ARTICLE

Memoir of the Fukui Prefectural Dinosaur Museum 17: 1–8 (2018)© by the Fukui Prefectural Dinosaur Museum

DESCRIPTION OF BIRD TRACKS FROM THE KITADANI FORMATION (APTIAN),
KATSUYAMA, FUKUI, JAPAN WITH THREE-DIMENSIONAL IMAGING TECHNIQUES
Takuya IMAI 1, 2, Yuta TSUKIJI 3 and Yoichi AZUMA 1,4
Fukui Prefectural Dinosaur Museum, 51-11 Terao, Muroko, Katsuyama, Fukui 911-8601, Japan
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan3
Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama,
Higashihiroshima, Hiroshima 739-8526, Japan
4
Institute of Dinosaur Reseach, Fukui Prefectural University, 4-1-1 Kenjojima, Matsuoka, Eiheiji, Fukui, 910-1195, Japan1

2

ABSTRACTThe Early Cretaceous ichnofossils of birds numerously occur in South Korea and China. Together with rich skeletal
record from northeastern China, they facilitate our understanding about distribution and diversity of birds during thetime. In contrast, in Japan, the fossil record of the Early Cretaceous birds is poorly known. Here, we report two ichnotaxaof the Early Cretaceous birds from the Aptian Kitadani Formation. We employ three-dimensional imaging techniques
to objectively document and describe the specimens. Specimens include FPDM-F-74 and FPDM-F-75 recoveredfrom an alternating sequence of fine sandstone and mudstone of the Kitadani Formation cropping out in the Kitadani
Dinosaur Quarry, Katsuyama, Fukui, Japan. FPDM-F-74 is large in size, bears slender digits with pointy ends (claw
impressions), and lacks hallux and webbing traces. These characters allow assignment of FPDM-F-74 to cf. Aquatilavipes
ichnosp. FPDM-F-74 is particularly large compared to other Aquatilavipes ichnospecies. FPDM-F-75 is smaller than
FPDM-F-74, lacks hallux trace, and has webbing traces between digits II and III, and III and IV. It is assignable to cf.
Gyeongsangornipes ichnosp. FPDM-F-75 is larger than most avian ichnotaxa with webbing traces. Its webbing traces
between digits II and III, and III and IV are characterized by their extension only up to the proximal half of the digits.
FPDM-F-75 differs from Gyeongsangornipes lockleyi from the Albian Jindong Formation, Gyeongsang, South Korea inhaving larger size and smaller divarication between digits II and IV, and the extension of webbing traces. These specimens
indicate the presence of possibly two medium-sized avian taxa in the Kitadani Formation and increase our knowledge of
the Early Cretaceous avifauna in Japan. Additionally, webbing traces in FPDM-F-75 and other late Early Cretaceous avian
tracks from Asia may suggest that such morphological feature was common among the fossil birds during the time.
Key words : bird track, ichnology, Tetori Group, Early Cretaceous, three dimensional imaging
今井拓哉・築地祐太・東 洋一（2018）3 次元計測手法による，北谷層（Aptian，福井県勝山市）から産出した鳥類足印化石の記録及び記載．福井県立恐竜博物館紀要 17：1–8．
前期白亜紀の鳥類足印化石は，韓国や中国から多数報告されている．これらの化石は，中国東北部で多産する鳥類骨格化石と合わせて，当時の鳥類の分布や多様性を理解する上で重要な資料となっている．一方で，日本における前期白亜紀の鳥類化石記録は非常に乏しい．本研究では，北谷層（Aptian）から産出した 2 種の前期白亜紀鳥類生痕タクサを報告する．研究には，より客観的な足印の記録，記載を行うために，3 次元計
測手法を用いた．標本はFPDM-F-74 及びFPDM-F-75 で，それぞれ福井県勝山市の北谷恐竜発掘現場に露出する北谷層の，細粒砂岩と泥岩の互層から産出した．FPDM-F-74 は足印長，足印幅が大きく，細長い趾と尖った趾遠位端（鉤爪跡）が見られる一方，第一趾跡や水かき跡がない．これらの特徴から，FPDM-F-74 はcf.
Aquatilavipes ichnosp.と同定される．FPDM-F-74 は他のAquatilavipes の種と比較しても大きなことが特筆される．FPDM-F-75 はFPDM-F-74 より小さい．また，第一趾跡が無いのは同じだが，第二趾と第三趾，及び第三趾と第四趾の間に水かき跡が見られる点で異なる．FPDM-F-75 はcf. Gyeongsangornipes ichnosp.と同定される．他の半蹼の鳥類足跡化石と比べて大きく，第二趾と第三趾，及び第三趾と第四趾間の水かき跡が趾全長の半分未満しか張り出さないことが特徴である．FPDM-F-75 は，韓国慶尚道のJindong層（Albian）から産出す
るGyeongsangornipes lockleyi よりも大きく，総指間角が狭い上，水かき跡の張り出し加減が異なる．これらの標本は，北谷層に 2 種の中型鳥類が存在することを示唆し，日本における前期白亜紀鳥類相に関する新たな知見となる．また，FPDM-F-75 や他の複数のアジア産鳥類足跡化石に水かき跡が見られることから，この形態学的特徴は珍しいものではなかったと考えられる．
Received July 29, 2018. Accepted November 16, 2018.
Corresponding author―Takuya IMAI
E-mail: t-imai＊dinosaur.pref.fukui.jp
1

