A newly discovered Paleocene species of Boreocomitas (Gastropoda: Pseudomelatomidae) from eastern Hokkaido, Japan, with implications for the biogeography of the Paleocene Bering Strait

Abstract

Two species of the gastropod family Pseudomelatomidae, genus Boreocomitas, including the new species B. inouei, are described from the Paleocene (upper Selandian to lowest Thanetian) Katsuhira Formation in eastern Hokkaido, Japan. These species represent the first Paleocene record of Boreocomitas. The paper discusses also the new recognition of species of Boreocomitas in Paleocene deposits of western Greenland and Denmark. Occurrences of some taxa, including this genus, are suggestive of faunal exchanges between the northern Pacific and the North Atlantic/Artic regions from the middle Paleocene (Selandian) to late Eocene. These exchanges could have resulted from direct marine connections between both areas via the Bering Strait.

Full text

A newly discovered Paleocene species of Boreocomitas
(Gastropoda: Pseudomelatomidae) from eastern Hokkaido, Japan,
with implications for the biogeography of the Paleocene
Bering Strait
Kazutaka Amano
Department of Geoscience
Joetsu University of Education
1Yamayashiki
Joetsu 943-8512, JAPAN
amano@juen.ac.jp
Krzysztof Hryniewicz
Institute of Paleobiology
Polish Academy of Sciences
ul. Twarda 51/55
00-818 Warszawa, POLAND
krzyszth@twarda.pan.pl
Robert G. Jenkins
School of Natural System,
College of Science and Engineering
Kanazawa University, Kanazawa City
Ishikawa 920-1151, JAPAN
robertgj@staff.kanazawa-u.ac.jp
ABSTRACT
Two species of the gastropod family Pseudomelatomidae, genus
Boreocomitas, including the new species B. inouei, are described
from the Paleocene (upper Selandian to lowest Thanetian)
Katsuhira Formation in eastern Hokkaido, Japan. These species
represent the first Paleocene record of Boreocomitas. The paper
discusses also the new recognition of species of Boreocomitas
in Paleocene deposits of western Greenland and Denmark.
Occurrences of some taxa, including this genus, are suggestive of
faunal exchanges between the northern Pacific and the North
Atlantic/Artic regions from the middle Paleocene (Selandian) to
late Eocene. These exchanges could have resulted from direct
marine connections between both areas via the Bering Strait.
Additional Keywords: Paleocene, gastropod, Boreocomitas, new
species, palaeogeography
INTRODUCTION
Two species of a Paleocene bathyal gastropod genus
including a new species are reported from eastern
Hokkaido, Japan. These species are here assigned to the
pseudomelatomid gastropod genus Boreocomitas.The
deep-sea gastropod genus Boreocomitas was proposed
by Hickman (1976) as an extinct subgenus of Comitas
Finlay, 1926, based on specimens from the Cowlitz and
Keasey formations in Oregon, western USA. Recent data
show that the age of the Cowlitz Formation is middle
to late Eocene and that of the Keasey Formation is
late Eocene to earliest Oligocene (Prothero, 2001).
Boreocomitas has also been recorded from the upper
Eocene Kovachinskaya Formation in western Kamchatka
(Gladenkov et al., 1991).
Many well-preserved molluscan fossils have been de-
scribed from the upper Selandian to the lowest Thanetian
Katsuhira Formation in eastern Hokkaido, northern Japan
(Figure 1; Amano and Jenkins, 2014, 2017; Amano and
Oleinik, 2014; Amano et al., 2015a, b, 2016, 2018). These
fossils occur mainly in small calcareous concretions (about
30 cm in diameter) with plant debris, considered to be
wood-fall communities (Amano et al., 2016a, 2018). From
the locality at Katsuhira along the Urahoro River, two
species of Boreocomitas including a new one have been
recovered. In this paper, we describe these species and
discuss their paleobiogeographical significance.
MATERIALS AND METHODS
Three specimens of Boreocomitas inouei new species and
one specimen of Boreocomitas species were obtained
from carbonate concretions (20 cm in diameter) with
many bored wood fragments included in dark gray mud-
stones of the Katsuhira Formation. These concretions are
exposed along the Urahoro River, 44 m south from the
mouth of the Kokatsuhirazawa River, Urahoro Town,
eastern Hokkaido (Figure 1; 42°59'10" N, 143°37'38" E).
Rocks in the direct proximity to the locality contain
dinoflagellate fossils indicating a late Selandian to ear-
liest Thanetian age (Amano et al., 2018). These species
were associated with 22 species of mollusks (Table 1) and
one species of echinoid. Among them, Myrtea ezoensis
(Nagao, 1938), and Thyasira oliveri Amano and Jenkins,
2018, were proposed as chemosymbiotic bivalves. As
mentioned by Amano et al. (2018), the paleobathymetry
of the Katsuhira Formation can be estimated as 200 to
500 m in depth. From these occurrences of mollusks, it is
possible to consider the two species of Boreocomitas as
members of the deep-sea wood-fall community.
All specimens of Boreocomitas are catalogued in the
University Museum of the University of Tokyo (UMUT).
The associated fauna is stored at Joetsu University of
Education (JUE). Classification at family level follows
THE NAUTILUS 132(3–4):117–123, 2018 Page 117

