Bull. Fukushima Museum, no. 37, p. 29−36, March, 2023

Original Article

A crocodyliform osteoderm from the Tamayama Formation（Upper Cretaceous）of Fukushima, Japan
Junki Yoshida＊・ Toshinobu Takahashi＊＊・ Satoshi Nabana＊＊＊

ABSTRACT
Crocodyliformes, such as gobiosuchids, goniopholidids, paralligatorids, and eusuchians, have widelydistributed in East Asia during the Cretaceous. In Japan, the Futaba Group in Fukushima Prefecture is oneof the most significant Cretaceous outcrops that have produced terrestrial vertebrate fossils, including
dinosaurs. However, a definitive crocodyliform has never been reported from Futaba Group. Here we reporta crocodyliform osteoderm from the Tamayama Formation（Coniacian–Santonian）of the Futaba Group. The
osteoderm is rectangular, wider than long, and bears a tall keel, which can be identified as a dorsal
paravertebral osteoderm. A comparison with other crocodyliforms suggests that it pertains to semi-aquatic
Neosuchia, but not to Goniopholididae or Paralligatoridae. The first definitive record of a semi-aquatic,
derived neosuchian（e.g., eusuchians）from the Futaba Group indicates their appearance before theConiacian age in East Asia.

INTRODUCTION

2 0 1 5 ; Sato et al., 2 0 0 5）.A definitive crocodyliform
has not been reported from the Futaba Group,
excluding a potential crocodyliform jaw from the
Ashizawa Formation（Hasegawa et al., 1987）. Herewe report a crocodyliform osteoderm from theTamayama Formation（Coniacian-Santonian）of
the Futaba Group.

Crocodyliformes, such as gobiosuchids, goniopholidids, paralligatorids, and eusuchians, havewidely distributed in East Asia during theCretaceous. In Asia, a variety of crocodyliform
families are known from the Cretaceous;gobiosuchids, goniopholidids, paralligatorids, andeusuchians（Storrs & Efimov, 2 0 0 1 ; Lauprasert eta l . , 2 0 1 1 ; K u b o e t a l . , 2 0 1 8 ; L i e t a l . , 2 0 1 9）.
Eusuchian has been found from the lower Upper
Cretaceous in Southeast Asia（Thailand, Aptian,
Kubo et al., 2 0 1 8）and Central Asia（Uzbekistan,
Turonian, and Tadzhikistan, Santonian, Storrs &Efimov, 2001）,whereas its first appearance in East
Asia is during the latest Cretaceous（Maastrichtian）（a crocodyloid Jiangxisuchus from the of Jilin,
China: Li et al., 2019）.
In Japan, the Coniacian-Santonian Futaba Groupin Fukushima Prefecture is one of the mostsignificant Upper Cretaceous exposures that haveproduced terrestrial and marine vertebrate fossils,
including dinosaurs and plesiosaurs（Hasegawa etal., 1 9 8 7 ; Saegusa & Tomida, 2 0 1 1 ; Ohashi et al,,＊Fukushima Museum＊＊

Iwaki Natural History Association

Geological setting
The Futaba Group is composed of the lowermost
Ashizawa Formation, the middle Kasamatsu
Formation, and the uppermost Tamayama
Formation. The Ashizawa Formation andKasamatsu Formation have produced a mammaliandentary（Kusuhashi et al., 2 0 1 6）and dinosaur
materials, such as hadrosauroid teeth and vertebrae
and a sauropod tooth（Hasegawa et al., 1987; Ohashi
e t a l . , 2 0 1 5）.T h e T a m a y a m a F o r m a t i o n h a s
produced a few sauropod teeth from theKohisagawa Member（Saegusa & Tomida, 2 0 1 1）
and a plesiosaur Futabasaurus（Sato et al., 2 0 0 5）
from the Irimazawa Member, which overlies the
Kohisagawa Member and ranges from the late
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Iwaki City Coal and Fossil Museum

Junki Yoshida・ Toshinobu Takahashi・ Satoshi Nabana

Figure 1. Discovery site of the crocodyliform osteoderm（modified from Inose 2019）.

