Paleocene protobranch bivalves from Urahoro Town in eastern Hokkaido, northern Japan

Abstract

Eleven protobranch species are described from deep-sea deposits of the Paleocene Katsuhira Formation. Among them, one new genus and three new species are included; Meganuculana n. gen., M. alleni n. sp., Neilonella alleni n. sp. and Tindaria paleocenica n. sp. Acila (Truncacila) hokkaidoensis, Pristigloma? sachalinensis, Ezonuculana and Menneroctenia survived the End-Cretaceous Mass Extinction and became extinct by the end of Paleocene. Such a pattern of extinction is similar to that of the deep-sea benthonic foraminifers.

Full text

Journal of ConChology (2017), Vol.42, no.5 283
PALEOCENE PROTOBRANCH BIVALVES FROM
URAHORO TOWN IN EASTERN HOKKAIDO,
NORTHERN JAPAN
KazutaKa amano1 & robert g. JenKins2
1Department of Geoscience, Joetsu University of Education, Joetsu 943–8512, Japan
2College of Science and Engineering, Kanazawa University, Kanazawa 920–1192, Japan
Abstract Eleven protobranch species are described from deep- sea deposits of the Paleocene Katsuhira Formation. Among
them, one new genus and three new species are included; Meganuculana n. gen., M. alleni n. sp., Neilonella alleni n. sp. and
Tindaria paleocenica n. sp. Acila (Truncacila) hokkaidoensis, Pristigloma? sachalinensis, Ezonuculana and Menneroctenia
survived the End- Cretaceous Mass Extinction and became extinct by the end of Paleocene. Such a pattern of extinction is
similar to that of the deep- sea benthonic foraminifers.
Key words Paleocene, protobranch bivalves, deep sea, Hokkaido, Japan
IntroducIon
Protobranch bivalves include many deep- sea
deposit feeders and chemosymbiotic species
(e.g. Dame, 1996). By examining their molecu-
lar phylogeny, it has been elucidated that they
consist of three orders, the chemosymbiotic taxa,
Solemyoida Dall, 1889, and the deposit feeders
Nuculida Dall, 1889 and Nuculanida Carter,
Campbell & Campbell, 2000 (Sharma et al., 2013).
According to Sharma et al. (2013), Nuculida con-
sists of one family, Nuculidae. On the other hand,
Nuculanida consists of two superfamilies and
nine families, including families Malletiidae H.
& A. Adams, 1858, Neilonellidae Schileyko, 1989,
Nuculanidae H. & A. Adams, 1858, Tindariidae
Verrill & Bush, 1897, and Sareptidae Stoliczka,
1870 treated herein. Sato & Sasaki (2015) sup-
ported this classification of higher taxa, based on
shell microstructure.
Paleocene fossils have been recorded in the
Russian Far East from South Sakhalin, western
Kamchatka, the Ilpinskyi Peninsula of eastern
Kamchatka, and the Koryak Upland (Dvjatilova
& Volobueva, 1981; Kalishevich et al., 1981;
Volobueva et al., 1994; Gladenkov et al., 1997).
In contrast, there are few Paleocene molluscan
records from Hokkaido and Shikoku. Ando et
al. (2001) and Ando & Tomosugi (2005) recorded
only Glycymeris sp. from the Paleocene part of
the Hakobuchi Formation in the Nakatonbetsu
area of northern Hokkaido. Kano et al. (2003)
listed some protobranchs identified as Acila sp.,
Nucula sp. and Portlandia sp. from the Paleocene
part of the Domeki Formation in Shikoku.
Recently, three new species have been described
from the Paleocene Katsuhira Formation in
eastern Hokkaido by the authors: the apor-
rhaid gastropod Kangilioptera inouei Amano &
Jenkins, the doubtful buccinid Urahorosphaera
kanekoi Amano & Oleinik and the arcid bivalve
Bentharca steffeni Amano, Jenkins & Nishida
(Amano & Jenkins, 2014; Amano & Oleinik,
2015; Amano et al., 2015). They were collected
mainly from calcareous concretions with many
plant fragments included in mudstone. Many
specimens of protobranch bivalves have also
been recovered from these concretions and host
rock mudstone. In this paper, we describe these
protobranchs and discuss their evolutionary
significance.
MaterIal
Many protobranch specimens were recovered at
29 localities in the upper part of the Katsuhira
Formation (Oda et al., 1959), in eastern Hokkaido,
northern Japan (Fig. 1). Eleven species have been
identified (Table 1; Figs 2–41). The age of the
upper part of the formation was assigned to the
early Selandian, based on planktonic foramini-
fer and calcareous nannofossil biostratigraphy
(Kaiho, 1984).
All specimens are stored at the Joetsu University
of Education (JUE).
Contact author : amano@juen.ac.jp

K amano & rg JenKins
284
SySteMatIcS
Class Bivalvia
Order Nuculida Dall, 1889
Superfamily Nuculoidea Gray, 1824
Family Nuculidae Gray, 1824
Genus Leionucula Quenstedt, 1930
Type species: Nucula albensis d’Orbigny, 1844
Leionucula yotsukurensis (Hirayama, 1958)
Figs 2–4
Nucula ventricosa Hirayama, 1955, p. 77–78, pl. 1,
figs. 12–15 (junior homonym).
Figure 1 Collection localities for protobranch bivalves
(based on the topographical map of “Katsuhira”,
scale 1:25,000, published by the Geographical Survey
Institute of Japan).
Table 1 List of protobranch bivalves from the Katsuhira Formation. Number in the list shows individuals.
Species Loc.1234567891011 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Leionucula yotsukurensis (Hirayama) 12 31 1 71 216111185
Acila (Truncacila) hokkaidoensis (Nagao) 11 2 1 2 12 1 1 3 3 1 12
Ezonuculana aff. obsoleta Tashiro 2 4
Maganuculana alleni sp. nov. 3 1 1 1 2
Malletiaporonaica (Yokoyama) 1 1 2
M. sp. 2 1
Menneroctenia plena Kalishevich 4 6 1 1 2
Neilonella alleni sp. nov. 1 9
Pseudoneilonella? sp. 4
Pristigloma? sachalinensis Salnikova 1 1 1 1 4
Tindaria paleocenica sp. nov. 4 53 1 3 4