the current taxonomy of Conoidea by Bouchet et al.
(2011, 2017).
SYSTEMATIC PALEONTOLOGY
Class Gastropoda Cuvier, 1797
Order Neogastropoda Wenz, 1938
Superfamily Conoidea Fleming, 1822
Family Pseudomelatomidae Morrison, 1966
Remarks: Boreocomitas was originally proposed as
a subgenus of the genus Comitas Finlay, 1926, and placed
in Turriculinae Powell, 1942, of Turridae H. Adams and
A. Adams, 1853 by Hickman (1976). Family Pseudome-
latomidae including the genus Comitas is characterized by
a smooth paucispiral protoconch.
Genus Boreocomitas Hickman, 1976
Type Species: Comitas (Boreocomitas)oregonensis
Hickman, 1976
Remarks: Boreocomitas is characterized by having
a medium to moderately large-sized fusiform shell with
axial nodes on the keel and fine spiral threads, which are
less prominent on its wide shoulder than anterior ward. Its
anal sinus is broad and moderately deep on the shoulder.
According to Hickman (1976), “the protoconch is missing
or worn on all specimens examined, but it is apparently
paucispiral”. The genus Comitas Finlay, 1926, has a more
slender shell, a narrower shoulder, a longer anterior canal
and weaker nodes on the periphery than in Boreocomitas.
It can be judged that these differences are enough to
separate the two taxa as distinct genera. Although the
Figure 1. Map showing the locality yielding the two species of Boreocomitas discussed.
Page 118 THE NAUTILUS, Vol. 132, No. 3–4

