. 5. Paratype(NMNS PM13227), left valve showing anterior adductor muscle scar (AAS) and anterior original point of pallial line (white arrow). 6, 11,
13. Holotype (NMNS PM13228). 6. Dorsal view. 11. Left valve, showing anterior adductor muscle scar (AAM) and anterior original
point of pallial line (white arrow). 13. Right valve, showing posterior adductor muscle scar (PAS) and posteriorly backward bent pallial
line (white arrow). 7. Dorsal view, NMNS PM28254. 10. Left valve, showing posterior adductor muscle scar (PAS) and posteriorly
backward bent pallial line (white arrow), NMNS PM28255. 14. Right valve, showing posterior adductor muscle scar (PAS) andposteriorly backward bent pallial line (white arrow), NMNS PM13256. Figures 4, 9, 12. Pleurophopsis uchimuraensis (Kuroda). 4. Leftvalve hinge, NMNS PM28257. 9. Left valve, NMNS PM28258. 12. Right valve, showing anterior adductor muscle scar (AAS) and theoriginal point of pallial line (white arrow), NMNS PM28259. Figure 8. Abyssogena phaseoliformis (Métivier, Okutani and Ohta), Holotype,
NSMT-Mo 64164, showing anterior adductor muscle scar (AAS) and the original point of a pallial line (white arrow). Scar bars 5 10 mm.

K. Amano et al., 2019

Page 51(except for P. hokkaidoensis), the hinge of Ectenagena has a
deep subumbonal pit.
Pleurophopsis matsumotoi new species(Figures 2, 3, 5–7, 10, 11, 13, 14)Akebiconcha uchimuraensis (Kuroda). Matsumoto and
Hirata, 1972: 755–757, pl. 1, ﬁgs. 1–8, pl. 2,
ﬁgs.1–2.
Adulomya? sp. Amano and Kiel, 2011: ﬁgs. 30–31.
Diagnosis: A large-sized, well-inﬂated and moderately
elongate Pleurophopsis; antero-dorsal margin short,posterior margin subtruncated, ventral margin concave;narrow hinge plate with thin anterior cardinal tooth (1)and rather thick posterior cardinal tooth (3b); posterior
end of pallial line turning toward to anterior before
reaching posterior adductor muscle scar.
Holotype: Internal mold of articulated specimen (NMNS
PM 13228), length 112.5 mm, height 39.3 mm.
Paratype: Internal mold of articulated specimen (NMNS
PM 13227), length 93.1 mm1, height 36.9 mm (from thetype locality).
Type locality: 1.5 km north of Mitsu, Muroto City in
Shikoku, Japan (Matsumoto and Hirata, 1972).
Material examined
locality.

Four specimens from the type

Description: Shell up to 112.5 mm long, elongate(height/length-ratio 5 0.34), equivalve and inequilateral,
well inﬂated (width/height-ratio 5 0.67–0.73). Surface
sculptured only by rough, irregular growth lines. Beaksituated anteriorly at about one-ﬁfth of shell length.
Antero-dorsal margin short and nearly straight, graduating
into rounded anterior margin; ventral margin concave;postero-dorsal margin straight, gently sloping, posterior
margin subtruncated. Escutcheon and lunule absent.
Hinge plate narrow, with two cardinal teeth in right valve;anterior cardinal tooth (1) very thin and inclined anteriorly;
posterior cardinal tooth (3b) rather thick and obliqueposteriorly; no subumbonal pit. Ligament occupying twoﬁfth of the postero-dorsal margin. Anterior adductor muscle
scar ovate, bordered posteriorly by thick ridge; posterior
adductor muscle scar ovate, less distinct than the anterior
scar; distinct ridge running from umbonal area to ventral sideof posterior muscle scar. Original point of pallial line locatedat posterior part of anterior adductor muscle scar, and pallial
line at posterior end bent toward to anterior before reaching
posterior adductor muscle scar.
Remarks: Pleurophopsis matsumotoi new species
represents the as-yet oldest record of Pleurophopsis (5Adulomya) in Japan.
Comparison: Pleurophopsis matsumotoi differs fromP. uchimuraensis (Kuroda) (Figure 4, 9, 12) in having amuch more higher shell (height/length-ratio 5 0.34
compared to 0.17–0.24 in P. uchimuraesnis; Kanno

et al., 1998, Amano and Kiel, 2011), an inﬂatedconcave ventral margin, and the pallial line turning
backward before reaching the posterior adductor
muscle scar. Pleurophopsis matsumotoi also shares theposteriorly backward bent pallial line with P. chitanii(Kuroda). However, P. matsumotoi has a larger shellthan P. chitanii (up to 70.4 mm in length) and aconcave ventral margin. The present new species
differs from other Pleurophosis such as P. akanudaensis(Tanaka), P. hamuroi Amano and Kiel, P. kuroiwaensis
Amano and Kiel from Japan in having larger, more inﬂated
shell and a concave ventral margin.
Distribution:

Only from the type locality.