PaleoCene ProtobranCh biValVes from hoKKaido 285
Nucula yotsukurensis Hirayama, 1958, p. 96.
Ennucula yotsukurensis (Hirayama). Kamada,
1962, p. 43–44; Nemoto & O’Hara, 1979, pl. 1,
fig. 1.
?Leionucula naibensis Kalishevich, 1969, p. 22–23,
pl. 2, fig. 3; Kalishevich et al., 1981, p. 95–96, pl.
12, fig. 5.
Material In total 51 specimens from the follow-
ing 15 localities: Loc. nos. 1, 3, 4, 7, 10, 11, 13, 14,
15, 16, 17, 18, 19, 20, 21. Of these, 11 specimens
are well preserved and could be measured.
Dimensions See Table 2.
Remarks This species was originally described
as Nucula ventricosa by Hirayama (1955), based
on specimens from the lower Oligocene Asagai
Formation in the Joban coal field. Later, Hirayama
(1958) noticed that this name was preoccupied
by many authors and proposed N. yotsukurensis
as a new name. The present species is character-
ized by its small to medium size (up to 15mm
in length for the type specimens), its trigonally
ovate shape, its umbo near the posterior end and
its escutcheon demarcated by a rounded ridge.
Most specimens obtained from the Katsuhira
Formation are small, less than 15mm long like
the type specimen, but they attain up to 19.6mm.
Leionucula naibensis Kalishevich, 1969 was
proposed from the lower Paleocene Sinegorsk
Formation in the area along the Naiba River
in South Sakhalin. Kalishevich et al. (1981)
described this species by reproducing the origi-
nal picture. This species is probably a synonym
of L. yotsukurensis because it shares a medium-
sized (up to 21mm in length), trigonally ovate
shell with a demarcated escutcheon.
Leionucula formosa was originally described
by Nagao (1930) as a species of Nucula from
the Upper Cretaceous Himenoura Group in
Kyushu. Topotype specimens were re- examined
by Tashiro (1976). Although this species is very
similar to L. yotsukurensis, it can be distinguished
from the latter by having an acute angle at the
posterior end and a narrower escutcheon.
Stratigraphic and geographic range Paleocene
Katsuhira Formation in eastern Hokkaido and
Sinegorsk Formation (?) in South Sakhalin; lower
Oligocene Asagai Formation in the Joban coal
field.
Genus Acila H. Adams & A. Adams, 1858
Subgenus Truncacila Schenck in Grant & Gale,
1931
Type species: Nucula castrensis Hinds, 1843
Acila (Truncacila) hokkaidoensis (Nagao, 1932)
Figs 5–7
Nucula (Acila) hokkaidoensis Nagao, 1932, p. 28–29,
pl. 5, figs. 17, 18; Nagao & Otatume, 1938, p. 37,
pl. 1, fig. 1.
Acila (Truncacila) hokkaidoensis (Nagao). Schenck,
1936, p. 52; Nagao & Huzioka, 1941, p. 118–119;
Tashiro, 1976, p. 35–36, pl. 1, fig. 11 (non figs
12–15); Hayami, 1975, p. 21; Salnikova, 1987, pl.
20, figs. 5, 6; Tashiro, 1992, p. 24–25, pl.1–11; Kiel
et al., 2008, p. 529–530, fig. 4A.
Table 2 Dimensions of Leionucula yotsukurensis (Hirayama) *PL = posterior length.
Specimens Length (mm) Height (mm) PL* Width (mm) Valve Loc.
JUE no. 15946 19.6 13.9 5.7 8.2 Both 1
JUE no. 15947 7.2 5.5 2.1 – Left 3
JUE no. 15948–1 11.5 10.9 2.0 – Both 10
JUE no. 15948–2 10.3 8.4 1.5 5.5 Both 10
JUE no. 15949 11.0 7.7 3.0 – Right 19
JUE no. 15950–1 13.7 11.0 1.9 6.5 Both 20
JUE no. 15950–2 15.3 11.3 2.9 7.8 Both 20
JUE no. 15950–3 14.1 11.5 1.9 8.0 Both 20
JUE no. 15950–4 9.7 + 8.0 – 4.6 Both 20
JUE no. 15950–5 12.6 9.0 2.4 – Left 20
JUE no. 15951 11.7 8.1 2.2 – Right 21

