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New fossil Bathymodiolus (sensu lato) (Bivalvia: Mytilidae) from
Oligocene seep-carbonates in eastern Hokkaido, Japan, with
remarks on the evolution of the genus
Kazutaka Amano
Department of Geoscience
Joetsu University of Education
Joetsu 943-8512, JAPAN
amano@juen.ac.jp
Robert G. Jenkins
JSPS Research Fellow
Faculty of Education and Human Sciences
Yokohama National University
Yokohama 240-8501, JAPAN
ABSTRACT
A new species of the genus Bathymodiolus (sensu lato) is
herein described from the lower Oligocene Nuibetsu Forma-
tion in eastern Hokkaido, Japan. This is the oldest species of
this genus in Japan and the second oldest world wide. Based on
occurrence and distribution of fossil Bathymodiolus (sensu
lato), we suggest that the “genus” spread to the whole world
by the late Miocene. This dispersal pattern is supported by
molecular studies and similar to that of the large vesicomyids.
Additional keywords: Oligocene, fossil, biogeography,
Mytiloidea
INTRODUCTION
Bathymodiolus (sensu lato) is one of the representative
members of chemosynthetic communities frequently
found at hydrothermal vents and cold seeps. Phylogenetic
relationships between Bathymodiolus (sensu lato) and
other modioline mussels are of considerable interest, since
Distel et al. (2000) hypothesized that Bathymodiolus
(sensu lato) in the seep and vent sites originated from
small wood- or bone-associated modiolines. The recent
description of Vulcanidas from a shallow vent site by
Cosel and Marshall (2010) suggests that pathways
of adaptation occurred at least three times in the
bathymodiolines. It is necessary to examine such pathways
of adaptation from the view point of the fossil record.
Six Recent species of this genus are known around
Japan (Sasaki et al., 2005). Morphologically, the genus
can be classified into the four groups Bathymodiolus
thermophilus,B. brevior,B.heckerae, and B. childressi
(see Cosel, 2002). Most molecular studies, however, indi-
cate that the genus is not a monophyletic group but
includes instead several distinct clusters (e.g., Miyazaki
et al., 2004; Jones et al., 2006; Iwasaki et al., 2006;
Samadi et al., 2007; Fujita et al., 2009). Based on these
studies and their morphological data, Cosel and Janssen
(2008) recognized the following three clades, the
B. thermophilus,B. aduloides, and B. childressi clades.
Moreover, the B. childressi clade was subdivided
into six groups, including the genus Gigantidas Cosel
and Marshall, 2003 as one group. Among them, the
B. thermophilus clade includes B. thermophilus,
B. brevior,B.heckerae groups of Cosel (2002). Recently,
Miyazaki et al. (2010) examined COI and ND4 genes
and divided Bathymodiolus into four groups that corre-
spond to three clades of Cosel and Janssen (2008) and
one clade of the genus Tamu Gustafson, Turner, Lutz,
and Vrijenhoek, 1998. Very recently, Lorion et al.
(in press) also suggested that Bathymodiolinae should
be split into the B. thermophilus and B. childressi
groups, based on the studies of COI mtDNA and 28S
rRNA. As a conclusion, the genus Bathymodiolus is not a
monophyletic group, but consists of two or three clades.
Strictly speaking, the genus Bathymodiolus should be
used only for the molecular clade including the type
species, B. thermophilus Kenk and Wilson, 1985. How-
ever, some authors used Bathymodiolus (sensu lato) to
the clades other than B. thermophilus clade (e.g., Cosel
and Janssen, 2008; Kiel et al., 2010). In this paper, we
use Bathymodiolus (sensu lato) for all species hitherto
described as Bathymodiolus.
The fossil record of Bathymodiolus (sensu lato) can be
traced back to B. willapaensis, which dates from the
middle Eocene (Kiel, 2006). In Japan, only three certain
and two doubtful fossil records of Bathymodiolus (sensu
lato) are known from cold-seep sites. One of these
records is from the middle Miocene Akanuda Limestone
of the Bessho Formation in central Nagano Prefecture.
Kuroda (1931) described Tamarindiformis akanudaensis
as a new species from Akanuda, Matsumoto City. Then,
Tanaka (1959) illustrated this species as Volsella.