Coniacian to early Santonian based on ammonoids,
inoceramids, and bivalves（Obata & Suzuki, 1 9 6 9 ;Ando et al., 1995; Inose 2019）.

Description
The osteoderm is rectangular in dorsoventral
view, 5 3 mm wide, 4 4 mm long, and wider thanl o n g（F i g . 2）.I t b e a r s a d o r s a l k e e l a l o n g t h e
midline on the dorsal surface, which is 1 1 mm
high. The keel forms an obtuse triangle in lateralview and runs slightly lateral to the midline closer
to its anterior margin. In anteroposterior view, theosteoderm is dorsally curved. The medial half isnearly flat and less curved than the lateral one.
The anterior margin is ambiguous in its outline
due to the damage; however, three other margins
are preserved. The posterior margin is concave inthe midline, having a space for contacting the
successive osteoderm. The medial margin isstraight and would contact the neighboring
osteoderm medially, while the lateral margin isdentate, indicating the absence of its neighboring
osteoderm laterally. Also, the medial margin islonger than the lateral margin. Based on thesemorphological characters, the osteoderm can beinterpreted as a left lateral-most osteoderm of thedorsal shield（Fig. 3）.
On the dorsal surface, the external large pits aredensely present, while the ventral surface issmooth and bearing some smaller pits which areexternal traces of vascular canals（Fig. 2）. The
pits closer to the edges are larger, elliptical, andinterconnected with each other, whereas those
closer to the center are smaller, more rounded,and separated from each other on the dorsal
surface. The deep pits are seen in ICFM-423360 asin some neosuchians such as paralligatorids andeusuchians（Rummy et al., 2019; Kubo et al., 2018;

MATERIAL AND METHODS

A crocodyliform osteoderm（ICFM-423360）was
collected from the Irimazawa Member of the
Tamayama Formation, the Futaba Group（lateConiacian-early Santonian, Late Cretaceous）atIrimazawa River in Iwaki City, Fukushima
Prefecture, Japan（Fig. 1）.ICFM- 4 2 3 3 6 0 wascompared with other crocodyliform osteoderms fortaxonomic identification. Also, the internal
structure of ICFM- 4 2 3 3 6 0 was examined by a CTs c a n n e r（i n s p e X i o S M X - 2 2 5 C T F P D H R ,SHIMADZU）in National Science Museum, Tokyo.
Institutional abbreviation- ICFM, Iwaki City Coaland Fossil Museum.

RESULTS

Systematic Paleontology
CROCODYLIFORMES Clark 1986
NEOSUCHIA Benton & Clark 1988
Gen. et sp. indet.
Material- A dorsal paravertebral osteoderm（ICFM-423360, Fig. 2）
Locality and horizon- Iwaki City, Fukushima
Prefecture, Japan. Irimazawa Member, Tamayama
Formation, Futaba Group（Coniacian-Santonian,Upper Cretaceous）.

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A crocodyliform osteoderm from the Tamayama Formation（Upper Cretaceous）of Fukushima, Japan

Figure 2 . A neosuchian osteoderm from the Tamayama Formation. ICFM- 4 2 3 3 6 0 is in dorsal（A）, ventral（B）, leftlateral（C）, and anterior views（D）. Scale bar = 1 cm.

DISCUSSION

Fig. 4）. The internal structure revealed by the CT
scanning shows that the inner cavities areconcentrated on the core region of the osteoderm（Fig. 5）. The porosity, the ratio of the vessels to
the total cross-sectional area, is 0 . 1 9 in the sagittal
midline（Fig. 5）, which is as high as semi-aquatic
crocodyliforms（Clarac et al., 2020）.