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TAKUYA IMAI, YUTA TSUKIJI AND YOICHI AZUMAINTRODUCTION
Recently, our understanding about distribution and diversity of
the Early Cretaceous bird tracks from East Asia was dramaticallyadvanced. A ubiquitous number of the Early Cretaceousbird tracks is described from the Albian Haman and Jindong
Formations of in the Gyeongsang, South Korea (Lockley et al.,2012). The bird tracks from South Korea represent a great variety
of size and forms and include at least ﬁve avian ichnofamilies. In
addition, the Lower Cretaceous of various regions across China
has yielded numerous bird tracks (Lockley et al., 2013). Together
with the rich skeletal remains of birds from northeastern China
and north China, the fossil record indicates that birds were amajor component of the ecosystem in the continental Asia during
the Early Cretaceous.
In contrast to these continental regions, fossil record of the
Early Cretaceous birds is quite limited in Japan and restricted
to the Berriassian to Aptian non-marine deposits cropping out
in central Honshu (main island). In this regard, the fossil recordof birds in the Tetori Group was first documented by Azuma(1993, Ph.D. dissertation) where seven separate bird tracks
from the Aptian Kitadani Formation, uppermost Tetori Group,
Katsuyama, Fukui are documented through photographs. Unwin
and Matsuoka (2000) reports fragmentary bird skeletons from theHauterivian Kuwajima Fromation, middle Tetori Group, Hakusan,
Ishikawa, and suggests they may belong to Enantiornithes. Azuma
et al. (2002) describes an assemblage of 37 avian tracks from theHauterivian Itsuki Formation, middle Tetori Group, Ono, Fukui,
and assigns them to a new ichnospecies Aquatiravipes izumiensis.
In addition, Imai and Azuma (2015) describes a fossil eggshellfragment, Plagioolithus fukuiensis, from the Aptian Kitadani
Formation and attributes it to an avian theropod.
The latter formation has been known to produce abundantvertebrate tracks (Azuma, 2003; Lee at al., 2010; Tsukiji et al.,2018), and putative avian tracks were occasionally recognizedamong them. Here, employing 3D imaging techniques asrecommended by Arakawa et al. (2002), we describe two different
morphotypes of bird tracks from the Kitadani Formation,
FPDM-F-74 and FPDM-F-75. Examination of the specimens adds
to our database of the Early Cretaceous avian fossil record andaids our understanding of avifauna during the Early Cretaceous ofJapan.
GEOLOGICAL SETTINGSThe Tetori Group, distributed in central Japan, consists ofmarine and non-marine deposits of the Middle Jurassic to the
Early Cretaceous in age. This group is subdivided into the
Kuzuryu, Itoshiro, and Akaiwa Subgroups in ascending order(Maeda, 1961). A variety of terrestrial vertebrates is yielded in the
Itoshiro and Akaiwa Subgroups (Fujita, 2003; Sano, 2015; Sanoand Yabe, 2016). The Kitadani Formation occurs as the uppermost
part of the Akaiwa Subgroup (Maeda, 1961), and was formed ina meandering ﬂuvial system (Yabe and Shibata, 2011; Suzuki etal., 2015). The depositional age of the Kitadani Formation has

FIGURE 1. (A) Location map of Fukui, Japan, with distribution of theTetori Group within the region. (B) Geological map around the Kitadani
Dinosaur Quarry (red circle), modified from Shibata and Goto (2008).
Keys: 1, Kitadani Formation and underlying Akaiwa Formation, Akaiwa
Subgroup, Tetori Group; 2, Omichidani Formation; 3, Hida metamorphic
rock; 4, Terrace deposit; 5, Nohi rhyolite; 6, Quaternary alluvial deposit; 7,
Quaternary fan delta deposit; 8, Andesitic rock.