Etymology: For Dr. Eiji Matsumoto who collected thetype material of this new species.
Genus Calyptogena Dall, 1891
Type Species: Calyptogena paciﬁca (Dall) (monotypy),
Recent, southeastern Alaska.
Calyptogena paciﬁca Dall, 1891
(Figures 15–24, Table 1)
Calyptogena paciﬁca Dall, 1891: 190; Dall, 1895: 713, pl.
25, ﬁg.4; Grant and Gale, 1931: 278–279, pl. 13, ﬁg.
13a, b; Otuka, 1937: text-ﬁg.; Woodring, 1938: ﬁg. 2b;
Otatume, 1942: 435–437, pl.16, ﬁgs.1–12; Okutani,
1966: 301, pl.27, ﬁgs. 1, 3; Boss, 1968: ﬁgs.16, 17, 19,
20; Keen, 1969: N664, ﬁg. E138, 11a, b; Tiba, 1972:
155, pl. 19, ﬁgs.6, 6a; Amano and Kanno, 1991: ﬁgs.
4.18, 4.19; Horikoshi and Hashimoto, 1992: pl. 1, ﬁg.
4a, pl. 2, ﬁg. 4b; Okutani et al., 1993: ﬁg. 7; Okutani,
2000: 997, pl. 496, ﬁg. 7; Amano, 2002: 27, ﬁgs. 3.4, 3.9;
Amano, 2003: ﬁgs. 3–14; Amano and Kanno, 2005:
204–207, ﬁg. 3; Krylova and Sahling, 2006: 362–368,
ﬁgs. 3–6; Suzuki, 2007: ﬁgs. III-3-4.5, III-3-4.6; Huber,
2010: 354; Amano and Jenkins, 2011: 166–169, ﬁgs.
3–16; Nevesskaja et al., 2013: ﬁg. 150-9; Okutani, 2017:
1233, pl. 528, ﬁg. 8.
Unio moraiensis Suzuki, 1941: 55–56, pl. 4, ﬁgs. 2–5.
Calyptogena (Calyptogena) paciﬁca Dall. Bernard, 1974:
11, ﬁgs.1A, 2A, 3A, 4A–D; Boss and Turner, 1980:
188–189, ﬁgs.10B, C; Kanno et al., 1989: ﬁgs. 1.7–1.15.
?Calyptogena sp. Tsuji et al., 1991: ﬁg. 25, 26.
Vesicomya (Calyptogena) paciﬁca (Dall). Coan et al.,
2000: 341, pl. 70.
Material Examined: Seven specimens. NMNS PM28260–
PM28266.
Remarks: The shells are rather small (less than 30.0 mm
in length) and ovate in outline (height/length 5 0.68,0.71). Their beaks are located at anterior one-ﬁfth to twoﬁfths of shell length (umbo% 5 21, 41). In the right valve,a posterior tooth (3b) is large and triangular with a small
thin anterior tooth (3a), showing U-shaped connection,and surrounding a small middle tooth (1). The pallial line
is entire. These shell and hinge characters are identical toPage 52

THE NAUTILUS, Vol. 133, No. 2

Figures 15–24. Calyptogena paciﬁca Dall from the upper Miocene Onnagawa Formation. 15. Left valve hinge, silicone rubber cast,
NMNS PM28263. 16, 17, 19. 16. Silicone rubber cast of inner part of left valve. 17. Left valve. 19. Silicone rubber cast of inner part ofright valve; NMNS PM28260. 18. Rubber cast of right valve hinge, NMNS PM28261. 20, 23, 24. 20. Dorsal view. 23. Left valve. 24.
Right valve; NMNS PM28262. 21. Inner part of left valve, showing pallial line (pl) and posterior adductor muscle scar (PAS), NMNS
PM28264. 22. Inner part of right valve, showing anterior adductor muscle scar (AAS), NMNS PM28265. Scar bars in 15–19 5 5 mm and
20–24 5 10 mm.