K amano & rg JenKins
286
Acila (Truncacila) longa Kalishevich, 1969, p.
28, 31, pl. 1, figs 10–11; Kalishevich et al., 1981,
p. 102–103, pl. 13, figs. 6, 7.
Acila (Truncacila) pecticostata Kalishevich, 1969,
p. 32–33, pl. 2, figs. 12–14; Kalishevich et al., 1981,
p. 104, pl. 13, figs. 10–12.
Acila (Truncacila) vereshagini Kalishevich, 1969,
p. 33–35, pl. 2, fig. 15; Kalishevich et al., 1981,
p. 104–106, pl. 13, figs. 13–14.
Acila (Truncacila) victoriensis Salnikova, 1980,
p. 22–23, pl. 8, figs. 4–9; Salnikova, 1987, pl. 20,
figs 7–9.
Material In total 49 specimens from Loc. nos.
1, 2, 3, 5, 6, 9, 12, 17, 21, 27, 29. Among these,
four specimens are well preserved and were
measured.
Dimensions JUE no. 15952–1, length, 11.5mm,
height, 8.6mm; JUE no. 15952–2 length, 11.2mm,
height, 7.8mm; Loc. 1. JUE no. 15953, length,
10.9mm, height, 7.8mm; Loc. 9. JUE no. 15954,
length, 22.5mm, height, 18.8mm; Loc. 29.
Remarks The present species has been known
only from the Upper Cretaceous (Coniacian to
Maastrichtian) formations in northern Japan
(Nagao & Otatume, 1938; Tashiro, 1992; Kiel et
al., 2008). As described and discussed in detail by
Kiel et al. (2008), it is characterized by a medium-
sized elongate shell (up to 30mm in length)
and by its ribs diverging once near the anterior
extremity.
In the Naiba area of South Sakhalin, Kalishevich
(1969) proposed Acila (Truncacila) longa and
A. (T.) pecticostata from the lower Paleocene
Sinegorsk Formation and A. (T.) vereshagini from
the Maastrichtian Krasnoyarsk Formation. These
species can be synonymized with the present
species because they have small to medium-
sized shells and ribs diverging once near the
anterior end. Moreover, despite its type local-
ity being located nearby in Hokkaido, he did
not compare his three species with A. (T.) hok-
kaidoensis. Salnikova (1980) proposed A. (T.) vic-
toiensis from South Sakhalin. She distinguished
this species from A. (T.) hokkaidoensis by hav-
ing a more quadrate outline, and a larger and
more inflated shell with the bifurcation of the
ribs located closer to the anterior end. However,
these differences are included in the variation of
A. (T.) hokkaidoensis.
Stratigraphic and geographic range Late
Cretaceous (Coniacian to Maastrichtian),
Krasnoyarsk Formation in South Sakhalin;
Upper Yezo Group and Hakobuchi Formation
in Hokkaido; Kuji Group and Sawameguri
Formation in Iwate Prefecture; Futaba Group
in Fukushima Prefecture. Paleocene Sinegorsk
Formation in South Sakhalin and Katsuhira
Formation in Hokkaido.
Order Nuculanida Carter, Campbell &
Campbell, 2000
Superfamily Nuculanoidea H. & A. Adams,
1858
Family Nuculanidae H. & A. Adams, 1858
Subfamily Nuculaninae H. & A. Adams, 1858
Genus Ezonuculana Nagao, 1938
Type species: Nuculana mactraeformis Nagao,
1932
Ezonuculana aff. obsoleta Tashiro, 1976
Figs 9, 14
Material Six specimens from Loc. nos. 1, 2.
Among them, one specimen is rather well
preserved.
Dimensions JUE no. 15955, length, 29.5mm,
height, 16.8mm; Loc. 1. JUE no. 15956–1, length,
23.0mm, height, 15.3mm, width, 11.8mm (with-
out shell); JUE no.15956–2 length, 14.2mm,
height, 8.9mm; JUE no. 15956–3, length, 5.7mm,
height, 3.9mm; Loc. 2.
Description Shell thick, elongate triangular,
large for genus (internal mould up to 29.5mm
in length), well inflated, equilateral and equiv-
alve. Umbo not very prominent, situated almost
in centre of dorsal margin. Antero- dorsal margin
nearly straight, continuing to rounded anterior
margin; postero- dorsal margin very slightly con-
cave and forming blunt angle with subtruncated
posterior margin; ventral margin nearly straight
to broadly arched. Blunt ridge running from
umbo to posterior corner. Surface smooth except
for fine commarginal riblets near umbo. Posterior
teeth small, more than 12; more than six anterior
teeth, larger than posterior ones; resilium very
small and shallow. Anterior adductor muscle
scar subquadrate; posterior adductor muscle scar
slightly smaller than anterior one, semi- elliptical.
Pallial sinus indistinct but shallow. Inner ventral
margin smooth.

PaleoCene ProtobranCh biValVes from hoKKaido 287
Figures 2–4 Leionucula yotsukurensis (Hirayama); 2. Right valve, JUE no. 15946, Loc. 1; 3. Left valve, JUE no. 15950,
Loc. 10; 4. Two valves spread apart, JUE no. 15950, Loc. 20. Figures 5–7. Acila (Truncacila) hokkaidoensis (Nagao); 5.
Left valve, JUE no. 15952–3, Loc. 1; 6. Right valve, JUE no. 15952–1, Loc. 1; 7. Right valve, JUE no. 15952–2, Loc.
1. Figures 8, 11–13, 16, 17. Meganuculana alleni sp. nov.; 8. Hinge of left valve, paratype, JUE no. 15960, Loc. 24; 11.
Dorsal view of anterior part, paratype, JUE no. 15958–1, Loc. 27; 12. Left valve, paratype, JUE no. 15959, Loc. 20;
13. Left valve, paratype, JUE no. 15960, Loc. 24; 16. Posterior part of left valve, paratype, JUE no. 15958–2, Loc. 27;
17. Right valve, holotype, JUE no. 15957, Loc. 17. Figures 9, 14. Ezonuculana aff. obsoleta Tashiro; 9. Interior surface
of left valve, JUE no. 15956–1, Loc. 2; 14. Left valve, JUE no. 15956–2, Loc. 2. Figure 10. Pseudoneilonella? sp. Right
valve, JUE no. 15974–1, Loc. 29. Figure 15. Malletia poronaica (Yokoyama) Right valve, JUE no. 15962, Loc. 28.

