ARTICLE

Memoir of the Fukui Prefecturel Dinosaur Museum 13: 17–23 (2014)© by the Fukui Prefectural Dinosaur Museum

FIRST REPORT OF FOSSIL WOOD FROM THE UPPER CRETACEOUSIZUMI GROUP IN AWAJI ISLAND, JAPANKazuo TERADA 1 and Kumiko HANDA 2
Fukui Prefectural Dinosaur Museum, 51-11 Terao, Muroko, Katsuyama, Fukui 911-8601, JapanThe Museum of Nature and Human Activities, Hyogo, 6 Yayoigaoka, Sanda, Hyogo 669-1546, Japan1

2

ABSTRACTA calciﬁed fossil wood from the Shimonada Formation (early Maastrichtian) of the Upper Cretaceous Izumi Group
collected at the coast of Nadachino, Minamiawaji City in the Awaji Island, Hyogo Prefecture is studied anatomically.
The fossil has the axial and radial intercellular (resin) canals and is identiﬁed as Piceoxylon sp. of Pinaceae. The fossil
is similar to fossil woods of Piceoxylon reported from the Upper Cretaceous of Hokkaido of Japan, Russian Far East
and Canadian Arctic Archipelago, but is different from them in having indistinct growth ring boundaries and variabletypes of cross-ﬁeld pitting. This represents the fossil wood reported from the Izumi Group for the ﬁrst time.
Key words：Awaji Island, early Maastrichtian, fossil wood, Hyogo Prefecture, Izumi Group, Piceoxylon,
Shimonada Formation, Upper Cretaceous
寺田和雄・半田久美子（2014）淡路島の上部白亜系和泉層群から産出した材化石の初報告．福井県立恐竜博
物館紀要 13：17–23.
兵庫県淡路島南部に分布する上部白亜系和泉層群下灘層（前期Maastrichtian）から産出した石灰化した
材化石について樹種を検討した．この化石は垂直・水平樹脂道を有し，マツ科のPiceoxylon sp.と同定した．
本化石は北海道や極東ロシア，カナダ北極諸島の上部白亜系から報告されているPiceoxylon の材化石に類似しているが，不明瞭な年輪界や非常に様々なタイプの分野壁孔を持つことで異なっている．本報告は和泉層群からの材化石の最初の報告である．

ammonites and crustaceans (e.g., Morozumi, 1985; Iwaki and
Maeda, 1989; Nanko, 1996; Karasawa, 1998), and some vertebrate
fossils such as dinosaurs, pterosaurs, mosasaurs and ﬁshes (Sasai,
1936; Nanko, 1996; Saegusa and Furutani, 2004; Tanimoto, 2005;
Kishimoto, 2014). The age of the group becomes younger towards
the east from western Shikoku Island (Campanian), across Awaji
Island (Campanian and Maastrichtian) to the Izumi Mountain
Range (Maastrichtian), based mainly on international correlationof ammonite faunas (e.g., Morozumi, 1985; Tanaka, 1989).
As for the plant fossils from the group, a few reports were
published (Matsuo, 1954, 1964, 1966; Iwaki and Maeda, 1989;
Nanko, 1996). The plant megafossils from the group wereﬁrstly reported by Matsuo (1954), who illustrated a small shootand a cone as Cunninghamia sp. that was later described as
C. izumiensis Matsuo (1966). He further reported some plant
megafossils including Zamites sp. of the gymnosperms and“Ficus”sp. of the angiosperms (Matsuo, 1966). This flora wascalled the Izumi Flora (Matsuo, 1964). Iwaki and Maeda (1989)

INTRODUCTION
The Izumi Group is one of representative Upper Cretaceous
strata of Southwest Honshu Island, Japan and is widely distributed
for about 300 km from western Shikoku Island eastward to the
Izumi Mountains of Honshu Island along the north border of theMedian Tectonic Line (e.g., Ichikawa, 1961; Tanaka, 1989). The
group constitutes a thick sequence of marine deposits, mainlycomposed of alternating beds of conglomerate, sandstone andmudstone of a turbidite facies (e.g., Ichikawa, 1961; Morozumi,
1985; Tanaka, 1989; Noda and Toshimitsu, 2009), and yields
a lot of rhizarian fossils such as radiolarian and foraminiferan(Yamazaki, 1987), invertebrate fossils such as mollusks,
Received July 23, 2014. Accepted October 7, 2014.
Corresponding author—Kazuo TERADA
Phone: +81-779-88-0001, Fax: +81-779-88-8710
E-mail: k-terada＊dinosaur.pref.fukui.jp
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KAZUO TERADA AND KUMIKO HANDAFIGURE 1. Locality of studied fossil wood on the geological map of the southern Awaji Island showing the distribution of the Izumi Group (modiﬁed from
Morozumi (1985)).

