The Cretaceous System in the Makarov Area, Southern Sakhalin, Russian Far East, edited by Y. Shigeta and H. Maeda,
National Science Museum Monographs, 31: 121–136, 2005

Maastrichtian Ammonoid Fauna from the Pugachevo Area,
Southern Sakhalin, Russian Far East
Haruyoshi Maeda1 and Yasunari Shigeta2
1

Department of Geology and Mineralogy, Graduate School of Science, Kyoto
University, Kitashirakawa-Oiwake-cho, Sakyou-ku, Kyoto 606–8502, Japan
E-mail: maeda@kueps.kyoto-u.ac.jp
2
Department of Geology and Paleontology, National Science Museum,
3–23–1Hyakunin-cho, Shinjuku-ku, Tokyo 169–0073, Japan
E-mail: shigeta@kahaku.go.jp

Abstract The stratigraphy and paleontology of the Cretaceous Yezo Group in the Pugachevo area
has been investigated. The group is divided into the Bykov and Krasnoyarka formations in ascendingorder, and its exposures in the area range in age from Middle Campanian to Maastrichtian. Canadoceras kossmati and Sphenoceramus schmidti, of Middle Campanian age, characterize the Bykov Formation, while the middle and upper parts of the Krasnoyarka Formation are characterized by the presence of Pachydiscus flexuosus and Gaudryceras makarovense, which are Late Maastrichtian in age. A
typical occurrence of this widespread Upper Maastrichtian ammonoid assemblage is confirmed in a
new section located between the Naiba and Makarov areas.
Preservation of the Upper Maastrichtian ammonoids ranks among the best in the world and isprobably unmatched in any other locality in southern Sakhalin. Specimens of P. flexuosus and G.
makarovense preserved in the large calcareous nodules mainly exhibit aragonite preservation. Theirphragmocones are usually free from compactional damage although the last two or three camerae aresometimes slightly crushed similar to the body chambers. As a matter of interest, it seems peculiarthat juvenile and immature specimens of P. flexuosus and G. makarovense less than 5 cm in diameterare absent in both the calcareous nodules and the surrounding muddy sandstone. The scarcity of immature specimens can not be explained simply by either hydrodynamic sorting by waves or currents,
or by selective dissolution of shells during diagenesis. This peculiarity may be a key to reconstructing
the depositional environment and paleoecology as well as the taphonomic processes of ammonoid
preservation.
Key words:

ammonoids, aragonite preservation, Pugachevo, Sakhalin, Upper Cretaceous

Introduction
Since the discovery of its remarkably wellpreserved and abundant Maastrichtian ammonoid fauna by previous authors, the Pugachevo area has been recognized as one of
the best reference sections for the Upper Cretaceous in southern Sakhalin (Yazikova, 1994).
Despite the taxonomic work of Zonova et al.
(1993) and Yazikova (1994), the exact locali-ties, stratigraphic positions, and modes of fossil occurrence remained very poorly known.
Japanese paleontologists were unaware of the
Pugachevo area until Masahiro Sato, an amateur paleontologist in Osaka, informed the authors of the excellent preservation of fossils in
the area.
The Pugachevo area is located in southern
Sakhalin about 150 km north of YuzhnoSakhalinsk, along the eastern side of the West

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Haruyoshi Maeda and Yasunari Shigeta

Fig. 1.

Map showing the study area in the Pugachevo area, southern Sakhalin.

Sakhalin Mountains. Exposures of the Upper
Cretaceous System occur at several localities
along the Pugachevka River and its tributaries.
Ammonoid preservation in these sediments
ranks among the best in the world and is probably unmatched in any other locality in southern Sakhalin.
To ensure precise stratigraphic distributionof the Maastrichtian ammonoid fauna, topographic and geologic assessment by satellite
photography was introduced. After reviewing
the topographic features of the region and pinpointing promising areas for fossil exploration, field expeditions were carried out by
the authors in 1993, 1994 and 1997 along the
Pugachevka River and a tributary, the Sen’ka
River (Figs. 1–3).
The purpose of this paper is to describe theCretaceous stratigraphy of the Pugachevo area,and discuss from a taphonomic perspective,
the manner of occurrence and preservation of

the Maastrichtian ammonoids, which is somewhat different from those in the Bykov Formation.
Repository of specimens: The following abbreviations are used to indicate fossil repositories: NSM, National Science Museum, Tokyo;
DGMKU, Department of Geology and Mineralogy, Kyoto University, Kyoto.
All fossils and samples utilized herein werecollected during the field expeditions and weretransported from Russia to Japan with permission from the Russian Government, the State
Government of Sakhalin, and other concerned
authorities. Locs. PC1001 and 1002 are now
protected by the Russian Government as a Paleontological Monument of Nature.
Stratigraphy
The Cretaceous Yezo Group consists of marine clastics that were deposited in a forearc

Maastrichtian ammonoid fauna from the Pugachevo area

Fig. 2.