K amano & rg JenKins
288
Remarks The present species is very similar to
Ezonuculana obsoleta Tashiro, 1976 from the upper
Campanian Member U- IIb of the Himenoura
Group in Kyushu. However, the present mate-
rial is slightly different in having a larger (up to
17.9mm in E. obsoleta) and lower shell (H/L = 0.57–
0.67; 0.66–0.75 for E. obsoleta). Although Tashiro
(1976) described the pallial sinus of E. obsoleta as
“pallial line simple” (implying that it probably
had no pallial sinus), the paratype specimen
KE 2055 has a small pallial sinus in front of the
posterior adductor muscle scar. In this character,
there is no difference between the Paleocene spe-
cies and E. obsoleta.
Stratigraphic and geographic range Only from
Katsuhira Formation in Hokkaido.
Genus Meganuculana n. gen.
Type species: Meganuculana alleni n. sp. (this
paper)
Diagnosis Large, moderately inflated, elongate
shell sculptured with many fine commarginal
grooves on upper half of disc. Two ridges run-
ning from umbo to posterior end and to postero-
ventral corner. Posterior outline concave. Hinge
consisting of two series of taxodont teeth and
small resilium. Escutcheon flat, bounded by
sharp ridge; lunule slightly depressed, narrow,
lanceolate, bounded by blunt ridge. Pallial sinus
obscure but seems to be shallow.
Description Shell large, to 32.0mm long, rather
thick, elongate, rostrate, with porcellanous inner
layer, moderately inflated, equivalve, inequilat-
eral. Antero- dorsal margin broadly arched, con-
tinuing to semicircular anterior margin; ventral
margin broadly rounded; posterior margin con-
cave, posterior end pointed, continuing to rather
straight postero- dorsal margin. Two ridges run-
ning from umbo to posterior end and postero-
ventral corner; both blunt in holotype but sharp
in smaller paratype specimens. Umbo prominent,
rather prosogyrate, located at anterior one-third
of shell length. Surface sculptured with fine, dis-
tinct commarginal grooves; grooves distinct on
only upper half of holotype but over whole sur-
face of smaller paratype specimens. Escutcheon
flat, bounded by sharp ridge; lunule slightly
depressed, narrow, lanceolate, bounded by blunt
ridge. Hinge plate narrow with two series of
small teeth; 11 anterior teeth; 16 posterior teeth.
Posterior adductor muscle scar small, ovate.
Pallial sinus obscure but seems to be shallow.
Comparison The present new genus differs from
Nuculana Link, 1807 in having a large shell with a
straight postero- dorsal margin, a broadly arched
ventral margin and a concave posterior margin.
Some species of the genus Ledella Verrill & Bush,
1897 resemble this new genus. However, Ledella
easily can be distinguished from the present
new genus by having a minute shell (a few mm
in length), a wider resilium, a pointed posterior
end, an opisthogyrate umbo and fewer teeth.
Meganuculana alleni n. sp.
Figs 8, 11–13, 16, 17
Type material Holotype, JUE no. 15957, Loc. 27;
Paratype, JUE no. 15958, Loc. 27; Paratype, JUE
no. 15959, Loc. 20; Paratype, JUE no. 15960, Loc.
24.
Type locality 0.9km upstream from Ponkatsuhira-
minami- zawa (Loc. 27); Katsuhira Formation.
Dimensions Holotype, JUE no. 15957, length,
32.0mm, height, 19.6mm, Loc. 27; Paratype, JUE
no. 15959, length, 5.4 mm + , height, 4.9mm, width
3.0mm, Loc.20; Paratype, JUE no. 15960, length,
6.8 mm + , height, 4.7mm, Loc. 24.
Diagnosis & Description Same as for the genus.
Etymology Named for John A. Allen (Emeritus
Professor of Marine Biology, University of
London).
Comparison Meganuculana alleni n. sp. can be
separated from Nuculana pernula (Müller, 1779),
the type species of the genus Nuculana Link, 1807
by having a larger but less elongate shell with a
straight postero- dorsal margin, a weakly pointed
posterior end and a shallowly concave postero-
ventral margin. The shell outline of Ledella bernardi
(Dautzenberg & Fischer, 1897) is somewhat simi-
lar to that of the present new species. However,
L. bernardi differs from the present new species
in being much smaller (up to 9.4mm; La Perna,
2008), with a less inflated shell, a more produced
posterior end, a centrally located opisthogyrate
umbo and a wider resilium.
Stratigraphic and geographic range Paleocene
Katsuhira Formation in eastern Hokkaido.