FIGURE 2. Photograph of the Kitadani Formation in the Kitadani Dinosaur
Quarry, and a stratigraphic section of the formation taken at the red arrowin the photograph (modiﬁed from Tsukiji et al., 2018). The horizon fromwhich the studied specimens were recovered is shown by the black arrowin the stratigraphic section.

recently been suggested as the Aptian (Sano, 2015; Sano andYabe, 2016).
The bird tracks described here come from the Kitadani
Dinosaur Quarry. The quarry, where the Kitadani Formation
crops out, is located along the left bank of Sugiyama River in the

BIRD TRACKS FROM THE KITADANI FORMATION, FUKUI, JAPAN

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Ichnogenus Aquatilavipes Currie, 1981
Diagnosis.̶Three slender functional pedal digits; average
total divarication angle > 100 , sharp claws; hallux impression
lacking; and digit IV longer than digit II.
cf. Aquatilavipes ichnosp. (Fig. 4A–C)

FIGURE 3. Measurement of tracks. See Leonardi (1987) for details.
Abbreviations: Div, divarication angle; DL, digit length; DW, digit width;
TL, track length; TW, track width.

northern part of Katsuyama, Fukui (Fig.1). Abundant vertebrate
fossils including dinosaurs were collected from the quarry since
1989 (e.g., Azuma and Currie, 2000; Kobayashi and Azuma,
2003; Azuma et al., 2016). Furthermore, numerous vertebrate
tracks, including those of amphibians (?), pterosaurs, and nonavian dinosaurs, were recovered (Azuma, 2003; Lee at al., 2010;
Tsukiji et al., 2018). The specimens in this study occur in analternating sequence of mudstone and ﬁne sandstone within the
Kitadani Formation along with amphibian (?), pterosaur, and nonavian theropod tracks (Fig. 2). Suzuki et al. (2015) interprets thissequence as a channel-ﬁll deposit.
MATERIALS AND METHODS
In total of seven putative bird tracks were collected in 1991
from the Kitadani Formation in the Kitadani Dinosaur Quarry.
Among them, two tracks, FPDM-F-74 (first documented by
Azuma, 1993) and FPDM-F-75 (new specimen) exhibited clear
topographic differences that could be captured with portable 3D
imaging devices (Artec Spider and Artec Eva, Artec 3D), andfurther examined in this study. Upon acquiring the 3D data, they
were processed to topographic images by image analysis software(VIVID2Rugle and 3D-Rugle, Medic Engineering Corporation).
For measurements of tracks, we followed Leonardi (1987) (Fig. 3).
Because digital pads are ambiguous in the studied specimens formeasurements of true digit lengths, we measured free length (sensu
Leonardi, 1987) instead.
SYSTEMATIC PALEONTOLOGY
Ichnofamily AVIPEDIDAE Sarjeant and Langston, 1994
Diagnosis.̶Avian track showing three digits all directed
forward; digits united or separate proximally; webbing lacking or
limited to the most proximal part of the interdigital hypex.

Materials.̶FPDM-F-74, single natural cast of left pes.
Horizon and Locality.̶Kitadani Formation, Akaiwa
Subgroup, Tetori Group, Kitadani Dinosaur Quarry, Katsuyama,
Fukui, Japan.
Description.̶FPDM-F-74 is 60 mm long and 94 mm wide, and
TL/TW ratio is 0.63. Lengths of digits II, III, and IV are 40 mm, 50mm, and 48 mm, respectively. Digit widths of digits II, III, and IV
are 4 mm, 5 mm, and 7 mm, respectively. Divarication angles are
56 between digits II and III, and 61 between digits III and IV,
resulting in the total divarication angle of 117 between digits II
and IV (Table 1). Pointy distal ends, claw traces, are observedon digits II, III, and IV; those on digits II and IV curve slightly
distally. Sediment fracture obscures portion of digit II, III, andIV proximally. In total of three digital pads are observable on
the digit IV. Web traces are not observable. Heel impressionis, while partially obscured, shallow-U shaped. Hallux trace isabsent. Three-dimensional scan data is available in STL format as
supplementary data (https://www.dinosaur.pref.fukui.jp/archive/
memoir/17_Imai_supplemental_materials.zip).
Ichnofamily incertae sedis
Ichnogenus GYEONGSANGORNIPES Kim, Kim, Oh, and Lee,2013
Diagnosis.̶Asymmetrical semipalmate, strong mesaxonic
bird track with three widely splayed thin digits impressions (II–IV) with sharp end, asymmetrical web traces proximal to mesial
between digits II–III–IV
cf. Gyeongsangornipes ichnosp. (Fig. 4D–F)
Material.̶FPDM-F-75, single natural cast of left pes.
Horizon and Locality.̶Kitadani Formation, Akaiwa
Subgroup, Tetori Group, Kitadani Dinosaur Quarry, Katsuyama,
Fukui, Japan.
Description.̶FPDM F-75 is 50 mm long and 71 mm wide, and
FL/FW ratio is 0.71. Lengths of digits II, III, and IV are 29 mm, 35mm, and 27 mm, respectively. Digit widths of digits II, III, and IV are
6 mm, 5 mm, and 7 mm, respectively. Divarication angles are
55 between digits II and III, and 46 between digits III and IV,
resulting in the total divarication angle of 101 between digits II
and IV (Table 1). Possible claw traces are present on digit III and
IV. Likely webbing traces between digits II and III, and digits
III and IV characterizes the specimen. Webbing traces extend
less than half the lengths of digits. Heel impression is, while
obscured, shallow-U shaped. Hallux trace is not observable.