those of Recent Calyptogena paciﬁca specimens. Tsuji
et al. (1991: ﬁg. 25, 26) identiﬁed and illustrated a
specimen as Calyptogena sp. collected from the Onnagawa Formation, near the fossil locality discussed here.
Although no pallial line and hinge features were shown,the outline of their specimen is similar to that of
Calyptogena paciﬁca illustrated here. (See Table 1 formorphometric data.)
Comparison: The oldest species of Calyptogena, C.
katallaensis Kiel and Amano, 2010 from the Oligocene
Kulthieth Formation in Alaska is most similar to C.
paciﬁca. However, C. paciﬁca differs slightly from the
Oligocene species by having a nymph with a rather abrupt

end and a long anterior cardinal tooth (3a) in the rightvalve.
Distribution: Upper Miocene: Morai Formation (Otatume, 1942; Amano, 2003) and Toyama Formation(Suzuki, 2007) from Hokkaido, Akaishi Formation(Amano and Jenkins, 2011) from Aomori Prefecture,
Onnagawa Formation (this study) from Akita Prefecture,
Nodani Formation (Kanno et al., 1989; Amano, 2002)
from Niigata Prefecture; Pliocene: Kurokura, Kawazume
and Nadachi Formations (Kanno et al., 1989; Amano andKanno, 1991; Amano and Kanno, 2005) from Niigata
Prefecture; middle Pleistocene: Wakimoto Formation(Otuka, 1937) from Akita Prefecture; Recent: Sea ofTable 1. Morphometry of Calyptogena paciﬁca studied.
Measurements (mm):NMNS PM28260
NMNS PM28262
NMNS PM28263

length (L)height (H)H/L

Umbo (%)*

Valve

18.0
30.0
29.6

12.3
21.3
118.9

0.68
0.71
-

41
21
-

left
left
left

* Ratio of anterior length (distance from umbo to anterior margin) to shell length.

K. Amano et al., 2019

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Okhotsk (Tiba, 1972), Dixon Strait, Alaska to Monterey
Bay, California (Coan et al., 2000).

BIOGEOGRAPHIC DISTRIBUTIONS OF
MIOCENE TO RECENT VESICOMYIDS IN JAPANIn this section, we review the geographic distribution ofvesicomyid species and genera in Japan from the Miocene
to the present (Table 2) in relation to tectonics, paleogeography, and climate. Following the appearance ofPleurophopsis matsumotoi new species around the early
Miocene, Pleurophopsis was the dominant vesicomyid
genus in Japan until the middle Miocene. The Japan Sea

was formed in the early Miocene and, initially, had deepwater connections with the Paciﬁc Ocean through thecentral part of Honshu, an area called the Fossa Magna
Region (Iijima and Tada, 1990; Ogasawara, 1994). The
ﬁrst vesicomyids to colonize the Japan Sea were Pleurophopsis chitanii and Pliocardia kawadai, with the oldest
record from the lower Miocene Kurosedani Formation in
Toyama Prefecture (Amano et al., 2019). Through theearly Miocene to early middle Miocene, Pleurophopsis
chitanii, along with Pleurophopsis uchimuraensis andPliocardia kawadai, occurred both in the Japan Sea andthe Paciﬁc side of Japan, while a remarkable number ofendemic species evolved in the Japan Sea, namely

Table 2. Age and distribution of the fossil vesicomyids without the Paleogene species in Japan. * Species from the Sea of Okhotsk.
** Bessho Formation in Nagano Prefecture is treated as the Japan Sea side.
SpeciesArchivesica sp.
Archivesica ? bosoensis (Kanie andKuramoch, 2001)
Archivesica shikamai Amano and Kiel,
2010
Archivesica kawamurai (Kuroda, 1943)

Archivesica kannoi Amano andKiel, 2010
Calyptogena veneriformis Amano andKiel, 2012
Pleurophopsis sp.
Archivesica shiretokensis (Uozumi, 1967)*
Archivesica nipponica (Oinomikado andKanehara, 1938)
Calyptogena paciﬁca Dall, 1891

Pleurophopsis kuroiwaensis (Amanoand Kiel, 2011)
Pleurophopsis akanudaensis (Tanaka,
1959)
Pliocardia ? tanakai Miyajima,Nobuhara and Koike, 2017
Pleurophopsis hokkaidoensis(Amano and Kiel, 2007)
Pleurophopsis hamuroi (Amanoand Kiel, 2011)
Pliocardia kawadai (Aoki, 1954)
Pleurophopsis uchimuraensis(Kuroda, 1931)
Pleurophopsis chitanii (Kanehara, 1937)
Pleurophosis sp.
Archivesica sakoi Amano, Jenkins,
Ohara and Kiel, 2014
Pleurophopsis matsumotoi n. sp

Age

PaciﬁcJapan Sea

Formation

Pliocene
Pliocene

1
1

Horinouchi F.
Shiramazu F.

Pliocene

1

Ikego F.

Pliocene–early
Pleistoscene

1early Pliocene

1

Tomioka F., Hitachi F., Na-arai
F., Kurotaki F., Ikego F.,
Imaizumi F., Hijikata F., Tamari
Siltstone, Shinzato F.
Kurokura F.

early Pliocene

1

Kurokura F.

early Pliocene
late Miocene
late Miocene–early
Pliocenelate Miocene–middle
Pleistocene

1

Kurokura F.
Rusha F.
Kubiki F., Araya F.