PaleoCene ProtobranCh biValVes from hoKKaido 289
Family Malletiidae H. Adams & A. Adams, 1858
Genus Malletia des Moulins, 1832
Type species: Malletia chilensis des Moulins, 1832
Malletia poronaica (Yokoyama, 1890)
Figs 15, 18
Nucula poronaica Yokoyama, 1890, p. 195–196, pl.
25, figs. 3a–c.
Malletia poronaica (Yokoyama). Kanehara, 1937, p.
159, pl. 15, figs.1–4; Uozumi, 1952, p. 20, pl. 16,
figs. 122a,b, 123, 124; Takeda, 1953, p. 72, pl. 8,
figs. 2–10; Honda, 1989, p. 49, pl. 1, figs. 1, 2, 3,
6; Gladenkov et al., 1991, p.150, pl. 28, fig. 19a, b;
Amano & Jenkins, 2007, fig. 4C.
Neionella poronaica (Yokoyama). Oyama, 1951, pl.
6, fig. 1; Oyama et al., 1960, p. 87–88, pl. 19, figs.
2a–d.
? Malletia (Minormalletia) poronaica (Yokoyama).
Shikama & Kase, 1976, p. 15–16, pl. 1, fig. 3.
Malletia (Malletia) poronaica (Yokoyama). Devy-
atilova & Volobueva, 1981, p. 20, pl. 18, fig. 5.
Malletia convexa Kalishevich in Kalishevich et al.,
1981, p. 91–92, pl. 11, figs. 8–12.
Material Four specimens from Loc. nos. 8, 10,
28.
Dimensions JUE no. 15961, length, 6.8 mm + ;
height, 4.8mm; Loc. 8. JUE no. 15962, length,
6.1mm; height, 4.0mm; Loc. 28.
Remarks Our specimens are characterized by
their small, elliptical, moderately inflated shell,
the anteriorly situated beak, the subtruncated
posterior margin and fine growth lines. They
are probably young shells of Malletia poronaica
(Yokoyama) whose length attains to 20mm in
adult. Kalishevich in Kalishevich et al. (1981)
described and illustrated Malletia convexa as
a new species from the lower Paleocene mid-
dle part of Sinegorsk Horizon in the Naiba and
Krasnoyarsk River area, South Sakhalin. Despite
the characteristics of his species coinciding
exactly with those of the present species, he did
not compare M. convexa with M. poronaica, but
only with M. inermis (Yokoyama, 1925). Thus, it
is possible that Malletia convexa is a junior syno-
nym of M. poronaica.
Stratigraphic and geographic range Paleocene
Sinegorsk Horizon (middle part) in South Sakhalin
and Katsuhira Formation in eastern Hokkaido.
Upper Eocene Olkhovskya Bed in the Koryak
Upland, Poronai and Tappu Formations in cen-
tral to northwestern Hokkaido. Lower Oligocene
Nuibetsu Formation in eastern Hokkaido. ? Early
Miocene Morozaki Group (Yamami Formation)
in Aichi Prefecture, central Honshu.
Malletia sp.
Fig. 19
Material Three specimens from Loc. nos. 20, 25.
Dimensions JUE no. 15963, length, 10.9mm;
height, 6.4mm; Loc. 20. JUE no. 15964, length,
10.4mm; height, 5.5mm; Loc. 25.
Remarks The available specimens differ from
Malletia poronaica in having a less inflated and
lower shell. Although they resemble Portlandia
in shape, the hinge seems to lack a resilium.
Genus Menneroctenia Kalishevich, 1973
Type species: Menneroctenia ornata Kalishevich,
1973
Cnestriella Tashiro & Otsuka, 1980, p. 47–48.
Remarks This genus was proposed by
Kalishevich (1973) based on specimens from
the Mastrichtian and Paleocene formations in
South Sakhalin. It resembles Portlandia Mörch,
1857 in outline but has oblique ridges crossing
the growth lines like those of Cnesterium Dall,
1898. However, it differs from yoldiids, includ-
ing Cnesterium, by having no resilium under
the umbo. Tashiro & Otsuka (1980) proposed
Cnestriella as a subgenus of Portlandia, based on
Portlandia cunesteriata Ichikawa & Maeda, 1958
from the Maastrichtian part of the Izumi Group.
However, Ichikawa & Maeda (1958) did not
describe the interior structure. When Tashiro &
Otsuka (1980) proposed the new subgenus, they
described P. cunesteriata from the Maastichtian
Himenoura Group in Kyushu. During our exam-
ination of the hinge of their specimens, we found
two specimens with continuous anterior and
posterior teeth series. Judging from this hinge,
Cnestriella belongs in Malletiidae and is a jun-
ior synonym of Menneroctenia Kalikevich, 1973.
Although Kalishevich et al. (1981) considered
this genus to be a junior synonym of Multidentata
Krishtofovich, 1964, the latter “genus” can be sep-
arated from the former by having a prominent,