Recently, Nobuhara et al. (2008) mentioned that the
species possibly belongs to the bathymodiolines, based
on the morphology of its juvenile shell. This species has
been also illustrated from a large seep carbonate of the
uppermost middle Miocene Ogaya Formation in Niigata
Prefecture (Amano et al., 2010). “Bathymodiolus” sp.
has been obtained and illustrated from siltstone of the
THE NAUTILUS 125(1):29–35, 2011 Page 29
Pliocene Tamari Formation in Shizuoka Prefecture by
Nobuhara (2003). In addition to these records, Katto
and Masuda (1978) illustrated one specimen from the
carbonate of the Oligocene(?) Muro Group in Wakayama
Prefecture as Modiolus sp. This species occurred in asso-
ciation with Conchocele cf. nipponica (Yabe and
Nomura, 1925) and Callista cf. hanzawai (Nagao, 1928)
(¼probably not Callista, but a vesicomyid). Moreover,
Amano et al. (2004) found several specimens of
Bathymodiolus? sp. in mudstone of the middle Miocene
Higashibessho Formation in Toyama Prefecture.
Fortunately, we could collect many bathymodioline
specimens of a new species from an Oligocene deposit
in Urahoro-cho, eastern Hokkaido. This is the oldest
record in Japan at the moment. In this paper, we
describe it and discuss its biogeographic significance
and evolutionary trend of Bathymodiolus (sensu lato).
MATERIALS
The new species described herein is from limestones
of the lower Oligocene Nuibetsu Formation which
crops out along the Atsunai River, 1.5 km east of Kami-
Atsunai railway station in Urahoro-cho, eastern Hok-
kaido (Figure 1). The limestone can be subdivided into
three parts from bottom to top; limestone yielding many
fossils and mudstone breccias (10m thick), laminated
limestone without fossils (2 m thick) and limestone
yielding pebbles of slate, plant debris and fossils (4 m
thick). Carbonate minerals of the lower limestone pre-
cipitated in an early diagenetic stage are depleted in
13
C
(d
13
C values as low as –49%vs. PDB; Pee Dee Belem-
nite standard), which indicates methane seep activity
(Peckmann and Thiel, 2004). Although both lower and
upper parts yield chemosynthetic bivalves, and carni-
vore or scavenging gastropods, Bathymodiolus (sensu
lato) was collected only from the parts, in association
with the thyasirid Conchocele bisecta (Conrad, 1849),
the vesicomyid Hubertschenckia ezoensis (Yokoyama,
1890), the solemyids Acharax aff. gigas (Kanno, 1960),
A. sp., the naticid Euspira meisensis (Makiyama, 1926)
and the buccinid Colus cf. fujimotoi Hirayama, 1955.
Terminology of description follows Gustafson et al.
(1998). All specimens are stored at the Joetsu University
of Education (JUE).
SYSTEMATICS
Family Mytilidae
Genus Bathymodiolus Kenk and Wilson, 1985
Type Species: Bathymodiolus thermophilus Kenk and
Wilson, 1985 from hydrothermal vent fields on the
Galapagos Rift.
Bathymodiolus (sensu lato) inouei new species
(Figures 2–6, 9, 11, 12)
Diagnosis: A small-sized Bathymodiolus (sensu lato)
with elongate shell, beak near anterior end, nearly
straight dorsal and ventral margin; blunt ridge running
from umbo to posterior corner.
Description: Shell of small size for genus, up to
45.4 mm long, modioliform, elongate (height/length
ratio ¼0.30–0.59; length/height ratio ¼1.71–3.31),
equivalve and inequilateral, moderately inflated, sculp-
tured by growth lines only. Blunt ridge running from
beak to posteroventral corner. Beak prosogyrate, situated
near anterior end (position of umbo; 2.3–6.9 % of shell
length from anterior end). Anterior margin broadly
Figure 1. Locality map.