reported a few leaves of the angiosperms such as Platanus? sp.
and Ulmus sp., which were identified by Dr. Hidekuni Matsuo.
Nanko (1996) showed some photographs of plant fossils including
cycadalean stems and leaves, coniferous shoots and cones,
ginkgoalean leaves of the gymnosperms, and an angiospermous
leaf, but no detailed information of the fossils was described.
As for the fossil wood from the group, no report has been
published. Only one guidebook showed one photograph of fossil
wood (Nanko, 1996). We had opportunities to study a fossil woodfrom the Izumi Group of the Awaji Island. The fossil is a calciﬁed(calcium-carbonated) wood. Although Nanko (1996) mentioned
his fossil as silicified wood, Nanko s fossil may be a calcified
wood because it was collected from the same locality as our fossil,where the marine deposits were distributed.
Here we report the result of anatomical study on the fossil
wood.
GEOLOGY
The Awaji Island is located in the eastern part of the Seto Inland

Sea between the islands of Honshu and Shikoku, and is separated
from Honshu Island by the Akashi Strait and from Shikoku Islandby the Naruto Strait. The Izumi Group in the island is distributedin the southern half of the island (Fig. 1), being over 10,000 m in
integrated thickness, overlies unconformably the acidic pyroclastic
rocks of the Sen-nan Group, and is divided into ﬁve formations;
the Seidan, the Anaga, the Kita-ama, the Nada and the Shimonada
formations in ascending order (Ichikawa, 1961; Morozumi, 1985).
Morozumi (1985) studied the ammonite fauna of the Izumi
Group in the Awaji Island biostratigraphically in detail andreferred to for analysis of the age of the group. The stratigraphic
position of the Shimonada Formation, which is distributed in anarrow area along the most southwestern margin of the island, is
not precisely determined, because the formation is separated fromthe main part of the group by a fault (Fig. 1). Morozumi (1985)
referred the formation to be synchronous with part of the Kitaama and/or Nada formations based on the contained ammonites.
The age of the Shimonada Formation is estimated as the early
Maastrichtian of the Upper Cretaceous within the Pachydiscus aff.
subcompressus Zone of ammonites (Morozumi, 1985).

FOSSIL WOOD FROM THE UPPER CRETACEOUS IZUMI GROUP

19

MATERIALS AND METHODSSpecimen.—MNHAH-D1029085
Repository.—Museum of Nature and Human Activities,
Hyogo, Hyogo Prefecture (MNHAH).
The specimen was collected as a washed-out rock on the coastof Nadachino, Minamiawaji City in the Awaji Island, where the
Shimonada Formation is distributed (Fig. 1). This specimen isa calcified fragment of secondary xylem and rather rounded byerosion, and 9 x 6.5 cm in diameter with 9 cm long (Fig. 2A).
This is probably derived from a mature stem due to the presenceof pith.
The specimen was thin-sectioned in cross, tangential andradial dimensions by a diamond saw, and ground to make thin
microscopic slides. These slides were studied and photographedwith an optical microscope.