Satellite photograph of the Pugachevo area, southern Sakhalin.

123

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Haruyoshi Maeda and Yasunari Shigeta

Fig. 3.

Locality map along the Sen’ka River.

Maastrichtian ammonoid fauna from the Pugachevo area

Fig. 4.

125

Lithology and faunal succession of the Cretaceous deposits along the Sen’ka River.

basin on the northwestern Pacific margin(Okada, 1979, 1983). These deposits are widely distributed in central Hokkaido and western
Sakhalin, and are composed of a thick, continuous sequence of Aptian–Maastrichtian marine sediments with abundant mega- and microfossils (Matsumoto, 1954; Vereshchagin,1977).
The Yezo Group is widely exposed along
the middle course of the Pugachevka Riverand the Sen’ka River, one of its tributaries. In
general, the strata strike in a N–S direction,and the dip varies from 5 to 30° westward. In
this particular area, the Yezo Group is divided
into two units, the Bykov and Krasnoyarka
formations in ascending order (Fig. 4), as defined by Vereshchagin (1961) along the Naiba
River.
Due to gentle dip of the strata in the Pugachevo area, outcrops representing the entire
stratigraphic range of the Yezo Group are restricted, and exposures are not as continuous.
However, the Maastrichtian outcrops are fresh,and most ammonoid fossils exhibit “aragonite

preservation” (Figs. 5–9).
Bykov Formation (Vereshchagin, 1961)
Stratotype: Naiba River, southern Sakhalin.
Locality: Lower course of the Sen’ka River(Loc. PC5027).
Thickness: Greater than 40 m.
Lithology: Only the uppermost part is exposed. It mainly consists of dark gray, intensely bioturbated mudstone interbedded withwhite vitric tuff layers ranging from 5 to 10 cm
in thickness. Lenticular or spherical calcareous
nodules, 30–60 cm in diameter, are commonly
embedded in the mudstone.
Fossils: Sphenoceramus schmidti (Michael)
is abundant and occurs in both the calcareous
nodules and the lower and middle parts of thehost rock. Canadoceras kossmati Matsumoto,Tetragonites popetensis Yabe, and Inoceramus
balticus Böhm occur in calcareous nodules in
the upper part of the formation.

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Haruyoshi Maeda and Yasunari Shigeta