Page 30 THE NAUTILUS, Vol. 125, No. 1
Figures 2–6, 9, 11, 12. Bathymodiolus (sensu lato) inouei new species. All specimens from type locality. 2, 3, 5. Holotype, length
28.7 mm, JUE no. 15873. 2. Right valve. 3. Left valve. 5. Dorsal view. 4. Paratype, length 8.7 mm, JUE no. 15874-5, right valve
(juvenile). 6. Paratype, length 28.4 mm, JUE no. 15874-4, left valve; AA, anterior adductor scar; PA, posterior adductor scar; PBR,
posterior byssal retractor scar. 9. Paratype, length 30.5 mm, JUE no. 15874-3, left valve. 11. Paratype, length 30.8 mm, JUE no.
15874-1, right valve. 12. Paratype, length 31.1 mm, JUE no. 15874-2, left valve. Figures 7, 10. Bathymodiolus (sensu lato)
willapaensis (Squires and Goedert). Topotype from Bear River deposit, Washington State, USA, collected by RJ. 7. Length
11.8 mm, JUE no. 15876-1, left valve. 10. Length 13.8 mm, JUE no. 15876-2, right valve. Figures 8, 13–15. Bathymodiolus (sensu
lato) akanudaensis (Kuroda). 8, 14, 15. Topotype specimens. 8. Length 29.6 mm, JUE no. 15882-1, dorsal view; 14. Length
22.4 mm, JUE no. 15882-3, right valve. 15. Length 24.2 mm, JUE no. 15882-2, left valve. 13. Length 15.4 mm, JUE no. 15883, left
valve, loc. Kita-Kuroiwa, Joetsu City.
K. Amano and R. G. Jenkins, 2011 Page 31
arched; ventral margin nearly straight; posterodorsal
margin very broadly arched, continuing into steeply slop-
ing posterior margin. Hinge edentulous. Nymph
extending from beak and occupying 55% of dorsal mar-
gin. Anterior adductor muscle scar distinct, small and
semicircular; posterior adductor scar large and ovate;
posterior byssal retractor scar long, thin, united with
posterior adductor scar.
Holotype: JUE no. 15873.
Paratypes: JUE no. 15874-1 to JUE no. 15874-5; all
from the type locality.
Type Locality: Outcrop along the Atsunai River,
1.5km east of Kami-Atsunai Railway Station in Urahoro-
cho, eastern Hokkaido.
Material Examined: Eighty specimens were exam-
ined. Among these, forty-six are articulated.
Measurements: See Table 1.
Remarks: The new species can be identified as
Bathymodiolus (sensu lato) because of its modioliform
shape and the occurrence from the cold seep site. Owing
to its terminal umbo and its continuous posterior retrac-
tor scar united posterior muscle scar, the new species
may be assigned to the B. childressi clade by Cosel and
Janssen (2008).
Comparison: The Eocene species Bathymodiolus
willapaensis (Squires and Goedert, 1991) (Figures 7, 10)
has very similar outline to juvenile form of the present
species, but has much smaller (27 mm long) and higher
shell than B. (sensu lato) inouei new species (figure 16).
Also, B. akanudaensis (Kuroda, 1931) from the Miocene
Bessho and Ogaya Formations (Figures 13–15) can
be discriminated by having distinctly higher and more
inflated shell than the new species (Figures 16, 17).
Bathymodiolus (sensu lato) inouei resembles Bathym-
odiolus (sensu lato) palmarensis Kiel, Campbell and Gail-
lard, 2010, from the “Oligocene” deposit of Colombia,
in its elongated shell and its beak located near anterior
end, but differs from it by having less distinct ridge and
less expanded posterior part. Volsella yokoyamai Hatai
and Nisiyama, 1952 described from the upper Eocene
Iwaki Formation in Fukushima Prefecture resembles
B. (sensu lato) inouei in having an elongate shell, but
differs by its well inflated shell, more posteriorly located
beak and more distinct ridge. One specimen illustrated by
Table 1. Measurements of Bathymodiolus (sensu lato) inouei new species.
Page 32 THE NAUTILUS, Vol. 125, No. 1
Katto and Masuda (1978) as Modiolus sp. from the Oligo-
cene? Muro Group in Wakayama Prefecture may be
inferred as Bathymodiolus (sensu lato) by the occurrence
from the carbonate in association with chemosynthetic
species. This specimen is similar to the new species in its
shell outline. However, owing to a few data on its speci-
men, it is necessary to collect additional specimens for
comparing with this new species in detail.