pseudomelatomid genus Nekewis Stewart, 1927, re-
sembles Boreocomitas,Nekewis can be distinguished from
Boreocomitas by having a long and straight anterior canal
with a fasciole and shallow anal sinus. Wenz (1938)
mistakenly considered Nekewis to be a synonym of the
raphitomid genus Clinura Bellardi, 1875. Clinura is very
similar to Boreocomitas in having biconic general outline
with distinct axial nodes on its periphery. However, Cli-
nura has a protoconch that is narrowly conical and di-
agonally cancellate. Its diagonally cancellate protoconch
demonstrates that Clinura belongs in Raphitomidae
Bellardi, 1875. Clinura also has an anal sinus whose apex is
located near the suture on the shoulder slope.
Boreocomitas inouei new species
(Figures 2, 3, 4)
Diagnosis: Moderate-sized, short, biconic Boreocomitas
species consisting of five and half whorls and at least one
smooth paucispiral protoconch. Surface sculptured with
23 spiral cords on base, nine on keel and six above wide
shoulder, and fine sinuous growth lines on last whorl. Apex
of anal sinus located at midpoint of shoulder.
Description: Shell moderate-sized, attaining 20.4 mm in
height, fusiform, consisting of 5.5 teleoconch whorls and
one protoconch whorl. Apical angle ranging from 59° to
73°. Protoconch smooth, paucispiral, and with large di-
ameter (d 51.6 mm in paratype UMUT CM 32942). Last
whorl very large, occupying about 74% of shell height in
holotype; spire low; subsutural band very weak; shoulder
slope broad and gently concave. Surface of penultimate
whorl sculptured with one fine spiral cord below keel, six
on keel, shoulder slope smooth, without growth lines, and
21 axial nodes on keel; keel of last whorl with 22 axial
nodes, 23 spiral cords on base, nine on keel, and six very
weak fine cords just above keel and five very faint on
shoulder below subsutural band; among spiral cords on
last whorl, two cords below keel stronger than other cords.
Anal sinus moderately deep, its apex located at midpoint
of shoulder slope on last whorl and just below subsutural
band on penultimate whorl. Aperture pyriform; outer lip
very thin; inner lip covered by thin, narrow callus. Anterior
canal short, slightly broken on anteriormost part, but
having slight siphonal fasciole.
Type Material: Holotype, UMUT CM 32793 (shell height,
20.4 mm1; diameter, 13.5 mm); Paratype, UMUT CM
32794 (shell height, 9.0 mm; diameter, 5.9 mm); Paratype,
UMUT CM 32942 (diameter, 7.5 mm1).
Type Locality: Cliff along Urahoro River, 44 m south
from mouth of Kokatsuhirazawa River, Urahoro Town,
eastern Hokkaido.
Remarks: This is the first record of Boreocomitas in the
Paleocene and from the northwestern Pacific region.
Boreocomitas inouei new species is similar to Bor-
eocomitas biconica (Hickman, 1976) from the middle to
upper Eocene Cowlitz Formation in northwestern Ore-
gon in having a similar size (shell height of B. biconica,
20.0 mm), a relatively low spire and a similar number of
nodes on the last whorl (20 in B. biconica). However, the
present new species has higher ratio of diameter (D)/shell
height (H) than the B.biconica species (D/H 50.66 for B.
inouei; 0.53 in B. biconica). Also, unlike B. biconica, the
present species has a spiral cord below the periphery and
no beaded subsutural cords.
Some previously identified species belonging to the
now invalid genus Pleurotoma Lamarck, 1799, are now
allocated to other genera. Among them, Pleurotoma
(Pseudotoma)brevior von Koenen, 1885 (p. 35–36, pl. 2,
figs. 5a–c), was described from the Selandian of Copen-
hagen, Denmark. Later, this species was re-described as
Genotia brevior by Ravn (1939, p. 93–94, pl. 4, fig. 11a–b).
Judging from the size, outline, sculpture and smooth
protoconch, P. brevior can be confidently allocated to
Boreocomitas. Boreocomitas brevior new combination
differs from B. inouei new species in having a more
slender, larger shell (ca. 30 mm in height), with fewer
nodes on the axial keel (18 to 19 in B. brevior) and distinct
spiral cords on the shoulder slope, and with more nu-
merous protoconch whorls (3.5 in B. brevior).
The genus Clinura Bellardi, 1875, has a similar shell
outline to Boreocomitas, with similar, moderately deep
anal sinus and many nodes on its keel. Among the species
of Clinura,Clinura sp. 1 from the Sonja Lens of the
Selandian Agatdal Formation in the western part of
Greenland (N ˆugssuaq) was illustrated by Kollmann and
Peel (1983, p. 97–98, fig. 220). This species is very similar
to the present new species in having a short biconic
outline, apex of the anal sinus located at the midpoint of
the broad shoulder slope, and very fine spiral cords above
Table 1. Mollusks associated species with Boreocomitas inouei
new species.
Species
Leionucula yotsukurensis (Hirayama)
Acila (Truncacila)hokkaidoensis (Nagao)
Ezonuculana aff. obsoleta Tashiro
Malletia poronaica (Yokoyama)
Menneroctenia plena Kalishevich
Pseudoneilonella? sp.
Tindaria paleocenica Amano and Jenkins
Propeamussium yubarense (Yabe and Nagao)
Limaria sp.
Myrtea ezoensis (Nagao)*
Thyasira oliveri Amano and Jenkins*
Astarte sp.
Cidarina? sp.
Naticidae gen. et sp. indet.
Kangilioptera sp.
Urahorosphaera kanekoi Amano and Oleinik
Admete katsauhiraensis Amano, Oleonik and Jenkins
Acteocina sp.
Retusa sp.
Biplica paleocenica Amano and Jenkins
Striodentalium sp.
Laevidentalium sp.
*Chemosymbiotic species.
K. Amano et al., 2018 Page 119