A

RESULT
The specimen (MNHAH-D1029085) was identified as
Piceoxylon sp. The technical terms for microscopic features
follow IAWA Committee (2004). Technical terms for microscopic
features particularly important to the distinction of Mesozoic
woods follow Philippe and Bamford (2008).
SYSTEMATIC PALEONTOLOGYClass PINOPSIDA Burnett, 1835
Order PINALES Dumortier, 1829
Family PINACEAE Lindley, 1836
Genus PICEOXYLON Gothan, 1905.
PICEOXYLON sp.
(Figs. 2, 3)
Description
Coniferous wood constituted of tracheids, ray tracheids, rayparenchyma, and epithelial cells of axial and radial intercellular(resin) canals. Whole tracheids in second xylem thick-walled likecompression woods. Growth rings indistinct, 3–5 mm wide (Fig.
3A, B). Transition from earlywood to latewood gradual (Fig. 3A.
B). Nests of sclereids in pith absent (Fig. 3A)..
Earlywood tracheids rounded-square, rounded-rectangularor rounded-polygonal in cross section, 20–75 x 25–52 µm intangential x radial diameters; thick-walled 7.5 µm thick, becauseof the formation of compression wood. Latewood tracheids
rounded-rectangular, radially ﬂattened in cross section, 25–58 x
15–30 µm in tangential x radial diameters; thick-walled, 7.5 µm
thick.
Tracheid walls slightly decomposed, spiral grooves of S2 ofsecondary wall layers distinct in tangential and radial sections.
Helical thickenings on the longitudinal tracheid walls absent (Fig.
3E, G).
Pitting on the radial walls contiguously uniseriate (Fig. 3F),B
FIGURE 2. Piceoxylon sp. (MNHAH-D1029085). A, Original material.
Scale bar: 2 cm. B, Close-up of the cut surface of cross direction of thefossil, showing indistinct growth rings and axial intercellular canals. Scalebar: 1 cm.

Terada and Handa Fig. 2
or rarely biseriate. Crassulae absent. The pits are circular or
somewhat elliptic, horizontally elongated, 10–25 µm in diameterwith round to oval aperture of about 5 µm in diameter.
Axial parenchyma absent. Axial and radial intercellular canals
present. Axial intercellular canals in all annual rings, mostly inearlywood, commonly occurring singly along rays, sometimes inpairs; usually round, 125–225 µm in diameter, with both thickwalled and thin-walled epithelial cells (Fig. 3A, B, C). Tylosoids
in resin ducts present, distinct. Traumatic resin canals absent.
Radial intercellular canals rather small, 55–87 µm in diameter,
thick-walled epithelial cells in biseriate, bi-triseriate, and triseriate
rays.
Rays consisting of ray parenchyma and ray tracheids, usuallyuniseriate or partly biseriate, 1–21 (commonly 4–8) cells tall
(40–575 µm) with 30–50 µm wide (Fig. 3D). The horizontal wallsof the ray parenchyma pitted (Fig. 3G, H. I). Abietineentüpfelungon tangential walls of ray cells present, distinct (Fig. 3E). Ray
tracheids sometimes present at margin of rays; possibly helicalthickenings present in ray tracheid walls (Fig. 3G). Pit bordersof ray tracheids not observed. Cross-field pitting variable, slitlike pinoid (Fig. 3H), half bordered taxodioid or cupressoid withoblique oriented elliptical apertures (Fig. 3I), or possibly large to20

KAZUO TERADA AND KUMIKO HANDA

A

B

C

D

E

F

G

H

I

A，Cross
FIGURE 3. Piceoxylon sp. (MNHAH-D1029085). A–I, Microphotographs
the fossil.
Teradaofand
Handa
Fig.section
3 showing the pith of wood, axial intercellular canals,
one indistinct growth ring boundary (blue arrow). Scale bar: 500 µm. B, Cross section around the ninth-year growth ring, showing one indistinct growth ringboundary (blue arrow) and axial intercellular canals. Scale bar: 250 µm. C, Cross section showing axial intercellular canals. Scale bar: 100 µm. D, Tangential
section showing uniseriate rays and a radial intercellular canal. Scale bar: 100 µm. E, Tangential section showing Abietineentüpfelung (red arrow) on ray cells.
Scale bar: 50 µm. F, Radial section showing predominantly uniseriate tracheid pitting in the radial walls. Scale bar: 50 µm. G, Radial section showing ray
tracheid with probably helical thickenings (white arrow), and pitted horizontal walls of ray parenchyma cells. Scale bar: 50 µm. H, Radial section showing slitlike pinoid cross-ﬁeld pitting. Scale bar: 50 µm. I, Radial section showing cupressoid to taxodioid cross-ﬁeld pitting. Scale bar: 25 µm.