Taxonomic assignment of the osteoderm
Preservation of a single osteoderm（ICFM4 2 3 3 6 0）m a k e s i t s t a x o n o m i c i d e n t i f i c a t i o nchallenging. However, as mentioned below, ICFM4 2 3 3 6 0 may belong to derived neosuchians（e.g.,
e u s u c h i a n s）, b u t n o t t o g o n i o p h o l i d i d s ,pholidosaurids, or paralligatorids, based on themorphological differences mentioned hereafter.
Although the anterior margin of ICFM- 4 2 3 3 6 0 is
poorly preserved, the lateral margin is smooth
ventrally and not abruptly bent vertically, which
differs from goniopholidids and pholidosaurids withvertically oriented lateral margin lateral to thedorsal keel. Moreover, the dorsal keel in ICFM4 2 3 3 6 0 does not reach the posterior margin of theosteoderm, unlike that in paralligatorids（Turner,
2015; Kuzmin et al., 2018; Rummy et al., 2022; Fig.
4）. Furthermore, in ICFM- 4 2 3 3 6 0 , the distinctive
ornamentation, posteriorly placed keel, zigzag
midline suture, and thickened posterior edge areabsent, which is different from gobiosuchids fromthe Late Cretaceous of East Asia（Osmolska et al.,
1 9 9 7 ; Buscalioni 2 0 1 7）.Therefore, ICFM- 4 2 3 3 6 0

Figure 3 . Inferred serial position of the osteoderm（ICFM- 4 2 3 3 6 0）in dorsal（A）and anteroposterior
views（B）.

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Junki Yoshida・ Toshinobu Takahashi・ Satoshi Nabana

Figure 4. Comparisons of dorsal osteoderm with the Cretaceous neosuchians.（A）ICFM-423360 of Tamayama Formation,

Futaba Group in Fukushima, Japan,（B）an eusuchian from Khok Kruat Formation, Thailand（redrawn from Kubo et al., 2 0 1 8）,（C）eusuchian Allodaposuchus from the Velaux-La Bastide Neuve site, France（redrawn from Martin et al., 2016）,（D）crocodyloid Albertosuchus
from Scollard Formation, Canada（redrawn from Wu & Brinkman, 2015）,（E）Susisuchus from Crato Formation（redrawn from Figueiredo
et al., 2011）, and（F）paralligatorid Rugosuchus from Nenjiang Formation, China（redrawn from Wu et al., 2001）. Not to scale.

belongs to a derived neosuchian species（e.g.,hylaeochampsids and eusuchians）, precluding thepossibility of Gobiosuchidae, Goniopholididae, andParalligatoridae. ICFM- 4 2 3 3 6 0 resembles theosteoderm of the lateral row of the tetraserial
dorsal shield in derived neosuchians, which iscongruent with the taxonomic assessment of thisstudy（Fig. 3）. ICFM-423360 resembles osteoderms
in some eusuchians（Allodaposuchus, Albertosuchus,
and the Khok Kruat eusuchian）in having the
pronounced median ridge and the large externalpits（Fig. 4）, but the further taxonomic identification
remains uncertain.

The paleofauna of the Tamayama Formation,
Futaba GroupThe Tamayama Formation was deposited in theshallow marine environment（Ando et al., 1 9 9 5 ;Inose 2 0 1 9）, and has previously provided bothmarine and terrestrial fossils; a plesiosaur（Sato eta l . , 2 0 0 5）, i n o c e r a m i d s（A n d o e t a l . , 1 9 9 5）,ammonoids（Obata & Suzuki, 1 9 6 9）, bivalves（Inose 2 0 1 9）,a dinosaur（Saegusa & Tomida,
2 0 1 1）and plants（Takahashi et al., 1 9 9 9 , 2 0 0 7）.
Since most neosuchians adapted to the semia q u a t i c h a b i t a t（Y o s h i d a e t a l . , 2 0 2 1 ）, t h eneosuchian osteoderm（ICFM- 4 2 3 3 6 0）might bewashed out with plant and dinosaur materials, and−32−

A crocodyliform osteoderm from the Tamayama Formation（Upper Cretaceous）of Fukushima, Japan

Figure 5 . Cross sectional CT image of the osteoderm（ICFM- 4 2 3 3 6 0）along the sagittal plane（A）and its bonyregions in black（B）. Scale bar = 1 cm.