K amano & rg JenKins
290
blunt ridge extending from the umbo to the
postero- ventral corner and many lamellate com-
marginal ribs. As shown by Amano et al. (2000),
Multidentata must be treated as a subgenus of
Neilo A. Adams, 1854.
Menneroctenia plena Kalishevich, 1973
Figs 21, 22, 25, 28
Menneroctenia plena Kalishevich, 1973, p. 25,
pl. 1, fig. 4.
Multidentata plena (Kalishevich, 1973). Kalishevich
et al., 1981, p. 87–88, pl. 10, figs. 8–11.
Material In total thirteen specimens from
localities nos. 1, 2, 13, 29. Among these, eight
specimens are well preserved and were
measured.
Dimensions JUE no. 15965–1, length, 13.9mm,
height, 8.5mm; JUE no. 15965–2, length, 20.7mm,
height, 12.5mm; JUE no. 15965–3, length,
16.2mm, height, 11.1mm; JUE no. 15965–4, length,
12.9mm, height 7.9mm; Loc. 1. JUE no. 15966–1,
length, 15.8mm, height, 9.8mm; JUE no. 15966–
2, length, 14.9mm, height, 8.4mm; Loc. 2. JUE
no. 15967–1, length, 12.6 mm + , height, 7.9mm;
JUE no.15967–2, length, 5.9mm, height, 3.6mm;
Loc. 29.
Remarks The species from Katsuhira has a
Portlandia- like shell with oblique lines crossing
the growth lines over the posterior part. The
ratio of shell height to length is about 0.61–0.62.
The umbo is located at the anterior third of the
shell length. The pallial sinus is small but deep.
The hinge consists of anterior and posterior taxo-
dont teeth. From these characters, the Katsuhira
species can be safely identified as Menneroctenia
plena Kalishevich, 1973.
Stratigraphic and geographic range Paleocene
Sinegorsk Horizon (middle part) in South
Sakhalin and Katsuhira Formation in eastern
Hokkaido.
Family Neilonellidae Schileyko, 1989
Genus Neilonella Dall, 1881
Neilonella alleni n. sp.
Figs 20, 23, 24
Type material Holotype, JUE no. 15969; Paratype,
JUE nos. 15970–1, 2, 3, 4, Loc. 21.
Type locality 550m upstream of Katsuhira- zawa
(Loc. 21 = Loc. 6 of Amano and Jenkins, 2014);
Katsuhira Formation.
Material examined In total eleven specimens
were collected from Loc. 3, 20, 21. Among these,
seven specimens are well- preserved and were
measured.
Dimensions Holotype, JUE no. 15969, length,
5.4mm, height, 4.0mm; Loc. 21. Paratype, JUE
no. 15970–1, length, 6.0mm, height, 4.3mm; JUE
no. 15970–2, length, 5.9mm, height, 4.8mm; JUE
no.15970–3, length, 5.8mm, height, 4.2mm; JUE
no. 15970–4, length, 7.9mm, height, 5.0mm; Loc.
21. JUE no. 15972, length, 5.2mm, height, 4.0mm;
Loc. 3. JUE no. 15973, length, 5.5mm, height,
3.9mm; Loc. 20.
Diagnosis Small, moderately inflated, ovate
shell sculptured with many fine, flat commarginal
riblets. Posterior margin narrowly rounded, not
pointed. Hinge consisting of two series of taxo-
dont teeth and no resilium. Pallial sinus shallow,
continuing to quadrate anterior adductor scar.
Description Shell small (up to 7.9mm long),
ovate, moderately inflated, equivalve. Antero-
dorsal margin rather straight, continuing to well-
rounded anterior end; postero- dorsal margin
straight, gently sloping, making right angle with
subtruncated posterior margin; ventral margin
broadly arcuate. Umbo located at anterior two-
fifth to half of shell length, slightly protruding.
Surface of shell sculptured with fine, flat com-
marginal riblets; riblets distinct on central part,
becoming obscure near dorsal margin. Hinge
consisting of two taxodont series without resil-
ium; eight anterior teeth, 12 smaller posterior
teeth. Pallial sinus shallow, continuing to quad-
rate posterior adductor scar.
Etymology Named for John A. Allen (Emeritus
Professor of Marine Biology, University of
London).
Comparison This new species is similar to
so- called Nuculana (Borissia) species from the
Russian Far East in having a shorter posterior
part than other Nuculana species. However, as
pointed by Kafanov et al. (2000, p. 59), this spe-
cies group belongs in Neilonella. Among them,
Neilonella alferovi (Slodkewitsch, 1938) most

PaleoCene ProtobranCh biValVes from hoKKaido 291
Figure 18 Malletia poronaica (Yokoyama) Hinge of left valve, JUE no. 15968, Loc. 10. Figure 19. Malletia sp. Left
valve, JUE no. 15963, Loc. 20. Figures 20, 23, 24. Neilonella alleni sp. nov.; 20. Right valve, JUE no. 15972, Loc. 3; 23.
Right valve, holotype, JUE no. 15969, Loc. 21; 24. Left valve, paratype, JUE no. 15970–2, Loc. 21. Figures 21, 22,
25, 28. Menneroctenia plena Kalishevich; 21. Left valve, JUE no. 15965–1, Loc. 1; 22. Right valve, JUE no. 15965–3,
Loc. 1; 25. Left valve, holotype, JUE no. 15965–2, loc. 1; 28. Right valve, JUE no. 15967–1, Loc. 29. Figure 26, 27,
29–32. Pristigloma? sachalinensis Salnikova; 26. Hinge of left valve, 27. Right valve, 30. Left valve, JUE no. 15976,
Loc. 29; 29. Left valve, JUE no. 16975, Loc. 24; 31. Right valve, JUE no. 15977–2, Loc. 29; 32. Right valve, JUE no.
15977–1, Loc. 29.