FOSSIL WOOD FROM THE UPPER CRETACEOUS IZUMI GROUP

small oopores (Fig. 3G, I); one to four per cross-ﬁeld (Fig. 3G, H,
I).
AFFINITY AND DISCUSSION
The fossil wood is prominently characterized by the presenceof (1) both axial and radial intercellular (resin) canals, (2) boththick-walled and thin-walled epithelial cells, (3) contiguously
uniseriate radial wall pitting, (4) Abietineentüpfelung on thetangential walls of ray cells, (5) pitted horizontal ray walls, (6)ray tracheids with possibly helical thickenings, (7) slit-like pinoid,
half bordered taxodioid or cupressoid, and possibly oopores crossﬁeld pitting.
These wood anatomical features were initially identified
having afﬁnities with the morphogenera of Pinaceae. Especially,
Abietineentüpfelung is one of the important microscopic features
for the distinction of the Mesozoic woods (Philippe and Bamford,2008), which was coined by Gothan (1905) to describe rounded
pits, areolate or not, occurring on the transversal wall of ray cells(Fig. 3G).
According to a key for the identification of the Mesozoic
coniferous woods by Philippe and Bamford (2008), themorphogenera which have both axial and radial intercellular
canals and Abietineentüpfelung limited the following four genera;
Palaeopiceoxylon Kräusel, Pinuxylon Gothan, Laricioxylon
Greguss and Piceoxylon Gothan. Although Pityoxylon Kraus
is described as having both axial and radial intercellular canals(Kraus, 1882, see Harland et al., 2007), this morphogenus nameshould not be used until its revision because of the problems of
typiﬁcation and specimen (Philippe and Bamford, 2008).
Palaeopiceoxylon is obviously different from our fossil inhaving araucarian, or araucarian and abietinean mixed pitting
on the radial walls of tracheids (Kräusel, 1949, see Philippe andBamford, 2008). Pinuxylon is apparently different from our fossil
in having thin-walled epithelial cells of intercellular canals andlarge window-like cross-ﬁeld pitting (Gothan, 1905, see Philippeand Bamford, 2008).
In wood anatomical features, Laricioxylon and Piceoxylon arevery similar to each other and share characteristics with extant
genera, Picea, Larix and Pseudotsuga of the Pinaceae. According
to a key of the identiﬁcation for the Mesozoic coniferous woodsby Philippe and Bamford (2008), they could be separated by thefeature of ray tracheid tips. The ray tracheid tips of Laricioxylon
are elongated, crooked, overlapping with one another (Greguss,
1967, see Philippe and Bamford, 2008). Contrarily, the raytracheid tips of Piceoxylon are short, straight, not overlapping(Gothan, 1905, see Philippe and Cantrill, 2007; Philippe andBamford, 2008). Furthermore, Harland et al. (2007) mention
Laricioxylon has a relatively high proportion of latewood cells
and displays characteristics similar to extant Larix wood. Our
fossil does not have distinctly elongated ray tracheid tips or a highproportion of latewood cells. Therefore, our fossil could be placedin the restricted genus Piceoxylon.
Although no genus name is mentioned in the key by Philippeand Bamford (2008), Pseudotsugoxylon Blokhina also has both

21

axial and radial intercellular canals and Abietineentüpfelung,which described from the Pliocene of the Southern Primorskii
Region in the Russian Far East (Blokhina and Bondarenko, 2004).
Pseudotsugoxylon is, however, different from our fossil in having
distinct helical thickenings on the longitudinal tracheid walls, andtypical picenoid cross-ﬁeld pitting in the diagnosis (Blokhina andBondarenko, 2004).
Therefore, our fossil is assigned to Piceoxylon of the Pinaceae.
Piceoxylon was firstly described as P. antiquius by Gothan(1905), which was designated its lectypification by Philippeand Cantrill (2007). Piceoxylon includes more than 20 reported
species from a large area of the Northern Hemisphere. Even justlimited to Upper Cretaceous to Paleogene woods of Piceoxylon,
the genus has been reported from Russian Far East (Sakhalin
and Kamchatka) (Shimakura, 1937; Nishida and Nishida, 1995;
Blokhina and Afonin, 2009; Afonin, 2012), Northern China (Sze,1951; Zheng et al., 2008), Japan (Shimakura, 1937; Nishida andNishida, 1995; Nishida et al., 1993, 1995) and Canadian Arctic
Archipelago (Harland et al., 2007). Most of the Upper Cretaceous
Piceoxylon in Japan have been described from the Yezo Group of
Hokkaido. Only one species, P. iwatense M. Nishida, H. Nishida
et Sugiyama, is described from the Kuji Group at Iwate Prefecturein Honshu Island (Nishida et al., 1993).
Among fossil woods of the Piceoxylon, the wood of our fossil
resembles those of P. scleromedullosum Shimakura (Shimakura,1937), P. takahashii M. Nishida et H. Nishida (Nishida andNishida, 1995), P. pseudoscleromedullosum M. Nishida, H.
Nishida, Yoshida et Kaiho (Nishida et al., 1995), and P. burejense
M. Afonin (Afonin, 2012) in having taxodioid to cupressoid,or oopores cross-field pitting without typically piceoid crossﬁeld pitting distinctive of extant Picea. Among them, our fossil
is similar to P. takahashii from the Yezo Group of Hokkaido, in
having lower rays, ray tracheids, tylosoids in axial intercellular
canals, and a few cross-ﬁeld pits. However, the wood of our fossil
is obviously different from that of P. takahashii in lacking distinct
growth ring boundaries and nests of sclereids in pith (Nishida andNishida, 1995).
Although it is probably a new species of Piceoxylon, theanatomical features may not be preserved well because of its
compression wood or its decomposition of tracheid walls (Forexample, helical thickenings on longitudinal tracheid wall).
Therefore, our fossil is identiﬁed as Piceoxylon sp. Our study adds
new geographic and stratigraphic information on Piceoxylon in
Japan.
Comparison with extant woods
The wood of our fossil is initially identified as having
afﬁnities with the Abietoideae form of Pinaceae due to the lackof typically“pinoid”cross-field pits and thin-walled epithelial
cells distinctive of Pinus. Among the Abietoideae, Abies, Cedrus,
Tsuga and Pseudolarix lack both axial and radial intercellular
canals, and Keteleeria and Nothotsuga lack radial intercellular
canals (Lin et al., 1995, 2000; IAWA Committee, 2004; Esteban
and de Palacios, 2009). Thus, our fossil has afﬁnities with Picea,