the derived neosuchians in East Asia before
Coniacian. The detailed taxonomic identification of
the osteoderm remains unclear within the derived
neosuchians, but future study on neosuchianosteology and phylogeny would resolve it（Turner,
2015; Turner et al., 2015, Tennant et al., 2016）.As
Japan was situated in the East margin of the
Asian continent and interconnected with North
American through the Beringian landbridge during
the Cretaceous（Russell 1 9 9 3 ; Fiorillo 2 0 0 8）,
Japanese fossils would provide important
information for understanding paleobiogeographyof Asian and pan-Pacific terrestrial faunas.

finally deposited in the shallow marine（brackish
to inner shelf）environment（Ando et al., 1 9 9 5 ;Inose 2 0 1 9）. The first semi-aquatic crocodyliform
from the Futaba Group in this study increases
diversity of the Coniacian Tamayama paleofauna.
Paleobiogeography of the Cretaceous neoschians
in Japan and Asia
The Cretaceous deposits of Japan have produced
a few crocodyliform remains [e.g., a goniopholidid
f r o m t h e E a r l y C r e t a c e o u s（A p t i a n - A l b i a n）
Kitadani Formation of Fukui: Kobayashi 1998]. Theo s t e o d e r m（I C F M - 4 2 3 3 6 0 ）f r o m t h e L a t e
Cretaceous（Coniacian-Santonian）Tamayama
Formation of this study provides the first recordof more derived neosuchian clade, such as
eusuchians, and increase crocodyliform diversity inJapan. It also suggests that non-goniopholidid
neosuchians have likely appeared no later than theearly Late Cretaceous in Japan.
In East Asia, a variety of neosuchian families
are known from the Cretaceous; goniopholidids,
paralligatorids, and eusuchians（Storrs & Efimov,
2001; Lauprasert et al., 2011; Kubo et al., 2018; Li etal., 2 0 1 9）. Pre-Coniacian eusuchians have beenfound in Southeast Asia（Thailand, Aptian, Kubo
e t a l . , 2 0 1 8）a n d C e n t r a l A s i a（U z b e k i s t a n ,Turonian, and Tadzhikistan, Santonian, Storrs &Efimov, 2 0 0 1）, but no definitive record of thederived neosuchians in East Asia. From postSantonian beds, a crocodyloid eusuchian was
discovered from the Maastrichtian Nanxiong
Formation in Jilin, China（Li et al., 2019）.The newderived-neosuchian osteoderm from the ConiacianSantonian of the Tamayama Formation fills aspatiotemporal gap and suggests the occurrence of

ACKNOWLEDGMENT

We thank Education Board of Iwaki City forspecimen collection, and Makoto Manabe, Takanobu
Tsuihiji, and Chisako Sakata（National Science
Museum, Tokyo）for providing technical assistancefor CT scanning. Masaya Iijima（Royal Veterinary
College）and Hiroaki Inose（Fukushima Museum）
improved the early draft of our manuscript. We
express gratitude to the editors of the Bulletin of
Fukushima Museum.

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福島県立博物館紀要 第37号 2023年 3 月 p. 29−36

福島県の玉山層（白亜紀後期）から産出したワニ形類の皮骨吉田純輝＊・ 高橋紀信＊＊・ 菜花智＊＊＊

要旨

白亜紀の東アジアでは、多様なワニ形類が分布し、ゴニオフォリス類、パラリゲーター類、ゴビオスクス
類、正鰐類などが存在した。日本において、白亜紀の陸生脊椎動物（恐竜を含む）化石を産する重要な地層の一つが、福島県の双葉層群である。しかしながら、双葉層群ではワニ形類の確かな証拠は見つかっていなかった。本報告では、福島県いわき市の双葉層群玉山層（コニアシアン期）からワニ形類の皮骨を報告する。この皮骨は幅広い長方形で、高いキールを背側にそなえており、背側板由来と考えられる。また、既知種との形態比較から、本標本はゴニオフォリス類やパラリゲーター類ではない、半水生の派生的な新鰐類の皮骨と考えられる。本研究による、双葉層群から初めて見つかった派生的な新鰐類の確かな証拠は、彼らが東アジアにおいて白亜紀コニアシアン期には既に出現していたことを示唆した。

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福島県立博物館＊＊

いわき自然史研究会＊＊＊

いわき市石炭・化石館−36−