Krasnoyarka Formation (Vereshchagin,
1961)
The Krasnoyarka Formation consists mainlyof sandstone and sandy mudstone, and it conformably overlies the Bykov Formation. The
top part is not exposed in the study area.
Stratotype: Krasnoyarka River, a tributaryof the Naiba River, southern Sakhalin.
Localities: Middle course of the Pugachevka River (Locs. PC1001, 1002); theSen’ka River (Locs. PC5001–5026).
Thickness: 770 m.
Lithology: The basal part of the formationconsists of greenish gray, poorly-sorted muddy
sandstone. Then it changes to bedded, fine tocourse grained sandstone and muddy sandstone in the middle part and to dark, greenish
gray, intensely bioturbated sandy mudstone
and muddy sandstone in the upper part. The
sandy mudstone in the upper part often contains spherical calcareous nodules, about
30–100 cm in diameter, some of which are fossiliferous.
Fossils: Pachydiscus flexuosus Matsumoto
and Nanonavis sp. were found in the finegrained sandstone of the middle part of theformation at Loc. PC5009. The sandy mudstone in the upper part is fossiliferous, and thefollowing very well preserved ammonoids
were found in calcareous nodules: Pachydiscus
flexuosus, Pseudophyllites indra (Forbes), andGaudryceras makarovense Shigeta and Maeda(Figs. 5–8).
Correlation
Ammonoids in the Pugachevo area are restricted to the Bykov Formation and the middle and upper parts of the Krasnoyarka Formation. The ammonoid and inoceramid zonation
applied in this area is based on the studies of
Matsumoto (1942, 1954, 1959, 1984), Zonova
et al. (1993), Zonova and Yazykova (1994),
Yazikova (1994), and Toshimitsu et al. (1995).
The presence of Sphenoceramus schmidti
and Canadoceras kossmati in the Bykov For-mation suggests the upper Lower Campanianstage. Similar faunas have been recognized in
the upper part of the Bykov Formation in the
Makarov area and the lower part of the Krasnoyarka Formation in the Naiba, Sinegorsk,
Vladimirovka, Gorbusha, and Kura areas insouthern Sakhalin (Matsumoto, 1942; Vershchagin, 1970, 1977; Poyarkova, 1987;
Shigeta et al., 1999; Kodama et al., 2000,
2002). This fauna is also similar to that of the
Yezo Group in Hokkaido (Matsumoto, 1954).
Pachydiscus flexuosus, of the Upper Maastrichtian stage, characterizes the middle andupper parts of the Krasnoyarka Formation.
The Upper Maastrichtian fauna in the Pugachevo area, which consists of Pachydiscus
flexuosus, Pseudophyllites indra, and Gaudryceras makarovense, is similar to that of theMakarov and Manui areas in southern
Sakhalin (Yazikova, 1994).
Mode of Maastrichtian ammonoid occurrence and preservation
The typical mode of Maastrichtian ammonoid occurrence can be observed at Locs.
PC1001 and 1002 (Figs. 2, 5), where darkgray, moderately bioturbated, bedded muddy
sandstone is exposed. These beds dip westward very gently and yield numerous large
spherical or ellipsoidal calcareous nodules,
30–100 cm in diameter (Fig. 5).
Features of nodules: The nodules aretightly cemented, which causes considerable
difficulty in extracting fossils, and sometimes,
small burrows such as Phycosiphon, cover the
nodule surface. Sediments in the nodules arealmost massive, and neither sedimentary structures nor trace fossils are discernible. Pieces ofwood, 5–30 cm long, are occasionally found in
the nodules, but smaller plant fragments are
few and shell debris is very rare. Besides thespherical nodules, nearly flattened or ellipsoidal calcareous concretions are also common, but they are usually barren. Unlike the
Bykov Formation or the Elk Butte Member ofMaastrichtian ammonoid fauna from the Pugachevo area

127

Fig. 5. 1, An outcrop of Upper Maastrichtian sandy mudstone of the Krasnoyarka Formation at Loc. PC1002
along the Pugachevka River. Many calcareous nodules are embedded in the sandy mudstone. 2, Vertical section of a large ellipsoidal calcareous nodule excavated from the outcrop. Note that a specimen of Pachydiscus
flexuosus Matsumoto is horizontally embedded in the nodule. 3, P. flexuosus just recovered after about 70 million years. Note that the shell exhibits aragonite preservation. Lens cap is 5.2 cm in diameter.

Fig. 6. Gaudryceras makarovense Shigeta and Maeda, NSM PM17304 from the Upper Maastrichtian muddy sandstone of the Krasnoyarka
Formation at Loc. PC1001 along the Pugachevka River. Scale bar5 cm.

128
Haruyoshi Maeda and Yasunari Shigeta

Maastrichtian ammonoid fauna from the Pugachevo area

129

Fig. 7. Gaudryceras makarovense Shigeta and Maeda from the Upper Maastrichtian muddy sandstone of the
Krasnoyarka Formation at Loc. PC1001 along the Pugachevka River. 1–3, NSM PM17305. 4–6, NSM
PM17306. Scale bar5 cm.

Fig. 8. Upper Maastrichtian Pachydiscus flexuosus Matsumoto from the Krasnoyarka Formation at Loc. PC1002 along the Pugachevka River. 1,
NSM PM17307. 2–3, NSM PM17308. 4–5, NSM PM17309. 6, NSM PM17310. Each scale bar5 cm. Upper left scale bar–1; Lower left bar–4
and 5; Central bar–2, 3 and 6.