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TAKUYA IMAI, YUTA TSUKIJI AND YOICHI AZUMAFIGURE 4. Photographs (left), topographic images (center), and line drawings (right) of the studied specimens. (A–C) FPDM-F-74. (E–F) FPDM-F-75. All
are in the same scale. Dotted lines denote ambiguous track outlines. A contour interval is 0.3 mm. Note that the specimens are natural casts and appear convex.

Three-dimensional scan data is available in STL format as
supplementary data.
DISCUSSION
Conventionally, Mesozoic tridactyl tracks with total
divarication angles between digits II and IV > 100 have been
assigned to birds (e.g., Currie, 1981). Lockley et al. (1992)
provides a list of criteria to distinguish fossil avian tracks. These
criteria include: 1, similarity to those of extant birds; 2, small
size; 3, slender digit impressions, with indistinct differentiation ofdigital pad impressions; 4, wide divarication angle (approximately
110 –120 ) between digits II and IV; 5, a posteriorly directedhallux; 6, slender claws; 7, distal curvature of lateral (II and IV)
claws away from the central axis of the foot; 8, track density; 9,associated fossils and feeding behavior; and 10, sedimentologicalevidence regarding track-bearing deposits. This interpretation
was recently challenged by Xing et al. (2015), which argues

that extant-large-bird tracks may exhibit divarication angles< 100 and small-non-avian-theropod tracks have been reported
to exceed total divarication angles of 100 . Xing et al. (2015)
recommends using the footprint length to pace length ratio to
confidently differentiate large-bird tracks and small-non-aviantheropod tracks. While the approach suggested by Xing et al.
(2015) is acceptable, FPDM-F-74 and FPDM-F-75 are isolated
tracks and do not permit measurements of the pace lengths.
Therefore, we assign FPDM-F-74 and FPDM-F-75 to birds basedon the following criteria after Lockley et al. (1992): 1, relatively
small size compared to majority of non-avian theropod tracks; 2,
slender digit impressions with indistinct digital pad impressions;3, total divarication angles between digits II and IV > 100 ; 4,
slender claws; and 5, distal curvature of lateral claws away fromthe central axis of the foot. In FPDM-F-75, this interpretation
is supported by the presence of webbing traces, which are notknown from non-avian-theropod tracks to date. We also plotted
total divarication against TL for selected small non-avian theropod

TABLE 1. Measurements of the studied specimens. Abbreviations: TL, track length; TW, track width.

BIRD TRACKS FROM THE KITADANI FORMATION, FUKUI, JAPANFIGURE 5. Scatter plot of TL (track length) against Div (divarication)
for selected non-avian theropod ichnotaxa, the Early Cretaceous avian
ichnotaxa, and FPDM-F-74 and FPDM-F-75 listed in Table 2.

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tracks and the Early Cretaceous avian tracks that were previously
described (Table 2), and added FPDM-F-74 and FPDM-F-75 tothe scatterplot. These specimens are plotted with the cluster ofavian tracks (Fig. 5), which is concordant with our interpretation.
Both FPDM-F-74 and FPDM-F-75 are characterized byrelatively-large size and shallow-U-shaped heel impressions.
On the other hand, major differences between the specimens
include divarication angles between digits II and IV and presenceof webbing traces only in FPDM-F-75. Nonetheless, thesedifferences do not necessarily indicate that different species left
the traces. For example, total divarication angles between digits II
and IV is known to vary over ± 10 in a large wading bird, Ardea
herodias (Xing et al., 2015) from the average. Thus, it is possible
the difference of divarication angles between FPDM-F-74 andFPDM-F-75 merely represents this variation. While FPDM-F-75
does not exhibit webbing traces, it is possible that the lack of the
webbing traces in FPDM-F-74 results from poor preservation.
Thus, while we assign them to different ichnotaxa, theirichnomorphological differences possibly reflect preservationalartefact.
FPDM-F-74 is characterized by its large size, which exceeds

TABLE 2. List of selected small non-avian theropod ichnotaxa, the Early Cretaceous avian ichnotaxa, and FPDM-F-74 and FPDM-F-75, with Div (divarication),
TL (track length), and TW (track width).