the keel. This species also has a smooth paucispiral
protoconch, which warrants its allocation to Boreocomitas,
not in Clinura. This species can be distinguished from B.
inouei new species by its larger shell (ca. 40 mm in height),
less numerous axial nodes, and by having only one spiral
cord below the keel on the last whorl.
Etymology: The new species is named for Kiyokazu
Inoue (Obihiro), who collected the holotype.
Distribution: Known only from the type locality, in the
upper Selandian to lowest Thanetian Katsuhira Forma-
tion, Urahoro Town, eastern Hokkaido.
Boreocomitas species
(Figure 5)
Description: Shell small, 13.1 mm in height, fusiform,
consisting of 5 teleoconch whorls; protoconch missing.
Apical angle 47°. Last whorl large, occupying about 69%
of shell height; spire high; subsutural band very weak;
shoulder slope broad and gently concave. Surface of
penultimate whorl sculptured with four fine spiral cords
below keel, three on keel, shoulder slope smooth, and 21
axial nodes on keel; keel of last whorl with 20 axial nodes,
12 spiral cords on base, three on keel, and no cord on
shoulder below subsutural band. Anal sinus moderately
deep, its apex located at midpoint of shoulder slope on last
whorl and just below subsutural band on penultimate
whorl. Aperture pyriform; outer lip very thin; inner lip
covered by thin, narrow callus. Anterior canal short.
Remarks: This species is similar to Boreocomitas inouei
new species as above described. However, Boreocomitas
species differs from B. inouei by having a slender form,
a higher spire, no spiral cord on the shoulder. Bor-
eocomitas oregonensis (Hickman, 1976), the type species
of the genus, is similar to Boreocomitas species in having
a rather higher spire. However, the type species has some
distinct spiral threads on its shoulder.
Distribution: Known only from the type locality of
Boreocomitas inouei new speices, in the upper Selandian
to lowest Thanetian Katsuhira Formation, Urahoro Town,
eastern Hokkaido.
DISCUSSION
The described species possiblylivedinawood-fall
community of upper bathyal depths. When Hickman
(1976) proposed Boreocomitas, this genus appeared to
be confined mainly to the middle Eocene to lower Oli-
gocene deposits in Oregon. As noted above, Gladenkov
et al. (1991) described and illustrated Comitas (Bor-
eocomitas) sp. from the upper Eocene Kovachinskaya
Formation in western Kamchatka. However, that shell is
poorly preserved, with weak nodes on the keel and
Figures 2–5. Boreocomitas species from the Katsuhira Formation. 2–4. Boreocomitas inouei new species. 2. Paratype, UMUT CM
32794; 2a. Apical view; 2b. Lateral view showing deep anal sinus of growth lines above shoulder slope. 3. Holotype, UMUT CM 32793;
3a. Apertural view; 3b. Abapertural view; 3c. Apical view. 4. Paratype, UMUT CM 32942, Apical view. 5. Boreocomitas sp., UMUT CM
32943; 5a. Apertural view; 5b. Side view; 5c. Apical view.
Page 120 THE NAUTILUS, Vol. 132, No. 3–4