130
Haruyoshi Maeda and Yasunari Shigeta

Maastrichtian ammonoid fauna from the Pugachevo area

the Pierre Shale in South Dakota, sideritic
concretions are never found in the Krasnoyarka Formation (Hayakawa, 2001; Landman& Waage, 1993).
Ammonoid occurrence: Near-adult specimens of Pachydiscus flexuosus and Gaudryceras makarovense are horizontally embeddedin the center of the large spherical nodules(Fig. 5). Pachydiscus flexuosus specimens,
20–30 cm in diameter, are most common, but
it seems peculiar that immature forms lessthan 5 cm in diameter and juveniles are neverfound. Unlike the fossiliferous nodules in the
Bykov Formation, each nodule contains just asingle ammonoid (Fig. 5). Nodules containing
plural ammonoids are rare. Generally, the nodules are two or three-times larger than the contained ammonoid. For example, a P. flexuosus
specimen 20 cm in diameter is usually foundinside a nodule exceeding 50 cm or more in diameter. In an extreme case, an immature P.
flexuosus specimen 6 cm in diameter was
found in the center of a huge nodule 70 cm indiameter. Benthic fossils such as bivalves, gastropods, and echinoids, which are common in
the Bykov Formation, are very rare in the
Krasnoyarka Formation.
Ammonoid preservation: Ammonoids fromthe Pugachevo area are exceptionally wellpreserved (Figs. 5–9), and a nacreous luster
is preserved on the shell tests of nearly all
specimens. The degree of such aragonite
preservation varies from one specimen to another; e.g., the color of the shell test ranges
from pearly white to reddish brown with arainbow luster (Fig. 8). In addition, an examination of these specimens from a taphonomic
perspective reveals several distinctive preservational features that are discussed below insome detail.
Generally, the body chambers of P. flexuosus are only partly preserved, and on average,
only a quarter of the body chamber remains.
The apertural margin is usually damaged severely due to biogenic or biostratinomic factors (Fig. 8.6). Specimens preserved with a131complete body-chamber and a perfect apertural margin have not yet been found. All bodychambers are crushed very slightly by compaction, even in the calcareous nodules, butthey are never flattened (Fig. 8.1).
Usually, the phragmocone is preserved with
its original geometry, however, only the outer
two or three whorls are preserved, and the central part of the umbilicus is mostly dissolved(Maeda, 1987; Maeda & Seilacher, 1996).
“Half-ammonoid preservation” is not found(Seilacher et al., 1976; Maeda, 1987, 2001;
Maeda & Seilacher, 1996).
The camerae are preserved with the septal
walls and the siphuncular tube intact, and areusually filled with drusy calcite formed during
diagenesis. However, the calcite infill is imperfect, and about half the volume of the camera
remains hollow. Camerae with sediment infill
are rare. The phragmocone is mostly free fromcompactional damage although the last two or
three camerae are sometimes crushed slightly
similar to the body chambers. Therefore, cementation of the calcareous nodules may havestarted just after the onset of shell-wall compaction (Maeda & Seilacher, 1996). However,crushed drusy calcite in such damaged camerae suggests that the drusy-calcite growth occurred prior to compaction of the shell.
Discussion
The significance of the Upper Maastrichtianammonoid assemblage, consisting of Pachydiscus flexuosus and Gaudryceras makarovense,in the Pugachevo area is summarized as follows:
1) A typical occurrence of this widespread
Upper Maastrichtian ammonoid assemblage isconfirmed in a new section located between
the Naiba and Makarov areas.
2) Exceptionally well-preserved specimens displaying aragonite preservation are
newly obtained.
3) Its taphonomic features are remarkably
different from those of ammonoid occurrences

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Haruyoshi Maeda and Yasunari Shigeta

Fig. 9. Scanning electron micrographs of the shell wall of Pachydiscus flexuosus Matsumoto, NSM PM17311
collected from the Krasnoyarka Formation at Loc. PC1002 along the Pugachevka River. Scale bar10 m m. 1,
Ventral wall at about 10 mm in shell diameter, consisting of the three layers: outer prismatic layer (op), nacreous layer (n) and inner prismatic layer (ip). 2, Close-up of outer prismatic layer and nacreous layer. Note thatthe original shell microstructure is well preserved.