Distribution: Known only from type locality. Lower
Oligocene Nuibetsu Formation in Hokkaido.
Etymology: Named after Mr. Kiyokazu Inoue,
Obihiro City, an amateur collector of molluscan fossils,
who found the locality of this new species.
DISCUSSION
Only four Paleogene species of Bathymodiolus (sensu
lato) are presently known. Among them, B. willapaensis
is the oldest species, having been reported from middle
Eocene to late Oligocene seep carbonates in Washington
State, USA (Goedert and Squires, 1990; Squires and
Goedert, 1991; Goedert and Squires, 1993; Goedert and
Campbell 1995; Kiel, 2006). Probably, the next oldest
record is the present new species from the lower Oligo-
cene. Kiel et al. (2010) described B. palmarensis from
the “Oligocene” in Colombia, but its precise age is uncer-
tain. Moreover, one specimen was illustrated as
Modiolus sp. from the Oligocene? Muro Group in Waka-
yama Prefecture, central Honshu by Katto and Masuda
(1978). All Paleogene species are small (less than 50 mm)
and have their beak located near the anterior end which
is one of the diagnostic features of Bathymodiolus
childressi clade.
In contrast, many records of Bathymodiolus (sensu
lato) are known from the Neogene deposits around the
world. Bathymodiolus akanudaensis and B.?sp.occur
in a middle Miocene deposit in central Honshu
(Kuroda, 1931; Tanaka, 1959; Nobuhara et al., 2009;
Amano et al., 2004, 2010). In the Caribbean area, two
bathymodioline species have been described from the
Miocene Freeman’s Bay Limestone of the late middle
Miocene Lengua Formation in Trinidad and from the
lower to middle Miocene Husto Clay Member of the
Pozon Formation in Venezuela (Gill et al., 2005).
Modiolus (Modiolus)exbrochii exbrochii Sacco and an
elongate Bathymodiolus-like fossil were reported from
the upper Miocene “Carcari a Lucina”atMontepetra,
Italy (Moroni, 1965; Taviani, 1994, 2001). Recently,
Saether et al. (2010) described Bathymodiolus (sensu
lato) heretaunga and Gigantidas coseli as new species
from the ?late early to the earliest late Miocene in the
North Island of New Zealand. Of Pliocene age is
“Bathymodiolus” sp. reported by Nobuhara (2003) from
siltstone of the Tamari Formation in the forearc basin
of Honshu.
Considering the geographic distribution of these
records, we suggest that the worldwide spread of
Bathymodiolus (sensu lato) might occur by the late Mio-
cene (Figure 18). This trend of geographic spread is the
same to that of the large vesicomyids which is another
characteristic taxa of chemosynthetic fauna (Taviani,
2001; Gill et al., 2005; Lucentte and Taviani, 2005;
Amano and Kiel, 2007; Amano and Kiel, in press; Kiel
and Peckmann, 2007; Campbell et al., 2008; Kiel and
Amano, 2010). Moreover, this pattern is consistent with
the molecular study of Miyazaki et al. (2008), who esti-
mated that the worldwide spread of Bathymodiolus
(sensu lato) took place during the middle Miocene.
Figure 17. Ontogenetic changes in shell length and width of
Bathymodiolus (sensu lato) inouei,B. (sensu lato) willapaensis
and B. (sensu lato) akanudaensis.
Figure 16. Ontogenetic changes in shell length and height of
Bathymodiolus (sensu lato) inouei,B. (sensu lato) willapaensis
and B. (sensu lato) akanudaensis.
K. Amano and R. G. Jenkins, 2011 Page 33
ACKNOWLEDGMENTS
We are grateful to Steffen Kiel (University of Go
¨ttingen)
for his review and useful comments to this paper. We
thank Kiyokazu Inoue (Obihiro City) for his information
on the fossil locality and two anonymous reviewers for
their critical reading of this manuscript and useful
advices. We also thank Tsuzumi Miyaji (University of
Tokyo) for her help on isotope analysis. This study was
partly supported by a Grant-in-aid for Scientific
Research from the Japan Society for Promotion of
Science (C, 20540456, 2008–2010) and the Nippon
Foundation-HADal Environmental Science Education
Program (HADEEP).
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