K amano & rg JenKins
292
closely resembles the new species. N. alferovi was
described from the uppermost lower to lowest
middle Miocene Tjushevskaya Formation in east-
ern Kamchatka and the lower Miocene Kholmsk
and Nevelsk Formations in South Sakhalin.
However, N. alferovi is larger (more than 10mm
in length), with a more protruding umbo and a
pointed anterior end.
Neilonella polunini (Devjatilova in Devjatilova
& Volobueva, 1981), described from the upper
Eocene Unelskaya Formation in Penjinsk Inlet,
has a similar small shell and outline to the pre-
sent new species. Its pointed posterior end,
slightly concave posterior margin and produced
umbo enable us to separate two species.
Stratigraphic and geographic range Paleocene
Katsuhira Formation in eastern Hokkaido.
? Genus Pseudoneilonella Laghi, 1986
Type species: Nucula pusio Philippi, 1844
Remarks According to La Perna (2007), the old-
est record of this genus dates back to the middle
Miocene. This genus is characterized by a small
ovate- trigonal shell with opisthogyrate umbo, a
single series of hinge teeth, an opisthodetic liga-
ment and a wide pallial sinus.
Pseudoneilonella? sp.
Fig. 10
Material Four specimens from Loc. no. 29.
Dimensions JUE no. 15974–1, length, 6.1mm,
height, 4.2mm; JUE no. 15974–2, length, 7.3mm,
height, 5.0mm; JUE no. 15974–3, length, 5.5mm,
height, 3.6mm; JUE no. 15974–4, length, 7.5 mm + ,
height 5.5mm; Loc. 29.
Description Shell large for genus, to more than
7.5mm long, rather thick, elongate triangular,
weakly inflated. Antero- dorsal margin broadly
arched, with rounded anterior margin; postero-
dorsal margin nearly straight; ventral margin
broadly arched; posterior margin subtruncated,
making obtuse angle with postero- dorsal mar-
gin. Blunt ridge running from umbo to postero-
ventral corner. Umbo opisthogyrate, located at
anterior two- fifth of shell length. Lunule very
small, shallow. Surface sculptured with coarse,
prominent commarginal ribs, obscure near ante-
rior and posterior margins. Hinge plate narrow,
bearing small chevron- shaped teeth without
resilium; 15 anterior teeth, more than 15 poste-
rior teeth. Inner ventral margin smooth. Anterior
adductor muscle scar triangular, crenulated by
fine threads; posterior adductor scar elliptical.
Pallial sinus shallow and wide.
Remarks This species has a similar shell out-
line to Pseudoneilonella species. Among these,
P. taurinensis La Perna, 2007 from the middle
Miocene deposits of the Turin Hills, northwest-
ern Italy, is the most closely similar species.
However, the present species differs from spe-
cies of Pseudoneilonella by its slightly larger (up
to 6.4mm in length for Pseudoneilonella) and less
inflated shell with coarser commarginal ribs.
Added to these differences, the fossil records
of Pseudoneilonella date back to only the middle
Miocene in the Mediterranean area. For these
reasons, the present species cannot be definitely
classified into Pseudoneilonella. Further research
on Neilonellidae is required to define the range
of Pseudoneilonella.
Family Pristiglomidae Sanders & Allen, 1973
?Genus Pristigloma Dall, 1900
Type species: Glomus nitens Jeffreys, 1876
Pristigloma? sachalinensis (Salnikova, 1987)
Figs 26, 27, 29–32
“Sarepta” sachalinensis Salnikova, 1987, p. 134, pl.
21, figs. 10–13.
“Sarepta” sachalinensis glomusiformis Salnikova,
1987, p. 134, pl. 21, fig. 11.
“Sarepta” sachalinensis Salnikova, 1987, p. 134–
135, pl. 21, figs. 12.
Material In total eight specimens from localities
22, 24, 26, 27, 29. Among them, three specimens
are well preserved and were measured.
Dimensions JUE no. 15975, length, 6.4mm,
height, 5.0mm, Loc. 24. JUE no. 15976, length,
7.5mm, height 6.1mm, Loc. 27. JUE no. 15977,
length, 8.4 mm + , height, 8.3mm, Loc. 29.
Remarks This species is characterized by its
weakly inflated, ovate shell, resembling tellinids.
The shell surface is smooth except for fine growth
lines. The umbo is located at about the centre of
the length or is slightly posteriorly located. The
hinge consists of anterior and posterior taxodont
teeth series with a small resilium. Both adductor
scars are ovate. The pallial line is entire. From

PaleoCene ProtobranCh biValVes from hoKKaido 293
these characters, the Katsuhira species can be
identified as Pristigloma? sachalinensis (Salnikova,
1987), described from Maastrichtian deposits in
the Naibuchi area of South Sakhalin. Salnikova
(1987) separated the species into two species and
one subspecies by their shell outline. However,
these differences can safely be included in the
variation of one species. Moreover, she described
this species doubtfully under the genus “Sarepta”.
Sarepta speciosa A. Adams, 1860, the type species
of the genus, has a larger shell (maximum shell
length 12.4mm), a narrow anterior part, and a
very shallow pallial sinus. Both Sakhalin and
Hokkaido specimens have a smaller and more
rounded shell more nearly resembling those of
species of Pristigloma Dall, 1900. Although we ten-
tatively include this species in Pristigloma rather
than in Sarepta, some characters of Pristigloma,
such as its elongate resilium and shallow pallial
sinus (see Okutani & Kawamura, 2002), cannot
be observed in the present material.
Stratigraphic and geographic range Maastrichtian
in South Sakhalin and Paleocene Katsuhira
Formation in eastern Hokkaido.
Family Tindariidae Verrill & Bush, 1897
Genus Tindaria Bellardi, 1875
Type species: Tindaria arata Bellardi, 1875
Tindaria paleocenica n. sp.
Figs 33–41
Type material Holotype, JUE no. 15978, Loc. 10.
Paratype, JUE no. 15979, Loc. 10; Paratype, JUE
nos. 15980–1, 2, 3, Loc. 29.
Type locality 5.5km upstream from Katsuhira-
kita- zawa (Loc. 10); Katsuhira Formation.
Dimensions Holotype, JUE no. 15978, length,
6.2mm, height, 5.4mm, Loc. 10. Paratype, JUE
no. 15979, length, 6.2mm, height, 5.4mm, Loc.
10. Paratypes, JUE no. 15980–1, length, 5.2mm,
height, 3.9mm, width, 2.9mm; JUE no. 15980–2,
length, 3.6mm, height, 3.2mm, width, 2.3mm;
JUE no. 15980–3, length, 3.2mm, height, 2.9mm,
width, 1.9mm; Loc. 29. JUE no. 15981–1, length,
4.6mm, height, 3.4mm; JUE no. 15981–2, length,
4.1mm, height, 3.2mm; Loc. 28.
Diagnosis Strongly inflated species of Tindaria
sculptured with many fine commarginal ribs.
Hinge taxodont with no resilium. Pallial line
entire.
Description Shell moderate- size for genus, to
6.2mm long, rather thick, nuculid- like, with por-
cellanous inner layer, well inflated (W/L = 0.56–
0.64 for paratype specimens), equivalve, inequi-
lateral. Prodissoconch rather large, attaining
about 580µm in length, smooth. Antero- dorsal
margin broadly arched, continuing to semicir-
cular anterior margin; ventral margin broadly
rounded; posterior end short, subtruncated, con-
tinuing to straight postero- dorsal margin. Blunt
ridge extending from umbo to postero- ventral
corner. Umbo prominent, rather prosogyrate,
located at anterior third of shell length to centre
of shell (AL/L = 0.35–0.47). Surface sculptured
with fine, distinct commarginal ribs; interspaces
between commarginal ribs each narrower than
one rib. Hinge plate narrow, with two series of
small teeth; nine anterior teeth; 15 posterior teeth.
Anterior adductor muscle scar small, ovate; pos-
terior adductor scar subquadrate. Pallial line
entire.
Etymology From Paleocene, the stratigraphic
occurrence of the species.
Comparison The present new species closely
resembles the Recent Japanese species Tindaria
soyoae Habe, 1953 in having a nuculid- like shell
and fine commarginal ribs. However, T. paleoce-
nica n. sp. differs from the Recent species by its
more acute posterior end, fewer teeth (eight to
13 anterior and 14 to 22 posterior teeth of T. soy-
oae) and no pallial sinus (a very shallow sinus
has been observed in T. soyoae). The outline of
Tindaria? sp. from the seep site of the upper
Oligocene Lincoln Creek Formation described by
Kiel (2006) is similar to the present new species.
However, the lack of information on its interior
structure prevents us from comparing them with
each other. Another Recent species, T. kennerlyi
(Dall, 1897), can be separated from the present
species by having a more elongate and less
inflated shell.
Remarks According to Moore (1969), the geo-
logical range of this genus is Tertiary to Recent.
Amano et al. (2004) listed Tindaria sp. resem-
bling T. kennerlyi from the mudstone of the
uppermost lower to lowermost middle Miocene
Higashibessho Formation in Toyama Prefecture,