22

KAZUO TERADA AND KUMIKO HANDA

Larix and Pseudotsuga which have normal axial and radial
intercellular canals surrounded by thick-walled epithelial cells(IAWA Committee, 2004; Esteban and de Palacios, 2009). As
Pseudotsuga may be readily distinguished from most of Picea andLarix by the presence of well-developed helical thickenings on
longitudinal tracheid walls (IAWA Committee, 2004). In the woodof our fossil, helical thickenings on longitudinal tracheid walls are
not preserved. Both the woods of Picea and Larix are very similar.
Noshiro and Fujii (1994) revealed that Larix was distinguishedanatomically from Picea by the presence of fusiform parenchyma
and marginal parenchyma in secondary xylem. In the wood of
our fossil, both of the fusiform parenchyma and the marginal
parenchyma are absent. According to wood anatomical studies
of Picea (Sudo, 1955; IAWA Committee 2004), the cross-field
pitting of Picea is relatively uniform, small slit-like piceoid tocupressoid. Our fossil is different from Picea in having large slitlike pinoid, half bordered taxodioid or cupressoid, and possibly
oopores cross-field pitting. Therefore, our fossil belongs to no
extant genera.
Recent cladistic analyses using cone characters suggest thatmany Pityostrobus species would be placed into intermediate
position between the extant genera of Pinaceae (e.g., Klymiuk andStockey, 2012; Ryberg et al., 2012). Some Pityostrobus species
are suggested to have abietoid affinity, but these are distinct
from the extant genera. The wood morphotaxa including our
fossil might be born at the tip of which such Pityostrobus cones.
Our result might suggest that there is the hidden diversity of the
Pinaceae during the Cretaceous.
ACKNOWLEDGMENTS
We would like to express our thanks to Mr. Fujiki Noda
for collecting and donating the studied fossil. Thanks to tworeviewers: Dr. Shuichi Noshiro (Forestry and Forest ProductResearch Institute, Tsukuba) and Dr. Toshihiro Yamada (Kanazawa
University), for their critical comments and useful suggestions on
our manuscript.
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＜
Anaga Formation ………… 阿那賀層
Awaji Island ………………… 淡路島
Yezo Group ………………… 蝦夷層群
Fukura …………………………… 福良
Izumi Group ……………… 和泉層群

23

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地名・地層名＞Kita-ama Formation ……… 北阿万層
Kuji Group ………………… 久慈層群
Minamiawaji City ……… 南あわじ市
Nada Formation ………………… 灘層
Nadachino …………………… 灘地野

Seidan Formation …………… 西淡層
Shimonada Formation ………… 下灘層
Sen-nan Group …………… 泉南層群
Sumoto ………………………… 洲本