in the Bykov Formation.
Upper Maastrichtian assemblage: Due toan increasing influx of coarse grained deposits, the Maastrichtian marine fossil record
becomes much more intermittent in Hokkaido,
Japan. Even in the Naiba section, the P. flexuosus-G. makarovense Assemblage is obscure(Kodama et al., 2002). In contrast, an alternative good Maastrichtian succession has been
discovered in the northern part of southern
Sakhalin, e.g., the Makarov section (Maeda etal., 2005; this volume).
The abundant occurrence of P. flexuosus
from the Pugachevo area has revealed a wide
range of morphological variation in thespecies. This variation becomes particularly
apparent when studying shell ornamentation,
whereby various morphotypes ranging fromalmost smooth (Fig. 8.6) to umbilically bullate(Fig. 8.2) exist in a population sample. At theextreme end, a strongly ribbed morphotype isoccasionally found (Maeda et al., 2005; Fig.
14.2, 14.5). The inner whorls of such strongly
ribbed morphotypes superficially resemble
Canadoceras, and thus, they could be mistakenly identified (Matsumoto & Morozumi,
1980, p. 7, pl. 2, fig. 1).
Detailed field observations reveal that theP. flexuosus-G. makarovense Assemblage iswidespread in various areas in southern
Sakhalin and Hokkaido. The discovery of anabundant occurrence of this fossil assemblage
in the Pugachevo area seems to suggest the existence of a wide distribution of this Upper
Maastrichtian assemblage in the North PacificRealm. Fragmentary fossil records of theMaastrichtian ammonoid fauna from the Pugachevo area

Maastrichtian in Hokkaido can now be stratigraphically reinterpreted by comparing themwith the succession in the Pugachevo area.
In contrast to the abundant occurrence of P.
flexuosus, which is widespread in the North
Pacific Realm, neither P. subcompressus Matsumoto nor Zelandites varuna (Forbes) arefound in the Maastrichtian sequence in the Pugachevo area. This lack of representatives
from the Indian province in the typical North
Pacific assemblage is also confirmed in the
Makarov area in another paper (Maeda et al.,2005: this volume).
Aragonite preservation: Preservation of
the Upper Maastrichtian ammonoids in the Pugachevo area ranks among the best in theworld (Fig. 9) and is probably unmatched inany other locality in southern Sakhalin or
Hokkaido. In the Naiba area, P. subcompressus
rarely exhibits aragonite preservation (Kodama et al., 2002, p. 378, fig. 8G).
Fossiliferous layers containing the P. flexuosus-G. makarovense Assemblage are also
traceable in the Makarov area. However,preservation in the Pugachevo area forms astriking contrast with that in the Makarov area,
in which P. flexuosus and G. makarovense
never display aragonite preservation (Maeda etal., 2005, Fig. 15).
The reason(s) for the excellent ammonoid
preservation in the Pugachevo area is still obscure. In the near future, well-preserved shellmaterial from Upper Maastrichtian ammonoid
specimens will be subjected to oxygen andcarbon-isotopic analyses.
Taphonomic features: The taphonomic aspects of the Upper Maastrichtian ammonoids
from the Pugachevo area largely remain to be
exploited. It seems rather odd that juvenile andimmature ammonoids less than 5 cm in diameter are absent, in both the calcareous nodules
and the surrounding muddy sandstone. This
scarcity of immature forms raises valid questions that may be explained by taphonomic
analyses.
Juvenile and immature ammonoids usually133

outnumber adults and near-adults in the Bykov
Formation, particularly in the fossiliferous calcareous nodules (Tanabe et al., 1977; Maeda,
1987, 1993). In the Bykov Formation, the sizedistributional pattern of ammonoids is usuallybimodal. Generally, the first mode consists ofimmature ammonoids less than 10 cm in diameter, and the second mode includes large
adults more than 30 cm in diameter. Ammonoids within the first mode usually occur incalcareous nodules, while those within the second mode are solitarily embedded in the hostrock (Maeda, l987). Ammonoids in the midrange of 20–30 cm in diameter are relatively
few in number.
The original size-distributional patterns ofammonoids may be biassed by taphonomic
processes, i.e., hydrodynamic sorting by currents and/or selective shell dissolution after
burial (Maeda, 1991). However, there are nosigns of strong currents or wave actions in surrounding deposits, and ammonoids from Locs.
PC1001 and 1002 appear not to have been accumulated as shell-lags, or reworked. Likewise, their exceptionally well preserved shelltests prevent us from concluding that strong
chemical dissolution has selectively eliminatedthe thin-shelled juveniles during diagenesis.
The scarcity of immature forms is one of
the unusual taphonomic features of certain uppermost Cretaceous ammonoids, e.g., Pachydiscus flexuosus. Upper Maastrichtian ammonoids from the Pugachevo area provide an
excellent opportunity for us to investigate thisphenomenon through the reconstruction of thedepositional environment and the examinationof ammonoid paleoecology as well as thetaphonomic processes of their preservation.
Conclusions
A typical occurrence of the widespread
Upper Maastrichtian ammonoid assemblage,
represented by Pachydiscus flexuosus andGaudryceras makarovense is confirmed in a
new section located between the Naiba and134