K amano & rg JenKins
294
central Honshu. The geological range of the
genus Tindaria has been extended back to the late
Oligocene by Kiel (2006), as noted above. From
the finding of the present new species, it is clear
that the genus appeared in the Paleocene.
Stratigraphic and geographic range Paleocene
Katsuhira Formation in eastern Hokkaido.
dIScuSSIon
Among the 11 species identified here, excluding
Malletia sp. and Pseudoneilonella? sp., the geo-
logical range has been examined. As a result, it
has become clear that five taxa have survived
the mass extinction at the end of Cretaceous:
the two species Acila (Truncacila) hokkaido-
ensis, Pristigloma? sachalinensis, and the gen-
era Ezonuculana, Menneroctenia and Neilonella.
Among these, Pristigloma? sachalinensis and
Ezonuculana are recorded here for the first time
from Paleocene rocks. In contrast, Malletia poro-
naica and Leionucula yotsukurensis appeared in
the Paleocene and extended to the Oligocene.
The oldest species of Tindaria is recorded here,
extending the genus back to the Paleocene.
At the K/Pg boundary, mainly shallow-
water foraminifers became extinct while deep-
water species survived (Kaiho, 1994; Thomas,
2007; Alegret et al., 2009). Following this, many
deep- water benthonic foraminifers suffered
Figures 33–41 Tindaria paleocenica sp. nov.; 33. Hinge of left valve, 36. Left valve, holotype, JUE no. 15978, Loc. 10;
34. Inner surface of left valve, paratype, JUE no. 15980–2, Loc. 29; 35. Dorsal view of both valves, 38. Enlargement
of beak, 41. Right valve, paratype, JUE no. 15980–3, Loc. 29; 37. Right valve, paratype, JUE no. 15980–4, Loc. 29;
39. Dorsal view of both valves, 40. Left valve, paratype, JUE no. 1980–1, Loc. 29.

PaleoCene ProtobranCh biValVes from hoKKaido 295
from extinction at the PETM (Paleocene- Eocene
Thermal Maximum). In contrast, few data exist
on the extinction of deep- sea molluscan fossils.
The survival of Acila (Truncacila) hokkaidoensis
and Menneroctenia is also recognized in South
Sakhalin (Kalishevich et al., 1981). The aporrhaid
gastropod Kangilioptera inouei demonstrates that
the aporrhaid also survived the End- Cretaceous
extinction (Amano & Jenkins, 2014). However,
Acila (Truncacila) hokkaidoensis, Pristigloma? sacha-
linensis, Ezonuculana and Menneroctenia became
extinct by the end of the Paleocene. These trends
are concordant with the extinction pattern of the
benthonic foraminifers. Thus, the environmen-
tal change at the PETM seems to have severely
affected deep- sea benthic animals.
acknowledgeMentS
We thank Alan G. Beu (GNS Science) for his criti-
cal reading of the manuscript and useful sugges-
tions. We also thank Graham Oliver (National
Museum of Wales) for giving us an opportunity
to write this paper to celebrate 90’s birthday
of Emeritus Professor John Allen (University
of London). We thank Kui Sato (Kanazawa
University) for his technical assistance with the
SEM observations. This study was partly sup-
ported by a Grant- in- aid for Scientific Research
from the Japan Society for Promotion of Science
(C, 26400500, 2014−2016) to KA and RGJ, and
Grant for Program to Disseminate Tenure
Tracking System (JST) to RGJ.
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