a narrow shoulder. It is uncertain whether this species
can be classified in Boreocomitas. Our study shows that
Boreocomitas dates back to the Selandian and it had
a broad distribution including Denmark, western Greenland
and eastern Hokkaido.
The aporrhaid gastropod Kangilioptera Rosenkranz,
1970 shows the same geographical pattern to Bor-
eocomitas.Kangilioptera has been recorded from the
upper Danian Kangilia Formation in western Greenland,
the Selandian Kerteminde Marl in Denmark and the
upper Selandian to lowest Thanetian Katsuhira Formation
in eastern Hokkaido (Rosenkranz, 1970; Kollmann and
Peel, 1983; Amano and Jenkins, 2014; Schnetler and
Nielsen, 2018) (Table 2).
The astartid bivalve Astarte paleocenica Amano and
Jenkins in Amano et al., 2018 from the Katsuhira For-
mation is similar to A. parvula Kalishevich in Kalishevich
et al., 1981, from the Danian Krasnoyarskaya Formation
in southeastern Sakhalin and A. trigonula Koenen, 1885,
from the Selandian Lellinge Greensand (see also Amano
et al. 2018; Schnetler, 2001).
The bivalve genus Conchocele Gabb, 1866, has its
earliest confirmed occurrences in the latest Cretaceous
(Maastrichtian) of Antarctica, the Danian of western
Greenland, and the Thanetian of Spitsbergen
(Rosenkranz, 1970; Little et al. 2015; Hryniewicz et al.,
2016, 2017). In Spitsbergen, the genus survived to the late
Eocene–early Oligocene (Thiedig et al., 1980). There are
so far no confirmed records of Conchocele from the
Cretaceous and Paleocene of the northern Pacific area.
However, some publications suggest it could have been
present in the Pacific during the Late Cretaceous. Con-
chocele cretacea has been recorded from the Upper
Cretaceous deposits from Vancouver Island area
(Whiteaves 1874, p. 11–12, figs. 2, 2a). Thyasira cretacea
(5Conchocele cretacea) has also been recorded from the
Coniacian(?) Funks Formation of Sacramento Valley area
in California (Anderson, 1958). Therefore, the absence of
Conchocele in the Upper Cretaceous and Paleocene of
northern Pacific area might be just an artifact of the fossil
record. The genus became very common in the northern
Pacific region after the Paleocene (Hickman, 2015;
Hryniewicz et al., 2017). This distribution pattern shows
that Conchocele populations could have interchanged
between the northern Pacific and the North Atlantic/
Arctic regions at least by the late Eocene, or probably
earlier.
Moreover, the oldest record of the deep-sea arcid bi-
valve genus Bentharca Verrill and Bush, 1898, has been
found in the upper Selandian to lowest Thanetian Kat-
suhira Formation in eastern Hokkaido (Amano et al.,
2015). As pointed by Amano et al. (2015), Barbatia (Acar)
hennigi Heinberg, 1978, from the Maastrichtian to
Danian deposits in Denmark is possibly an ancestor of this
genus.
Adding to Boreocomitas, the four deep-sea mollusks
above mentioned occur in both northern Pacific and
North Atlantic/Arctic, indicating the sea connection be-
tween both areas during the Paleocene, although most
paleontologists believe that the Arctic was isolated from
the Pacific from the Albian to the latest Miocene
(Marincovich et al., 1990, 2002; Thiede et al., 1990;
Marincovich, 1993; Marincovich and Gladenkov, 1999;
Beard and Dawson, 1999). Deep-sea genera commonly
have broad geographic distribution (e.g. Amano et al.
2015a, b; Amano and Jenkins 2017), and similarities be-
tween the fauna of the Paleocene Katsuhira Formation
from Hokkaido and that of the North Atlantic/Arctic
Paleocene mollusk faunas could be partially caused by
that. However, an alternative explanation is that the
Bering Strait was to some extent open and connected
northern Pacific and Arctic oceans, allowing for faunal
interchange between both areas during the Selandian to
late Eocene time. When Brikiatis (2014) reviewed the
early Cenozoic paleogeography around the Arctic region
based on the land mammals and flora, he showed no land
bridge around the Arctic Ocean during the Selandian
in his Figure 8. Gleason et al. (2009) also reconstructed
the early Eocene paleogeographic map without any
land bridge surrounding the Arctic Ocean, based on the
Table 2. Taxa of Paleocene faunas shared between the Northwestern Pacific, Arctic and Northern Atlantic regions.
Region NW. Pacific Arctic/ N.Atlantic
E. Hokkaido SE. Sakhalin Ellesmere Is. Spitsbergen Is. W. Greenland Denmark
Taxa Age L. Selandian2
E. Thanetian
Danian Paleocene Selandian Danian2
Selandian
Danian2
Selandian
Bentharca 1 1?
Small Astarte 11 1
Thyasira 11 1
Conchocele 11
Kangilioptera 111
Drepanocheilus 11 1 1
Aporrhaidae 1
Boreocomitas 111
Reference This study Kalishevich
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and Nielsen (2018)
K. Amano et al., 2018 Page 121

examination of Nd-Sr isotopes in fossil fish debris from
Lomonosov Ridge. Although these data support our hy-
pothesis on the Paleogene temporary opening of the
Bering Strait, more data are needed to confirm it.
ACKNOWLEDGMENTS
We acknowledge Alan Beu (GNS Science) and Geerat
J. Vermeij (UC Davis) for providing comments to an
earlier version of the manuscript. We thank Carole S.
Hickman (UC Berkeley) for her information on the type
specimens of Boreocomitas species. We thank Anton E.
Oleinik (Florida Atlantic University) and Kai I. Schnetler
(Denmark) for their information on fossil species from the
Paleogene in Kamchatka and Denmark. We also express
many thanks to Kiyokazu Inoue (Obihoro in eastern
Hokkaido) who found the holotype and kindly donated
Boreocomitas specimen. This study was supported by
a Grant-in-aid for Scientific Research from the Japan
Society for Promotion of Science (C, 17K05691, 2017–
2019) to KA and RGJ. Financial support to KH was
provided by the Polish National Science Centre (NCN)
research grant 2014/15/B/ST10/04886 “The influence
of Paleocene/Eocene Thermal Maximum on oceanic
chemosynthesis-based ecosystems”.
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