11latest middle Miocene

1

Onnagawa F., Morai F., Toyama F.,
Akaishi F., Nodani F.,
Kawazume F., Nadachi F.,
Kurokura F., Wakimoto F.
Ogaya F.

middle Miocene

1

Bessho F.**

middle Miocene

1

Bessho F.**early middle Miocene

1

Chikubetsu F.

early–early middle
Mioceneearly–middle Miocene

1

Higashibessho F.

11early–middle Miocene

11

early–middle Miocene

1

1

middle Miocene
early Miocene

11

Honya F., Kurosedani F.,
Higashibessho F., Nupinai F.
Bessho F.**, Takinoue F.,
Shikiya F.
Mizunoya F., Kamenoo F., TairaF., Morozaki G., Nupinai F.,
Kurosedani F.
Aokiyama F.
Shikiya F.

late Oligocene–early
Miocene1

Hioki Complex

Page 54

Pleurophopsis hamuroi, P. hokkaidoensis, P. akanudaensis, and P. kuroiwaensis (Table 2).
The vesicomyid faunas both sides of Japan became more
distinct from the late Miocene onward, when Archivesica
and Calyptogena became the dominant vesicomyid genera.
Several species of Archivesica have been documented fromthe Japan Sea and the Paciﬁc side of Japan, but none of thosespecies occurs on both sides of Japan (Table 2). However,there is no late Miocene record of Archivesica in Japan,
partly because the late Miocene on the Paciﬁc side ischaracterized by strata barren of molluscan fossils innortheastern Japan and by a hiatus in southwestern Japan(Chinzei, 1986). Fossils of Calyptogena are only known fromthe Japan Sea, including the extant C. paciﬁca. Remarkably,
the timing of the faunal change from the dominance ofPleurophopsis to that of Archivesica/Calyptogena coincides
with the tectonic inversion from tension to compression in
Northeast Japan (Sato, 1994). However, potential reasonsfor this coincidence remain unclear. Vesicomyids became
locally extinct in the Japan Sea due to deep-water anoxia
during the Pleistocene glacial period; their absence from thepresent-day Japan Sea might be because they have been
unable to pass through the shallow straits from the Paciﬁc
Ocean (Amano, 2001, 2007; Amano and Jenkins, 2011).
Based on the areas of origin and the ecology of the two
extant vesicomyid species that have fossil records in Japan(Calyptogena paciﬁca Dall and Archivesica kawamurai;
Amano and Jenkins, 2011), and their relatives, we postulate that the observed paleobiogeographic patterns
might be broadly related to temperature preferencesamong Calyptogena and Archivesica. Present-day
Calyptogena paciﬁca occurs mainly in the cold watersof the North Paciﬁc Ocean (Coan et al. 2000). The genus
Calyptogena ﬁrst appeared in the Oligocene of Alaska(Kiel and Amano, 2010) and migrated southward to theJapan Sea in the late Miocene, potentially related to thelate Miocene climate cooling (Amano and Jenkins, 2011).
The Japan Sea at that time was connected to the Paciﬁc
Ocean mainly through its northern straits (Iijima andTada, 1990; Ogasawara, 1994), and was probably blockedfrom inﬂux of warmer waters from the south, thus providing a suitable habitat for Calyptogena species.
The oldest species of Archivesica, A. sakoi Amano, Jenkins,
Ohara, and Kiel, 2014, is from the lower Miocene Shikiya
Formation in Wakayama Prefecture on the Paciﬁc side ofsouthern Japan. From the late Miocene onward Archivesica
also occurs in the Japan Sea, but during the same time it ismore diverse and more widely distributed on the Paciﬁc side,with extant Archivesica kawamurai and three further
Archivesica species distributed from central Honshu tosouthwestern Japan (Amano and Jenkins, 2011; Table 2). The
Paciﬁc side of central Honshu to southwestern Japan has
been inﬂuenced by warm-water currents since early Miocene(Ogasawara, 1994) and has been situated at a subduction
zone, resulting in constant methane seepage (Amano andJenkins, 2011). Today, Archivesica includes at least ten
species living in the warm-water area from central Honshu tosouthwestern Japan on the Paciﬁc side (Okutani, 2017) and is
the most diversiﬁed genus among the vesicomyids.

THE NAUTILUS, Vol. 133, No. 2

ACKNOWLEDGMENTS
We thank Takuma Haga (National Museum of Nature andScience) for his help examining the Akebiconcha uchimuraensis specimens, collected by Matsumoto and Hirata(1972). We also thank Crispin Little (University of Leeds)
and Krzysztof Hryniewicz (Paleobiology Institute ofPolish Academy of Sciences) for their critical reviews that
improved the manuscript. This study was supported by aGrant-in-aid for Scientiﬁc Research from the Japan Society for Promotion of Science (C, 17K05691, 2017-2019)
to KA and RGJ.

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