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TAKUYA IMAI, YUTA TSUKIJI AND YOICHI AZUMA

other Mesozoic Aquatiravipes ichnospecies. Nonetheless, evenlarger Early Cretaceous avian ichnotaxa, such as Shandongornipes
muxiai Li, Lockley, and Liu, 2005, and Limiavipes curriei McCrea
and Sarjeant, 2001, are known (Table 2). Thus, FPDM-F-74 is
not exceptionally large among the avian tracks from the Early
Cretaceous. FPDM-F-74 exhibits the diagnostic characters
for Aquatilavipes erected by Currie (1981) (i.e., three slender
functional pedal digits, average divarication angles > 100 , sharp
claws, and no hallux impression, digit IV is longer than digit
II). However, with only a single track, it is difﬁcult to determine
whether FPDM-F-74 is conﬁdently assignable to Aquatilavipes.
Among known Mesozoic bird tracks with webbing traces,
FPDM-F-75 is distinguished from all ignotornids by the lackof hallux impression (Kim et al., 2006). In addition, the size of
specimens clearly exceeds those of other ignotorid ichnotaxa. The
specimens are comparable in size to Sarjeantopodus semipalmatus
Lockley, Nadon, and Currie, 2004, medium-sized webbed bird
tracks from the Maastrichtian Lance Formation, Wyoming, U.S.A.
However, in S. semipalmatus, the webbing traces between digits
III and IV extends more than half the digit lengths, while not in
FPDM-F-75. Dongyangornipes sinensis Azuma, Lu, Jin, Noda,
Shibata Chen, and Zheng, 2003, is another bird ichnotaxon of
middle-sized semi-palmate tracks whose webbing traces extend
from the distal end of digits II and IV. FPDM-F-75 is larger insize and has both webbing traces from less than half the length ofeach digit. FPDM-F-75 differs from Gyeongsangornipes lockleyi
Kim, Kim, Oh, and Lee, 2013 from the Albian Jindong Formation,
Gyeongsang, South Korea in having larger size and smaller
divarication. In addition, unlike G. lockleyi, FPDM-F-75 has both
webbing traces extending less than half the length of digits.
The webbing trace in FPDM-F-75 is small in comparison
to those of other fossil and extant birds, and its function for
propulsion on or in the water is questionable. Indeed, according
to Leonardi (1987, p. 47), (interdigital) web is regarded as ...
characteristic of partially or wholly aquatic animals. However,in extant birds, interdigital web is present even in non-aquatictaxa such as grouse and some domesticated Galliformes (Kochan,1994). The web of these birds is smaller than those in aquaticand semi-aquatic birds and comparable to the one inferred for
FPDM-F-75. Thus, interdigital web does not necessarily imply
water-related habitat and incipient web can be present in fossil
birds for other (yet unknown) functional purposes.
Discovery of FPDM-F-74 and FPDM-F-75 in the Kitadani
Formation indicates the presence of possibly two medium-sizedavian taxa and increases our knowledge of the Early Cretaceous
avifauna in Japan. Additionally, web traces in FPDM-F-75 andother aforementioned late Early Cretaceous avian tracks from
Asia may suggest such morphological feature was not uncommon
among the fossil birds during the time. The lack of trackways
does limit our understanding about the ichnotaxonomy andichnomorphology of the avian tracks in the Kitadani Formation,
which should be elucidated with further excavation efforts in theformation in the future.

ACKNOWLEDGMENTS
We thank members of the 1st Kitadani Dinosaur Excavation
Project where the studied specimens were discovered.
Especially, S. Hayashi contributed to the initial data collection
and documentation of the specimens during the project. M.
Shibata and S. Kawabe provided constructive comments to themanuscript. Access to Artec SpaceSpider was kindly provided by
DataDesign Co., Ltd. (Nagoya, Aichi, Japan). We appreciate S.
Ishigaki and T. Kubo for their thorough review and comments that
greatly improved the manuscript.
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: in Chinese with English abstract: in Korean with English abstract: in Spanish with English abstract: in Japanese with English abstract