Haruyoshi Maeda and Yasunari Shigeta

Makarov areas. Preservation of the Upper
Maastrichtian ammonoids ranks among thebest in the world and is probably unmatched inany other locality in southern Sakhalin or
Hokkaido. Aragonite preservation of the shelltests and the scarcity of immature ammonoids
are suggested as distinctive characteristics ofthis Upper Maastrichtian Fauna. These features will be exploited in the near future froma taphonomic perspective.
Acknowledgments
We are very much grateful to K. F. Sergeev,
Director of the Institute of Marine Geology
and Geophysics, Yuzhno-Sakhalinsk, and G.
S. Steinberg, Director of the Institute ofVolcanology and Geodynamics, YuzhnoSakhalinsk, for their supervision of this research program. We also thank the various
staff personnel of these Institutes for their kind
help and cooperation throughout the field survey. A. V. Solov’yov (Institute of Volcanology
and Geodynamics) and members of the JointResearch Program, T. Kase and K. Uemura(National Science Museum, Tokyo), K. Kodama (Kochi University), T. Okamoto (Ehime
University), T. Hasegawa (Kanazawa University), M. Futakami (Kawamura Women’s University), and T. Takeuchi (Anjo Gakuen HighSchool) gave us kind advice and help in various ways during the course of this study.
Thanks are also extended to the Mayor andother area authorities, as well as the citizens of
Makarov, and the State Government of
Sakhalin for their assistance during the work.
This study was financially supported by theJapanese Government’s Ministry of Education,
Science, Culture, and Sports (Project Nos.
5041068, 8041113 and 9041114), and by the
Fujiwara Natural History Foundation’s supportof H.M. (Project No. 12-3) and Y.S. (ProjectNo. 7-21).

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Haruyoshi Maeda and Yasunari Shigeta

ロシア極東・サハリン南部・プガチェボ地域のマストリヒチアン階アンモノイド化石群：
プガチェボ地域の白亜系蝦夷層群について，層序学的研究と古生物学的研究を行った．本層群は，下位からブイコフ層とクラスノヤルカ層に区分され，カンパニアン期中期からマスト
リヒチアン期の地層が露出する．ブイコフ層はカンパニアン期中期の Canadoceras kossmati やSphenoceramus schmidti で特徴づけられる．一方，クラスノヤルカ層中部と上部はマストリヒ
チアン期後期の Pachydiscus flexuosus と Gaudryceras makarovense で特徴づけられる．ナイバ地域とマカロフ地域の間に位置する本セクションから，マストリヒチアン期後期を特徴づける
アンモノイド化石群が典型的に産出することが確認された．マストリヒチアン期後期のアンモノイド化石の保存状態は，世界的に見ても最良の部類に属し，他の南部サハリン各地における化石の保存状態を凌駕している．大型の石灰質ノジュール中から産する P. flexuosus と G.
makarovense の殻は，多くの場合アラレ石保存を示す．最後の 23 の気室は，住房部と同様にわずかに圧密を受けているが，それ以外の気房部は圧密の影響を全く受けていない．また，殻の直径が 5 cm 未満の P. flexuosus と G. makarovense の幼年殻が，石灰質ノジュール中にも母岩の泥質砂岩中にも見られないことは注目に値する．この幼年殻の少なさは，単なる波浪や水流による淘汰，あるいは続成の過程における選択的な殻の溶解では説明できない．このような特異な現象は，化石化作用の解明と同様に，堆積環境やアンモノイドの古生態を復元する上で，重要な鍵となると考えられる．
前田晴良・重田康成